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RH (Relative Humidity) is the measure of water vapour in the air relative to its temperature – or its ability to hold that water vapour.

At 100% the air  can no longer hold any additional moisture – called dewpoint

To decrease the RH you can do 2 things

  1. Increase the temperature
  2. Dilute the air by adding dryer air ( from outside)



Average interior environment of a home

21 C (Degrees Celcius) with 30 % RH

The Dewpoint is 2.78 C and any surface colder than 2.78 C will have condensation

Mould will grow at a minimum of 70% RH


If your house is 21 C with 30 % RH and the outdoor temperature is -25 C, the temperature next to some exterior walls such as inside closets, at windows, etc will have very cold surfaces due to limited insulation values. At 7.87 C, the RH will be 70%, and from 7.87 C to 2.78 C the RH will be between 70-100 % or enough moisture to grow mould on surfaces that have a food source  (drywall, dust on windows, etc)

Lowering the temperature overnight to 16 C, will cause the RH to be 42 %, or lower the air’s ability to hold moisture. So what will the inside temperature of a closet or window drop to if the outdoor temperature is very cold, say -20 C?


Here are some Do’s and DON’T’s

  • Don’t block heat vents
  • Don’t place clothes or other items against exterior walls 
  • Do leave closet doors open when outside is really cold
  • Do open blinds and curtains to let heat access the windows
  • Do TURN ON your Principal Exhaust Fan when outside is really cold
  • Better yet – upgrade to an HRV if your buying new or renovating 
  • Do buy a hygrometer and keep your interior humidity at an appropriate level to prevent condensation

In simple terms, when air cannot hold anymore humidity (vapour) it begins to turn to liquid, or sometimes referred to as “sweating”

Condensation on windows is an indicator of high humidity in a home. There are many sources of humidity that can cause your windows to sweat.

If your windows are sweating (have condensation forming) then likely you have too much humidity in your house.

Many people find that when they buy a  new home or upgrade their old windows to new ones they get more condensation – That is true. Why?

Why new homes have more window condensation . . .

Most new homes are built very energy efficient. That means they have a sealed combustion furnace and are built very air tight. That also means minimal fresh air from outside coming in to dilute the humidity in the house.

If the house is built energy efficient and air tight you also won’t run the furnace as much which is how you get fresh air into the home.  Unless you switch on the principal exhaust fan (located in the hallway near the thermostat) you won’t be able to reduce the humidity.


HRV (Heat Recovery Ventilator) 

If you have an HRV, that unit will run all the time and bring fresh air in and exhaust stale air out. It keeps the humidity steady and indoor air quality high. Most new homes SHOULD have one, but most homebuyers are not aware of these units and are more interested in granite counter tops than fresh air until the first winter when the windows start sweating.

Here are the maximum humidity levels you can expect to run your home before the windows sweat.



Dual Glaze windows         Triple Glaze Windows

-30 Celsius  and Colder                   16% at 21 Celsius               32% at 21 Celsius

-29 Celsius to -24 Celsius                22% at 21 Celsius               38% at 21 Celsius

-23 Celsius to -18 Celsius                24% at 21 Celsius               45% at 21 Celsius

-17 Celsius to -13 Celsius                28% at 21 Celsius               50% at 21 Celsius

-12 Celsius to -6 Celsius                 33% at 21 Celsius                62% at 21 Celsius

-5 Celsius to +2 Celsius                  40% at 21 Celsius                68% at 21 Celsius


NOTE 1 – Minor sweating may still occur along bottom edge of window glass depending upon the spacer insulation and window material

NOTE 2 – Wind will have a dramatic effect on the glass temperature and decrease the indoor maximum relative humidity

NOTE 3 – air circulation across the window glass will decrease condensation on the glass as it warms the glass surface

NOTE 4 – Always leave curtains open, leave blinds up 10-12”, do not place furniture in front of windows blocking floor vents

*This table was compiled by comparing Canadian Window manufacturers data

When air from the house leaks into the attic, it condenses and freezes.

