Insulate floor over garage
Hi, Everyone. I read Martin’s article about insulating a cold floor, but still have some questions. Thanks in advance for any guidance!
Situation: we have 3 bedrooms above the garage. The floor is cold in the winter and AC cannot keep up in summer. The joists are 2×10 and there is duct work and water supply lines in the floor system. There is no insulation right now. I am planning dense-pack cellulose (I’m not a fan of spray foam). The water heater and furnace are in the garage (atmospheric venting).
Questions:
1. I’ve heard that dense-pack cellulose also air seals. Should I still air seal the rim joist?
2. OK to just do cellulose or do I need rigid foam around the rim joist first?
3. Is it necessary to air seal the duct work with mastic?
4. Do water supply lines need to be wrapped in pipe insulation before cellulose?
5. What air sealing of the floor (not rim joist) should be done? At seams in subfloor?
6. Does the duct work need insulation sleeve before cellulose?
Thanks,
Matt
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Replies
Matt,
1. Dense-pack cellulose definitely slows air flow, but should not be considered an air barrier. If possible, air seal everything before installing cellulose.
2. What climate zone are you in? You don't need foam at the rim joist, and cellulose in a 9 1/4" cavity should give you about R-33, which meets or exceeds code in all climate zones except 7 and 8, which require R-38 to meet code. On the other hand, if you are concerned about the cellulose settling(which shouldn't be a problem with a good installer, but it depends on the situation), adding foam the full height of the rim joist could be good insurance against a thermal short-circuit. Adding additional insulation usually pays off in higher comfort and lower heating bills, but the equation can get tricky if you factor in the environmental cost of some insulation types (mainly foam).
3. Duct work should always be sealed with mastic or tape, with certain requirements. (M1601.4.1: https://codes.iccsafe.org/public/document/IRC2015/chapter-16-duct-systems)
4. The only code requirement for insulating water lines is a minimum of R-3 for hot water lines (N1103.5.3: https://codes.iccsafe.org/public/document/IRC2015/chapter-11-re-energy-efficiency). Your cellulose should suffice for this. Cold water lines are often insulated to protect against condensation, but that should not be a problem if wrapped in cellulose.
5. At minimum, you should have one good air barrier between your garage and the living space, but the more airtight the assembly, the better, especially with AV fuel-burning appliances in the garage. Use a flexible caulk or acrylic tape at all seams and joints between materials.
6. Yes, because ducts within the insulation are not "completely within the thermal envelope," R403.3.1: https://codes.iccsafe.org/public/document/IECC2015/chapter-4-re-residential-energy-efficiency.)
Note that if you're in a colder zone, your pipes may be in danger of freezing unless they are located close to the top of the floor assembly.
What is your location (US DOE climate zone)?
I have a similar situation and have wondered if an ideal solution would be to tear our the drywall in the garage ceiling so you can insulate the interior side of the outer walls, then reassemble the ceiling with a layer of rigid foam followed by sheetrock.
I am in Portland, OR. Marine zone 4, I think.
All the water lines above garage are cold. I will be replacing them while the drywall is off, so can position them close to conditioned space. I will not be moving the ductwork though.
John - yes, I am leaning toward fully removing drywall. I contemplated just blowing in cellulose without removing it, but I also want to air seal, and get mastic on the ducts, and replace water supply lines. So I am planning to fully remove and replace drywall. But as of now I had not been planning a layer of rigid foam (cost, garage ceiling height).
In Portland OR the garage is never going to be cold enough that temperature striping of the floor is going to be an issue, no need for a layer of rigid foam.
Your 99% outside design temp is +27F (yes I know it gets colder than that, but for only 87 total hours in a typical year), and the garage isn't fully outdoors, and will be several degrees warmer than the outdoors with the garage door closed. Water pipes don't freeze every often even in vented crawlspaces in your area. Even though an air-leaky garage can get colder than a vented crawlspace, at essentially ANY depth within the insulation layer the pipes will be inherently freeze protected- they don't need to be right under the subfloor.
When it's 65F at the subfloor, and 20F in the garage during an extended cold snap it'll be warmer than 40F at the midway point in the insulation between the subfloor & ceiling gypsum. Just don't leave the garage door open when it hit's single digits outside (something that happens now and again, but not every winter.) The all time record low for Portland OR is -3F (back in 1950), which would be what it takes to bring the temp mid-way through the insulation layer to 32F, and it would have to be that cold for many hours to actually freeze the pipes.
There is no need to use rigid foam on the band joists, but it IS a good idea to air-seal the band joist with polyurethane caulk sealants &/or can-foam, even if dense-packing.
As long as there is at least 2.5" of cellulose under the ducts, they don't need to be further insulated, but they do need to be mastic-sealed (to the extent possible.) It's possible to AeroSeal them without replacing the ceiling gypsum, which could save a lot of time, but probably not a lot of money. It's pretty expensive compared to buckets of mastic- figure on a couple grand to do your whole duct system.
Thank you, everyone, for the incredible and quick answers. Folks like me are lucky to have you and this site as a resource.