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.

OUTDOOR TEMPERATURE            MAXIMUM INDOOR RELATIVE HUMIDITY       

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

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:

1. LOCALIZED –

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.

2. GENERALIZED –

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

IS IT NORMAL?

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.

WHAT CAUSES IT?

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.

HOW TO FIX IT?

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.

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.

FREE 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

RELATIVE 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.

ABSOLUTE HUMIDITY

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.

DEW POINT

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.

1. ENERGY

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.

EXAMPLE

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!

2. MOISTURE

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

EXAMPLE

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.

information pending