That could be caused by:

Air Leakage from house into attic:

  • High interior humidity
  • High interior air pressure from stack or mechanical pressure
  • Inadequate air sealing techniques
  • Disconnected Vents – Fans, Dryer, Kitchen Hood, Etc
  • Exterior high humidity entering the attic space through soffiting – IE Front door under large vented soffit area

To stop the build-up you need to prevent the humid air from entering.

Where is leak from? With windows, the leaks can come from a variety of locations. However many times they are not from the actual penetration itself. Window nailing flanges rarely leak, therefore usually the leak is from above or from the window itself. 


Where is leak from? Water entry into the wall cavity does not always present as a leak.

Stucco cracking and discoloration, drywall cracking, wet carpet, staining on ceilings also provide indications that water is entering.


DON’T WAIT – get it checked!


Installation or window unit . . .  To rule out the window, seal it off from the penetration by taping poly to the frame. After the test  the poly can be removed and the window  itself can be tested.

Upper windows that are installed with the lower paper flap behind the wall membrane allows entry into the lower window via joints in sheathing.

We use a blower door to depressurize the house when doing water tests to simulate wind against the building.


Soffits  weather barriers missing from behind soffits and insulation stops are a common source of windows leaks. The house to the left had windows leak when there was wind driven rain

After water testing we determined the soffits were the source. To confirm we did final destructive test and found the paper was lapped incorrectly as seen below.


Verify to reduce destructive testing when not necessary verification can be done using infrared thermography, moisture meter or straight visual inspection.


Destructive Testing

After we are sure of the water path a destructive test can be conducted by opening the location and viewing both the source of the leak ( i.e. lap issues with paper) or the damage resulting. This can be documented for forensics or repairs.

Attic rain is when you have an accumulation of frost from air entering the attic from the house.

Usually takes extended periods of cold weather.

Once the air warms or the sun shines on bare shingles, the frost melts quickly, raining down into the insulation and sometimes leaving staining and dripping from lights.

Adding ventilation WILL NOT PREVENT it.

Newer homes with higher insulation levels have limited  radiant heat loss. Warm air is required to absorb moisture from the attic. Cold attic means air can’t absorb much moisture. Meaning ventilation wont stop it.    ** ventilation will help dry the attic after

Older homes can experience attic rain after adding a new furnace or additional insulation.  New furnace creates positive pressures and added insulation eliminate radiant heat loss needed to absorb moisture.


There are two types of attic rain:


Usually from a larger hole or penetration in the attic space.

Unsealed exhaust vents (bath fans and dryers) let air escape into the attic.

If you check your attic during cold weather, you will see frost accumulating around the air leakage area.



Usually occurs throughout the attic

High interior relative humidity exacerbates the issue

Also often affects the walls, although you may not see the evidence in all cases

OSB resin staining often related to excessive air leakage into walls and attic spaces during cold weather, forming frost and when melting, absorbs the resin from the OSB sheathing



It is common to have some condensation or frost in attics and inside walls, but it should be small amounts. This is not an issue,  as these small amounts it will usually sublimate (change directly from frost to vapour) before it melts.

At times there can be significant accumulations of frost. When this melts, it causes stains in the ceiling, dripping inside lights or in windows, and the most common giveaway is stains on exterior cladding.



Attic rain is caused by the rapid melting of  excessive frost in the attic or walls. It is caused by a poor understanding of building science.

There are two main factors. Holes & Pressures.

Newer homes, renovated homes or if simply upgrading your furnace make you susceptible to attic rain.  WHY?


OLDER HOMES had a chimney or “B” vent to exhaust hot combustion gases. When air leaves the house through that chimney, it creates a negative interior pressure. That creates a slight suction, which will pull outdoor air into the building. The air outside is dry, having very low amounts of vapour, therefore as is comes into the building it absorbs moisture, acting as a natural drying mechanism every time your furnace runs.

