Insulating a stepped concrete wall
canada_deck
| Posted in General Questions on
My house is from the 1980s and is a bit of an energy hog. I live in British Columbia – similar climate to Seattle. I’ve been chipping away at projects. This year, I am going to replace the electric furnace with a heat pump. Currently, the furnace is in a crawl space underneath a living room. The wall is stepped. At one end, it’s almost entirely a wood wall and at the other end, it’s almost entirely a concrete wall. Currently the space has no external ventilation and shares air with the rest of the house through a vent and through the operation of the furnace (very leaky ducts.) I haven’t yet decided what I will be doing with air flow once my project is complete (the new ducts will be air tight.)
When I put in the heat pump, I am going to have to add a lot of new ducts so there will be a short window of time (after the furnace and existing ducts are removed and before the heat pump air handler and new ducts are installed,) when it will be very easy for me to upgrade the walls in this space.
My initial thought was just to remove the existing vapor barrier, build a 2*4 wall in front of the existing wall, and then fill it with batt insulation and apply a new vapor barrier. However, then I realized that I’d be trapping that concrete wall and the new wooden wall behind a vapor barrier which could end poorly.
A few ideas:
– Build the 2*4 wall to the full height of the crawl space but keep it away from the concrete by a small gap (perhaps use some pressure treated furring strips.) Fill it with rockwool insulation. Use an air barrier like Tyvek instead of a vapor barrier for this crawl space.
– Remove the vapor barrier and place rigid foam over the concrete and over the existing wall.
– Remove the vapor barrier. Place furring strips from floor to ceiling and then re-install the vapor barrier. That would mean there would be a 1/2″ air gap in front of the concrete portions of the wall and that air could get up and dry to the outside of the building. Then build a 2*4 wall in front and fill with batt insulation. Use Tyvek on the inside side of that wall.
What do you all think?
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Replies
Picture of the wall in question:
canada_deck,
Assuming the framing is 2"x6", the wood walls are (minus drywall) the same as all the other exterior ones in the house, so I'm not sure it's worth monkeying with them. The low hanging fruit is the exposed concrete.
The stem-walls below are best insulated with rigid foam board glued to the concrete. Depending on what services are going to remain in the crawlspace, that foam may or may not need to either have a protective covering or have an approved fire rating. See BCBC 9.10.17.10.(1) & 9.10.8.9.
Thanks Malcom,
Re the firecode, it looks like that won't be an issue if I am interpreting this correctly: https://www2.gov.bc.ca/gov/content/industry/construction-industry/building-codes-standards/building-code-appeal-board/building-code-appeal-board-decisions/bcab-1726
Re the foam: That does sound like a fairly good approach. I am concerned about my ability to achieve an airtight seal against the concrete wall since it is poured so rough. See attached picture.
canada_deck,
If the furnace goes, I think you are fine for exposed foam.
https://boabc.org/wp-content/uploads/2015/10/code_idx_980011.pdf
You could bring down the high points with a grinder, but with a bead of foam friendly adhesive around the perimeter of each piece, I don't think air-tightness will be much of an issue.
Hmm interesting. The furnace is going but is being replaced with a central ducted heat pump unit so it may count as a service room. Drywall would be a lot of work...
canada_deck,
Talk to your BI and see where they stand. Thermax foam does not need any protection.
The problem with insulating the concrete with permeable insulation, like rockwool, is that the warm interior air can make its way through it and condense on the inside of the stem-walls.
https://www.greenbuildingadvisor.com/article/three-ways-to-insulate-a-basement-wall