Wall covering choices for attached garage
Sublux
| Posted in Energy, Heating & Insulation on
My garage is under my house so the thermal envelope of the house goes under my kitchen and DR and then down between my basement and unheated garage. I’m sure the insulation is substandard so this spring I will be gutting the garage ceiling and walls and reinsulating. After careful airsealing, I’ll do fiberglass batts and foam. My question is whether gypsum is a poor choice in case of condensation inside the walls. I thought about painted plywood with caulked seams. Is that overboard? What do you think?
Replies
Put a vapor barrier under the drywall. It will be more airtight, and fire resistant. Use 5/8 fire rated drywall.
But on a common wall any vapor barrier should be on the house side of the insulation.
Not impossible to do, but you're right.
The IRC requires some sort of fire seperation between garages and living space. I doubt plywood will cut it. In the jurisdictions I've worked in, the garage side had to be rocked with at least one layer of 5/8 in. Type X drywall, with all seams taped.
I think the consensus is towards fire rated drywall and I don't disagree but if it were me I would want to overlay the bottom 4 feet of the wall with a bathroom panel that I caulked extensively so that I could run the hose in there without wetting the drywall. They make vinyl panels and fiberglass in 4x8 sheets that can be glued up. One I have used has a pebble finish and another is basically smooth. I wouldn't necessarily opt for the fake tile look but hey you might like that or get overruled by the wife.
Good point on fire rated wall, I'll check code but probably required. My concern is that normally an exterior wall is sheathed with a material that can pass water vapor but I imagine gypsum is more of a vapor barrier so condensation could collect and cause moisture problems. My poorly insulated walls didn't have this issue before
You only need thje fire code on the walls adjacent to the living space. If you have the vapor barrier on the warm moist side of the insulation, moisture should not be a big deal. Are you planning on storing things ion these walls? There are some good storage systems out there now.
Only my ceiling is adjacent to living space but the old oak floor of the above rooms doesn't stop much air. I've seen details in articles on this site that show insulation and foam below living space yet at the same time that detail on an exterior wall is debated because of condensation concerns. Is the key using foam that is really thick to keep the dew point from occurring in the ceiling thus preventing condensation and figuring water vapor will dry to the living space?
Your foam question requires a more complex answer than I can give. The key is that you need enough R from your foam on the outside of a framing assembly to prevent its interior face from reaching the dew point. There's a new detail in the code that allows something like an R5 of foam on the outside of wall studs, with batt insulation between them, that's specced for Zones 5 and lower. The issue there is that the R5 may be fine in Zone 5, but wholly inadequate in Zone 3 or 4. It's tough to write code.
Now, keep in mind that dew point is a function of both temperature and humidity, so the risk is smaller if the interior humidity is well managed. And remember also that if you hit the dew point for an hour once a year, eh, your assembly can probably handle that. But if it's at the dew point constantly, you could be SOL. With your leaky floor above, I'd be concerned. You could install rigid foam against the floor and seal the edges with can foam or acoustical sealant, or you could have an inch or two of spray foam applied. These would minimize air infiltration, which is where most of the moisture is likely to come from. Then insulate with batts (I'm a big fan of mineral wool - I find it much easier to install well than FG, although it's itchier and costs more). Then I'd be a lot less worried about a layer of foam on the bottom of the joists followed by the Type X drywall.
How I'd Do it
I'd certainly cover the house / connecting wall with atleast an hours' worth of drywall. This usually means TWO layers.
The house wall is often also a 'shear' wall.In new construction, youwill often find 3/4" plywood under that drywall.
The other walls? Fiberglass insulation should be fine. I'd cover that with plywood at least 1/2" thick - thick enough to hold screws, for all the stuff that will get hung there.
I'd cover any exposed (visible) wall face with FRP as well. These fiber-reinforced plastic, pebble-grained panels are absolutely waterproof, rot proof, and easy to clean. They're also pretty affordable.
I'd install permanent receptacles every 4th or 5th stud bay, at about counter height. Say, 40" off the floor. When the walls are completed, I'd mount several 4-ft. plug strips.I find the ones made by Wiremold, carried by HD, to be superior to the others.
With whatever wall covering you choose, I'd make sure to leave a 3/4" gap at the bottom- to prevent moisture from wicking from the floor. The sillplate will cover this gap easily.