(as air warms its relative humidity goes down. Same amount of moisture, but warmer air can hold more of it, hence it is relative to temperature)


NEWER HOMES have no chimney. They use what is called a sealed combustion system. You have a sealed pipe into the furnace for combustion air and a sealed pipe out for exhaust. No Pressure change. You do however require a fresh air intake into the return air ducting to ventilate the home. Now, every time the furnace runs, it brings air into the house creating a slightly higher interior pressure, which forces air outward. That is where the holes come in. Limiting holes equals limited condensation. Big holes, which never mattered in older homes, due to the drying when the furnace ran, now are critical to keep small.

The irony is, that you can reduce this by turning on the switch under the thermostat labeled “Ventilation Fan”. This activates a fan that removes the same amount of air that the furnace is bringing into the home, hence equalizing the pressures. You actually can control the attic rain in many instance by simply running that fan all winter, but many builders and most homeowners don’t know this. Even mechanical contractors are not aware of this phenomenon. (we test over 50 new homes a year for this problem)

The other irony is that if you run that fan 24/7 through the winter your heating bill will outpace the savings of that high efficient furnace.

A less expensive operating system is an HRV (Heat recovery Ventilator) that operates balanced and saves money. (other than a small upfront cost to install during construction) An HRV brings in equal amounts of air as it discharges, preheating the incoming air with the heat from the exhausted air. It provide higher indoor air quality as well.



First you must find the holes (air leakage locations). To do that you need to induce air movement. This can be accomplished by using a blower door to depressurize the home. Under a negative pressure, cooler air drawn into the home through the holes will leave a signature that can be found using an infrared camera. These holes should be verified by wetting the back of your hand and placing it next to the opening. You can then also feel the cool air. In the summer, this air will often be warmer, since it may be hotter in the attic than the house.

Now you can seal the holes.

If you can keep the total square inches of total openings into the attic to less than 4 square inches, you should not have issues. (as long as there are no abnormally high interior pressures)  Potlights and bathroom fans are number 1 leakers.

Next you need to equalize the interior pressures. Test the mechanical system as a whole, with the ventilation fan on and then also off, as the furnace runs. Also do a pressure test on each bedroom. If there is insufficient return air to the furnace, a room can have too much supplied air and not enough return, causing a higher pressure. A transfer grill or undercut door can increase the flow. Use a diagnostician that understands these issues and how to test for them.


Attic rain is NOT the result of lack of roof vents.

Over ventilating the attic can cause the wind to “suck” air from your house, into your attic, making the problem worse.

With higher energy efficiency comes higher risks of moisture. Why?

How do you dry your clothes? Places them in an airtight bag and keep them cold? No, you let lots of warm air get to them to absorb the moisture quickly.

New homes have less heat loss (higher insulation values) and are more air tight. They don’t dry very well if they get wet. Then there are basements . . .

. . . frost walls in a basements.

Actually there are only two kinds of frost walls, when you insulate from the interior:

  1. Frost walls that condensate 
  2. Frost walls that will condensate 

Homeowners have experienced more basement condensation issues in recent years due to several issues.

The main reason is that we insulate the inside of a concrete wall. Just to make sure it’s really really cold, so when air from the house reaches the surface, it for sure sweats.

Next, is that a lot of the 1000 gallons use to make that concrete wall is still in it. It was probably poured 3 months ago in cold, nondrying weather conditions. Then we used temporary propane heat while we did all the rough in mechanical and electrical and added water back into the concrete.

Finally we put a vapour barrier from top to bottom so it can’t dry. Then we assume that the upper part of the concrete sticking out of the ground will dry it. Nope. In fact when the sun shines on that concrete, it creates what is called a vapour drive and forces the moisture inside. You may notice this on the interior side of the poly on a warm day. You can see moisture. In fact sometimes it’s so much it runs down and looks like a basement wall leak. Nope, just sweating.


