A colder basement after air sealing the attic?
Hey all, been following for some time here. We just finished a complete re-roof and insulation (R6 > R49) and part of that was a significant air sealing undertaking that I was able to finish while the 3:12 roof decking and old batt insulation was removed. The attic floor should be basically air tight at this point, with only the (insulated, weather stripped) access panel open to the attic now. The late 60’s house was around 3150 CFM50 before any sealing was undertaken, and there were some huge gaps in the attic.
I would have thought that the basement would be much warmer now, but it seems to be colder than ever. The (unusable) chimney has been blocked and sealed. The foundation walls in the basement definitely need some insulation (as do all the walls), and the rim joists and sill plates aren’t even leaky. The perceived temp in the basement is definitely colder, it almost feels breezy, but there is no air flow. Thermal imaging reveals no significant spot sources of cold. It’s even warmer outside now than it was earlier in the season, before the work was done.
Any thoughts on what could be causing this, or where to attack next in terms of air sealing or insulating for comfort?
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Replies
Is your furnace or boiler in the basement? It could be that it's running a lot less so the basement is getting less heat.
DC,
That's a great observation. Leaky ductwork and/or uninsulated boiler piping adds quite a bit of heat to a basement. Shorter and less frequent duty cycles would certainly result in less heat delivered to the basement.
So, is the colder basement causing problems? You might be saving a lot on energy, but at the cost of a colder basement. If this is a problem, the fix is to insulate the basement - rim joists and walls are generally best. That will warm up the basement a bit, bringing the heat loss back into balance. A fast fix that seems counterintuitive is to add some heat back to the basement. If the attic insulation and air sealing was successful, then adding a bit of heat back to the basement will increase the heating costs a bit but not nearly to the pre-insulation levels. Of course the best solution (in terms of energy costs) is to insulate the basement properly, then add what heat is necessary for comfort.
Thanks for the replies.
Yes, the furnace is located in the basement, and I hadn’t considered it, but in addition to it not cycling as frequently, or running as long, I made a point to seal seams and gaps in the uninsulated ductwork with foil mastic tape, where I could get to it (most of the main trunk runs the length of the unfinished basement space). I guess I did a good enough job that excess heat isn’t making its way into the basement.
Insulating & air sealing the basement walls & band joist is what it's going to take. The fact that it was somewhat warmer when the furnace was running a higher duty cycle only meant that there was more distribution loss heat going into the basement (and into the Great Outdoors via the heat leaks of the basement.)
If the basement ceiling is insulated and the walls are not, the distribution losses are fully lost, but if the walls get insulated and it's still too cool down there it won't be a large energy hit to remove the ceiling insulation, passively heating the basement with heat coming from the first floor.
Thanks, Dana. That makes sense. The basement ceiling is definitely NOT insulated. I can see in between most of the joists from the half of the basement that is unfinished. I know the outside walls are a big heat loss (minimal insulation with lovely era-appropriate wood paneling, where finished at all). The front half of the basement is below grade, and back half (finished half) is at grade, all cinderblock wall with exterior fiber sheathing and brick. It glows brightly at night on the thermal cam.