Oh, that’s gonna smell…

What is it?

What is it?

 

Sometimes you just get a lot of the same thing around the same time. The past couple of weeks it’s been the wrong way to insulate concrete walls. Out of the three instances recently, two were a bit, shall we say, aromatic. That’s a high percentage, but on those two I was called because of the odor. I think that increased the odds.

What is this dastardly method? Just the most common one. You frame a 2X4 wall as close to the concrete wall as you can get it, install fiberglass batts (other permeable insulation would work too), cover it with drywall and wait. If you can add an aquarium or a hot tub you won’t have to wait as long.

You could put a sheet of plastic under the drywall if you want to intensify things, too. That just slows down any drying potential. Remember, unless it’s perfect (really, really hard) plastic isn’t going to stop all the air from getting in.

Let’s look at the ingredients. We have a heated space (that’s why you insulated), warm air holds moisture better than warm air. We have a high mass (concrete) wall that’s relatively cool. Separating them we have an assembly that only slightly slows air movement. So the air in the room infiltrates the wall, condenses on the cool concrete and stays there. This is pretty much the recipe for mold growth. Whether you end up with a smelly mess depends on the amount of water and the speed of infiltration. If there is another place for the moist air to go more easily it will. Not to disparage air, but it always takes the path of least resistance.
Quiz time. Is the infrared image above one of the stinky walls or the non-stinky one. Hints: The darker colors are cooler. The studs have a lower R-value than the insulation. Water has a lower R-value than wood.

Tick.

Tock.

Tick.

Tock.

Time’s up! This wall does NOT stink. It’s not that wet. Check the temperature spread on the right of the image. The total range is just over 5F. Not very large given the temperature difference between inside and out, which was about 30F. The pattern shows some air infiltration at the top of the wall (the was no blower door, for you geeks) and a splotch in the middle that could be a compressed batt. So that was kind of a trick question.

But how do you make sure your wall won’t stink? Install rigid insulation* against the concrete. At least R-10, preferably double that. Then put 3/4” strapping over that, there are numerous attachment systems to fasten it to the wall, and screw the drywall to that. You’ll have to carve the electrical boxes into the foam (don’t overdo it) or use 2X stock on flat instead. Make sure that the seams are tight and tape them. Offset them too. No moisture is going to make it to the concrete, and the surface of the foam will almost certainly be above the dew point. No water, no smell*.

 

*I prefer XPS, but Polyisocyanurate will work too.

*Apologies to Bob Marley

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I love buildings. I love thinking about how they are made and how they work. I am a building geek. I love to talk about buildings too. Get in touch.
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