If you have a new home, YOU HAVE high moisture and condensation in the basement frost walls. Even if you can’t see it yet, you have it.

Most people find when, in the middle of winter, they pull of the poly to develop the basement and low and behind, the concrete has a thick layer of frost.

This combined with “old” or “we’ve always done it this way” techniques cause issues. These old ways are, unsealed poly, air space at the top of the walls between concrete and frost wall, etc.

How to reduce condensation behind the wall and maintain long term protection and performance.

First – Insulate the exterior of the concrete – don’t’ insulate the interior. Warm concrete cannot condensate and as it sounds, is warm, meaning comfort. That giant heat sink maintains nice even comfortable temperatures. Sure there are other ways, but they don’t work.
Second – If you already are committed to doing interior insulation, then mitigate potential issues by doing the following.
1. Seal the upper portion of the frost wall between the concrete.
2. Seal the vapour barrier as you would seal it in the upper portion of the home. Joints on solid backing secured in place by something to prevent air from getting behind it.
3. Use a PWF treated bottom plate on the frost wall. Seal it to the floor with 2 beads of caulking when you put the wall in place. This will stop any condensation behind the frost wall from getting under the frost wall. The materials behind the frost wall are pretty resistant to moisture (reasonable levels of moisture, below 70% RH and/ or 20% moisture content by weight)
4. Use a Smart Vapour Barrier™ Such as CertainTeed Membrane™ which become vapour permeable at high humidity allowing better drying. Alternately, you could remove the lower 1/3 of the vapour barrier and apply an air barrier like housewrap, as long as it is sealed.
5. If you are developing your basement WAIT for 1-2 years after construction to allow the walls to dry. If you can’t wait, cut off the lower 1/3 of the poly when you install the drywall. The drywall acts as an air barrier and stops the humid air from reaching the concrete and the drywall with 2 coats of latex paint will allow for drying of the wall assembly. Since gravity brings all moisture to the bottom.

Rules of performance
– Air barrier is 100 times more important that vapour barrier

– Vapour barriers prevent drying, NEVER double vapour barrier

– If it gets wet, dry it, quicker is better

Insulation values provide some indication of the efficiency of a home. However there are other factors. If air leaks through the fiberglass is makes a great filter but the R-value drops to 3-4. It also can contribute to condensation in the walls and attic.

Thermal bridging ( heat transfer to cold through a material)  – Studs have an insulation value of 1.2 per inch so each stud value is only R-7. If you have a 3 ply 2X6 post supporting your roof inside a closet, and the closet is closed during cold weather you could expect to see frost on that wall. Sometimes if you pile things up against that wall and the warm air cannot access it, it will actually freeze to the wall.

Don’t pile things against exterior walls

Do keep closet doors open

Adding exterior insulation is always a good option. You increase the house warmth, reduce thermal bridging, but better yet, you decrease the risk of condensation and frost inside the wall.


Different materials are available for insulating your home

Current Code minimum

Currently the minimum required insulation levels for new homes built to current Alberta Building Code are as follows:

(Nominal rather than effective)

  • Exterior walls R-12
  • Cantilever floors ( including bonus rooms) R-20
  • Attics R-34
  • Basement walls R-8

Most new homes generally use

  • Exterior walls R-19
  • Cantilever floors ( including bonus rooms) R-28
  • Attics R-40
  • Basement walls R-12

By the way – in 2013 the building code proposals will  increase energy requirements and ratings will be in the R-22 effective range for walls ( nominal about R-24-26) 

Using a nominal R-19 fiberglass batt insulating and building with 24 inch on centre spaced studs provides an effective R value of about R-17 while 16″ on centre spacing is around R-16.  ( depending on how much solid lumber thermal bridging is in a wall)


Mineral wool batt insulation

Mineral wool insulation provides sound barrier and better insulation value as well as being fireproof.

