FROST on Frost Walls

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 is 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, etc.

Maximum moisture content of concrete by weight is 6%. This image shows saturated concrete, typical of when vapour barriers are installed before concrete is dried.
Click to enlarge what the Alberta Building Code says about Condensation
Temporary propane or natural gas heaters produce high moisture levels. They are often used when rough ins occur.
You can see the result of heating a basement with temprioary heating. Heavy condesation on t ehconcrete will be absorbed, inhibiting drying.
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
Note that condesation is normal. Excessive condesation is not.
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