I’m building an actively cooled wine cellar in my basement. I’ve read a lot of relevant threads on WB, and I’ve read the Richard Gold book. The consensus I’ve seen is that when your cellar is actively cooled, that your vapor barrier goes on the outside/warm side of the insulation, and that you definitely do not want two vapor barriers (due to mold concerns).
Because of the way the joists above my cellar are configured, it is going to make the most sense to use foam board insulation on my cellar ceiling. I’m planning to use 4 inches (about R-20) screwed to the face of the joists. That foam board will also act as the vapor barrier. However, that will leave voids between the joists.
I would like to fill those voids between the joists to provide better insulation from the warm room above, but I’m not sure if I should, or what material should be used. Can I fill those voids with unfaced fiberglass insulation? (Unfaced so as to avoid a second vapor barrier.) Should I cut foam board to width and fill the voids with that? (Sounds like a pain.) Or should I just leave the voids empty? (Unfortunately, blow-in foam is not an option here.)
Any advice would be appreciated. Thanks.
Charles,
With R20 insulation and not a large temperature delta between your cellar and the room above (assuming that room is conditioned space, not sure how warm you mean when you say “warm”) you will see diminishing returns from adding insulation in the joist bays. If you do decide to add insulation, mineral wool or unfaced fiberglass is fine. I am assuming you are going with 2 layers of 2" foamboard. If so, I would tape the first layer closest to the joints, including all the seams and the perimeter where it meets the walls to make a continuous air and vapor barrier. For the second layer, I would just offset the seams from the first layer.
I did a basement wine cellar (actively cooled) project about five years ago, maybe a little less, but here in California, where moisture is not a huge factor compared to other places since our summers are not humid. That said, I installed vapor barrier by wrapping it around the studs and joists on the two sides that were already built, then put in fiberglass insulation, and that seems like plenty. The room above is “conditioned space” and not the most exposed to heat from sunlight, and everything else is already below grade with very slight temperature variation. It’s a lot quicker to use fiberglass than cut something else, since it is sized to the space between your studs and joists. Just my thoughts from my one-off project.
Agree with Jon above.
Another approach would be to screw foam board to the floor above, and then seal the corners with the small cans of foam you can get. Less of a mess - just use a dropcloth.
I had a similar situation with my cellar. I stuffed the joist bays with the thickest Rockwool insulation they had and filled them completely, then I installed 2" foil faced poly-iso insulation. Then drywall atop that. The foil insulation goes all the way around my cellar and is foil taped, so it is the vapor barrier. The ceiling to the floor above is the biggest loss of heat averaged over a year. Outside walls will operate on a gradient to the ground temp and of course change with the seasons. But the ceiling will be fighting 72F of whatever you have the thermostat on year round. So the extra insulation is worth it there.
With active cooling you will want to vapor barrier the floor. I have a passive cellar so I left my floor bare concrete to maximize cooling effect. But with active cooling you will be constantly pulling out humidity which works the unit much harder and can cause early failure.
Best of luck.
Thanks for the feedback everyone.
To answer Jon’s question about how warm the room above is: The room above is air conditioned, but in the summer we often leave the sliding glass door open and close just the screen because people are going in and out to the deck all the time. So while 72° is the usual temperature of the room above, 78°-82° isn’t uncommon.
Thanks again, everyone. This weekend’s project is taking final measurements and making a parts list!