  • 5 1/2″ wallNominal R-22 ( effective R 20.5)

You can also blow insulation into the walls

R values are an effective R-20 to R-22



  • Average dual glaze windows are about R-2
  • add Low E and argon R-3
  • Triglaze Low E argon R- 6



Attics have several options

  • Fiberglass batt
  • Fiberglass blow-in
  • Cellulose blow-in


Spray Foams

Spray foam insulations come in 2 main varieties:

  • Closed cell – 2 pound density –  acts as vapour barrier, air barrier, insulation  R- 6/7 per inch
  • Open cell – 1/2 pound density –  acts as air barrier, insulation (requires vapour barrier) R-4 per inch

Very air tight if you also seal the framing connections ( studs to plates, between plates, etc.)

An HRV or HEAT RECOVERY VENTILATOR provides fresh air to your airtight energy efficient home.

It controls humidity levels so you don’t have sweaty wet windows.

It reduces indoor air pollutants especially from new carpet, hardwood, paint, cabinets, doors & trims, plastics, insulation, etc, etc.

If you are sensitive or have allergies you will find these units very helpful in reducing congestion.

And it operates in a balanced way, so there is equal amounts of air into the home as are exhausted out.

Prior to the changes to the building code requiring energy efficient designs, houses typically had a PVF ( Principal Ventilation Fan). The fan would only activate when you manually switched it on.

Without it on, the furnace of a sealed combustion system (high efficiency heating) will only intake air, producing a high indoor static air pressure. This can force interior humid air outward into the walls and attic spaces.

Always run this switch through the winter – 24/7

Without it you are subject to ATTIC RAIN.

You can also upgrade from a PVF to an active HRV.


You will spend about $3500 on an HRV.

But wait – you get more than just clean fresh air, it also saves you enegy.

If you have a $200,000 mortgage and add a $3500 HRV your mortage goes from $1163.21 to $1183.57 (based on a 25 year, 5 yr fixed term @ 5%)

That increases your monthly payment by $20.36.

BUT – your yearly energy costs drop by $255.70 or $21.31 per month SAVINGS.

Net saving each month day 1 is $0.95 or a free HRV!

Houses built to Building Code are designed for a maximum interior Relative Humidity at 21 Celsius of 35% during winter.

Humidity is the measurement of moisture in the air in a gas vapor form

There is Relative Humidity and Absolute Humidity



Warmer air holds more humidity (vapor) than cool air. This does not change the amount of vapor in the air (absolute humidity), warm air can hold more vapor so it is relative to the air temperature. If your Relative Humidity is too high, increase the temperature, which will allow the air to hold more humidity before it deposits it in the form of condensation.



The amount of vapor in the air does not change as the temperature changes. It is absolute, until you add more humidity by evaporation or other sources or we reduce the humidity by removing or diluting with dryer air.



The maximum amount of vapor in the air at a given temperature before it cannot hold any more humidity and it turns to liquid.

If your home is at 20 degrees Celsius and 30% Relative humidity, the dew point is around 3-4 degrees Celsius. In other words, surfaces that are colder than 3 – 4 degrees will condense.

As you increase the temperature the relative humidity is reduced.

However here is the WARNING – if you reduce the temperature overnight to a cooler temperature, i.e. 15 degrees Celsius, you will have more moisture on the windows in the morning because the air cannot hold that moisture and because the air is not keeping the glass warm.


Too Much Humidity?

At times a house can have too much humidity.

When outdoor temperatures are cold you want to lower the indoor humidity.

For instance at -20 Celsius the maximum relative humidity level with Dual Glaze windows is about 25% before condensation. Dropping to -30 means a maximum humidity of 15%.

To keep track, it is a good idea to have a hygrometer which tells you the humidity and temperature.

To reduce the humidity, you can increase the air temperature or you can dilute it by adding dryer outdoor air.


Use the ventilation fan switch under the thermostat

This fan is designed to remove air, it turns on the furnace which brings dryer air in, and that air is mixed with the house air to reduce humidity.

Watch the humidity and run the fan when it gets too high. Buy a hygrometer ( left) to track relative humidity levels.

