Insulating an above-ground basement wall
The basement is mostly above grade. there is about 16″-24″ of concrete foundation wall with 2×4 framing above. to be clear, much of the concrete is not below grade.
The homeowners would like to do the insulating themselves. the budget and anecdotal evidence have made them leery of spray foam. the framing is mostly done so installing full sheets of rigid foam would be difficult if not impossible. the lower 3′ of the wall is bumped out (to allow for 8″ concrete wall) thus giving them a continuous ledge.
I have read about the GBA’s preferred wall stack-up for below-grade basement walls but wonder if this particular scenario deserves different treatment. the house is in Vancouver, BC, and while the basement appears dry, the adjacent neighbours have have wet basement issues in the past.
Thanks again…
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Replies
Erik,
The sections of the wall that are made of concrete should be insulated on the interior or exterior with sheets of rigid foam. If you are insulating on the interior, don't forget to install horizontal rigid foam above the ledge at the top of the wall. For more information, see How to Insulate a Basement Wall.
The sections of wall that have wood framing should be insulated like any other above-grade wood-framed wall. As with any type of insulation, it is essential that you pay attention to airtightness.
Since the wood-framed portions of the wall aren't as thick as the poured concrete, you have the opportunity to beef up the insulation of the wood-framed portion by adding a layer of rigid foam to the interior. This will reduce thermal bridging through the studs and improve the thermal performance of the wall.
martin, the short wall framing that covers the concrete is set off the concrete by 1/4" or less: there is no room to install a continuous foam layer. this is not code-compliant. short of ripping out the framing and mechanicals, does it make any sense to foam the gaps between concrete and framing and then fill the stud bays with rigid foam using cut and cobble?
also, when using latex paint as a vapour retarder: are one coat of primer and two coats of colour of regular latex paint sufficient or should a branded vapour barrier paint be used? the bcbc allows paint to be used as a vapour barrier but there is only one product that passes the relevant standards testing and it is a pain to source it out. the said, all the manufacturers sell a 'vapour barrier paint.' are you supposed to apply the paint as thick as possible? the bcbc requires a third party wet mil test if you choose this route..
Erik,
Q. "Does it make any sense to foam the gaps between concrete and framing and then fill the stud bays with rigid foam using cut and cobble?"
A. That approach wouldn't be as good as demolishing the wall framing and installing a continuous layer of rigid foam of an adequate thickness. But we all make compromises sometimes. Only you can decide whether you want to cut corners on this job.
Q. "When using latex paint as a vapour retarder: are one coat of primer and two coats of colour of regular latex paint sufficient or should a branded vapour barrier paint be used? the bcbc allows paint to be used as a vapour barrier but there is only one product that passes the relevant standards testing and it is a pain to source it out."
A. This question is best answered by your local building inspector. Personally, I think that those who obsess about the vapor permeance of their interior finishes are obsessing about the wrong issue. (Airtightness matters much more than vapor permeance.) If your local inspector is happy, that's all that matters.
If it's a pain to find the right type of paint, you can always purchase and install MemBrain, a smart vapor retarder.