The gift that keeps on giving: the wafer of cold air between floors in my house
Hi again, GBA Forum:
Two and a half years ago, I had the roof ripped off my house and a second story put on. I paid for the envelope including door/window installs, the plumbing, and additional ductwork runs, and did the electrical, insulating of the second floor, and finish work myself.
Since the floor of the original attic was 2×4 construction (1920s bungalow) and there has been some settling over the years, the new second floor sits on a new sturdier deck that’s been built to be level, spanning the width of the house on LVLs, about a foot or so higher than the previous one. Insulating in that area was in the contract, and indeed that was done: By the time the subfloor was screwed down and the job was “done” and ready for me, there was an insulating job performed that covers the newly-made space. So between the old attic floor and the bottom plates of the new second floor. 2″ (or so) thick pieces of polyiso were foamed into place between where the new LVL met the rim joists. Raise your hand if you already see a problem. I certainly didn’t know this was what was going to happen; I guess I used to expect professionals to be professional and do their jobs well, so that I can go do my job, instead of sitting and micromanaging people in an area where I don’t personally have a lifetime of experience.
Ductwork and plumbing were run in this ostensibly-conditioned new space between the first and second floors. The plumbing access on the second floor is a small doorway to this space. A wall in the first floor was opened to run ductwork up from the basement (which I finally started to close up yesterday).
The problem is, when the first winter came (I’m in Minnesota), it became clear that there was still cold air penetration –significant– coming in below where the polyiso had been foamed in (which ended at the top surface of that previous attic floor). I started hanging indoor/outdoor digital thermometer leads into the between-floor space that first winter and measured temperatures of (if I remember correctly) about 40F. I’ll be calling this my cold air wafer. If you like, you can think of it as an Oreo cookie (I don’t care) where the sandwich cookies are conditioned space and the filling is a layer of The Great Outdoors.
A team of insulators came in and dense-packed the perimeter of my cold air wafer to about 18″ in. Maybe it helped. I’m not completely convinced–seems like there’s still room for cold air penetration. But it’s two years later and I’ve had a couple of energy audits, the house is mostly comfortable, the house has long since been sided and the flooring has long been down on top of the subfloor and I live upstairs and I deal with it.
So that’s what it’s been up to this point. Fast forward to yesterday: I finally got around to starting to close up the first floor interior wall where the supply ductwork runs from the basement up to that between-floor space, to feed the second floor. This wall is in the center of the house. With a piece of sheetrock up I can now feel cold air streaming out through those smaller gaps around that sheetrock. (Previously, with an entire open 27 sf piece of wall, it wasn’t apparent.) I’d tried foaming up around the top plate where the ductwork penetrates into the wafer but there’s still cold air. So while that’s unfortunate, now it seems that I have a new issue: this uninsulated ductwork (since it was run in an area that was supposedly inside the envelope) was previously kept in a (relatively) warm space, in part thanks to the open wall. Once I finish sealing this wall up, I’m going to be sending this warm air inside the duct in this leaky interior wall up into the cold air wafer, through a space colder than it has been. I’ll pour far less heated air into this cold wafer –that’s great– but I’ll also be up against the unknowns of how my uninsulated ductwork and my plumbing are going to do, once we learn how cold it really turns in there, when I’m not sending hot air in.
I’m curious to hear what to expect and how I can mitigate any problems at this point. I could go in under the top plate here again and try to foam the bejeezus out of it, which helps with the air leakage between the first floor and the wafer, but I’m concerned about how cold it’s going to get in there. Short of dense-packing the entire wafer or removing the siding on the entire house in a poor attempt to get back in and extend the insulation down around the perimeter, neither of which is really feasible, I’m not sure how to help this problem, nor what problems to really expect.
And, finally, since I mentioned the plumbing access on the second story, above, I may as well come back to it: that’s an uninsulated piece of plywood hatch in the middle of the second floor layout, between my bedroom and the cold air wafer. My bathtub is the metal membrane between my second-floor bathroom and the cold air wafer. My second-floor floor is the 3/4″ ply partition –with an additional 1/4″ of flooring for luxurious insulating warmth– between my conditioned space and the cold air wafer.
I look forward to hearing your opinions on how I should deal with the weirdly invasive cold air problems stemming from a construction job that passed inspection in 2008, and what to expect with my HVAC and plumbing running through that space once I turn it even more outside the envelope by closing up my wall. Thank you.
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