If your humidity does not reduce you may need to check the air volume of the ventilation system. At times these are undersized and are the cause of high humidity.


You can record how the home is operated by placing a data logger in the house for several weeks. You can place one outside to see the exterior temperatures as well. 

It can record temperature, relative humidity and dew point.

This can be useful in helping a homeowner see where they can reduce humidity levels through ventilation.

Below is a sample graph showing the operational choices for this home. You can see where the levels spike each day due to showers and when they drop when everyone leaves the house.

The dew point is at almost 10 degrees Celsius – in other words any surface colder than this temperature will condense.


Tests conducted with a blower door are fast and safe. The house is either pressurized or depressurized by installing a large cloth frame into a doorway and using a fan to move air in or out of teh building.

No one has to leave and the pressures created are to simulate an approximately 25 km/hr wind.


A blower door is useful in testing many variables in a home: 

  1. Air Leakage Volumes
  2. Depressurization Problems
  3. Air Infiltration or Exfiltration Issues
  4. Wind driven water leakage (IE windows)
  5. with Infrared Thermography
  6. Zone Pressure Diagnostics

Energuide, Energy Star, LEEDS, and many other energy programs for homes require a blower door test to calculate the volume of air leakage to provide the energy rating

There are three basic components of a blower door:

  1. Door frame /cloth
  2. Fan
  3. Manometer

A variety of diagnostic testing can be conducted with a blower door.

A simple test that can be conducted uses the building’s interior volume and can calculate the air change rate of the house, including whether the openings are made up of several larger penetrations or hundreds of tiny ones.

Zone pressure diagnostics calculates which surfaces have the highest levels of air leakage. This is especially useful in multi family buildings where party walls can be a cause of higher leakage.  


Pre-Drywall Air Testing – Spray Foam Application

RCI Alberta is a local company in Central Alberta that contracts us to perform air tightness testing on the house after the foam is applied.

First the blower door is installed and set to depressurize the building.

Next, using an Infrared Camera we check the exterior surfaces and verify if there is missing insulation or air leaking at temperature changes  (shows dark in winter and light in summer) showing on the Infrared Camera.

Finally, these areas are marked and the contractor returns and touches up these areas. (usually very minimal)

This provides incredible air tightness and assurance that there should be no moisture issues associated with air leaking outwards onto cold surfaces such as OSB sheathing.

Do you really need to keep your relative humidity at 40% or higher if you have a hardwood floor?

Well, what does the science indicate?

Whatever relative humidity you keep a piece of wood in, will reach a moisture content equilibrium with that humid air – sometimes call acclimatizing.

Below is a chart using the following formula from US Forest Products Laboratory: 

Moisture content (MC) is a measurement of the percentage of the water in the wood by weight.

Different species are slightly different, but average saturation of the cellular materials is 28% by weight.

If you look at the following chart, you can figure that wood kept in a room at 20 C with a 35% relative humidity (RH) will reach a moisture content of 7%.

Therefore, if you install a piece of hardwood at 7% MC over a subfloor with 7% MC in a building that is kept at 20 C and 35% RH, you have to maintain 35% RH to maintain its stability. Higher humidity will cause swelling, while lower humidity will cause shrinking.

The wood you choose should be stored and acclimatized in the local climate where you are building.

Wood shipped from Ontario to Alberta and installed without reaching local equilibrium will cause you grief and you will have to maintain 40-50% RH.

Your house operates as a system. You can’t simply have high humidity without addressing other issues. 

If you plan on hardwood, get better windows and a good ventilation system that can properly maintain the correct humidity.

  1. Check with your local supplier as to the MC of the wood you are buying
  2. Make sure you check that your building is stable and the subfloor is dry – use measurement tools!
  3. Site finished will require more care and attention.
  4. Don’t be adding moisture before installing your flooring. Don’t pour concrete or be taping & painting.
  5. Deliver the wood early and allow it to reach equilibrium only once the building is dry and going to stay dry.
  6. Use a data logger to record the RH the weeks prior to installing your hardwood, and adjust your plans accordingly.
  7. Use a moisture meter and record the MC of the subfloor before installing the hardwood.
  8. To keep your hardwood stable, you need to maintain the moisture and humidity it was originally installed at.
  9. If you are installing hardwood in your home make sure you design the home to handle the required humidity.
  10. Install an HRV and a good humidifier. Also install a climate control that will measure and adjust the indoor humidity based on the outdoor temperature.
  11. Install good triple pane windows if you install hardwood.



Installing high MC hardwood on high MC subfloor in high humidity and you will have to maintain that high humidity.

In cold climates this can be a real problem due to the risks associated with high humidity.


MAXIMUM HUMIDITY to prevent window sweating 


Dual Glaze windows         Triple Glaze Windows

-30 Celsius  and Colder                   16% at 21 Celsius               32% at 21 Celsius

-29 Celsius to -24 Celsius                22% at 21 Celsius               38% at 21 Celsius

-23 Celsius to -18 Celsius                24% at 21 Celsius               45% at 21 Celsius

-17 Celsius to -13 Celsius                28% at 21 Celsius               50% at 21 Celsius

-12 Celsius to -6 Celsius                 33% at 21 Celsius                62% at 21 Celsius

-5 Celsius to +2 Celsius                  40% at 21 Celsius                68% at 21 Celsius


The above chart shows the maximum humidity levels at various temperatures.

Below the recommended RH level should keep your windows from sweating too much.

These temperatures are weather dependent,  wind will require a lower RH, as it will cool the windows further.

Keep the blinds and drapes open and get the air in th e house to move over the windows – circulation.

During prolonger cold outdoor weather, turn down the indoor humidity. Lowering the RH for a week or 2 will not significantly affect the hardwood flooring

The higher the humidity is in the winter months the higher your risk of having moisture inside your walls or attic.


For Energy considerations 35% of your heating requirements can be caused by air leaking!

So you want a very tightly built house. . . BUT

Some builders focus heavily on air tightness and forget that just because it is airtight, you need fresh air also.


Wrap yourself in a plastic bag to keep warm on a cold day. Where does the moisture go?

With a tight, energy efficient home, you get good energy efficiency. But how to you get rid of the humidity that can build up in the house?

Especially when it is too cold to open a window but the mechanical ventilation provided by the furnace is not bringing in fresh air because the furnace doesn’t run due to minimal heat loss from a very efficient home.

ANSWER: Active HRV (Heat Recovery Ventilator)

It will run independent of the furnace constantly bringing in fresh air and preheating that air with the exhausted stale air.- NOW seal that house tight!



Frost in your attic?

95% of the time it is NOT poor ventilation

95% of the time it IS air leaking from the house into the attic space. That air condenses and freezes and over time accumulates



My neighbour this winter had a huge accumulation of frost in his attic, to the point that when it warmed up to above freezing, water from dripping from several doorways on the second floor.

With a thermal imager we found several “pools” of water in the attic.

During our testing, the Attic access lid and several main plumbing stacks had significant air leakage.

These were all sealed with a material that would stay in place (Not Acoustical Caulk)

2 weeks later the frost returned. Upon further investigation we found another significant opening next to the furnace “B” vent that went 2 storeys to the basement.

Once sealed and after 3 weeks of significant cold weather – no more frost in the attic.


Testing for Air Leakage

A blower door test is a fan that is placed in an exterior doorway and removes some air, placing the house under a negative pressure.

Using a Manometer, the pressure difference created can be measured and calculated when entered into the software to determine the ACH or air leakage.

A blower door test will generally only take 15-20 minutes to collect the data.

We can also use this tool at the pre-drywall stage when you have just insulation and poly in place. This can be useful to identify and correct air leakage before drywall is installed.

When used with a thermal imaging camera you can literally “see” the leakage areas as air leaking back into the building is cold and cools surfaces that can now be seen with the infrared camera.


Air Sealing

Here are some of the areas to consider when looking at air sealing a home.

Air Leakage, Sealing

What is Infrared Thermography?

An infrared thermal imaging camera sees emitted energy in the form of heat from objects that it is pointed at.

It cannot see air

It cannot see through glass

It cannot see through walls

It cannot see moisture

It cannot see mold

It CAN see the effects of heat signatures of these items. Therefore, it is like your doctor looking at an X-ray. A certain level of training is required to diagnose and know what your camera sees.

And you need to confirm what the camera sees with another tool such as a moisture meter or smoke pencil.

An infrared picture is called a thermogram.

Darker on a thermogram indicates cooler while lighter is warmer.

The camera is so sensitive the temperature differences it records are as fine as 100th of a degree.



If the exterior is colder or hotter than the interior and we use a blower door to depressurize the house we can see where air leakage inwards can cool or warm the surface it is moving across.



If there is a wet area it will either register hotter or colder than the surrounding areas and has a specific signature. But to confirm another tool is required to test the moisture of the material that appears to be wet. A moisture meter ( either pin or non-destructive type) is used to check this.

Mechanical pressure imbalances can lead to all kinds of moisture problems and also produce very poor comfort zones in your home.

Testing can determine:

  • if there is enough heat being provided
  • If it is properly distributed
  • If the heat loss calculation is accurate
  • If you are pressurizing a zone or the entire building
  • What needs to be done to bring the pressures into balance and increase the comfort.


Sealed Combustion Furnace Pressure Testing 

-using the blower door manometer

 Older homes used a “B” vent or chimney to exhaust hot air for heating the house. This created a slight negative pressure that “sucked” air in through openings, penetrations, pot lights, etc. This provided drying and reduced air leakage into walls and attic spaces.

 Sealed combustion furnaces (High Efficiency) only have a fresh air intake into the return air system. The combustion is sealed and has no effect on the air pressures. Since the furnace brings fresh air in, but has nothing discharging air, it creates a slight positive pressure. Sometimes the pressures can be excessive leading to problems. This can push humid air into the exterior walls and attic spaces.


Average pressures created in new homes is 3 Pascal’s as an average. In some cases, tested homes were as high as 21 Pascal’s and the air forced into walls and attic spaces is tremendous.

The higher pressures mean the more “perfect” the interior air barrier (poly) must be.



Principal Exhaust Fan Pressure Testing 

-using the blower door manometer

The principal exhaust fan is the way to get fresh air when the furnace is not running. It is also a way to reduce excessive interior positive pressures in many cases.

A properly balanced Principal exhaust fan is required by the Alberta Building Code 

If the Principal exhaust fan has been balanced to the intake, the pressures will be balanced when the ventilation fan is running.

This fan is not automatically run when the furnace is running and must be manually turned on by the occupants.

information pending

information pending

Complete heat loss and heat gain calculations are required for all heating and cooling distribution systems designs. These calculations ensure that the mechanical system is tailored to the requirements of each zone in a building and provide the information required for the optimal sizing of the mechanical system for your specific project’s scope and budget.

information pending

Contact Us

Getting in touch with us is easy, call at one of our office numbers or fill out the form below:

Calgary: 403-251.0683
Toll-Free: 1-866-241-6804

Edmonton: 587-991-8257

Medicine Hat: 403-251-0683

Central Alberta: 403-251-0683

British Columbia

Vancouver Island: 778-441-3838

Lower Mainland: 1-866-241-6804

Revelstoke: 1-866-241-6804

Okanagan: 1-866-241-6804

Smithers: 1-866-241-6804


Greater Toronto Area: 1-866-241-6804

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Qualistat Building Performance Consultants - Head Office Address
514, 3208 8 Avenue NE
Calgary, AB

Phone: 403-457-7828
Office Hours: 8 am - 5 pm, Mon - Fri