Installing Shiplap Siding Directly to Studs
I posted a question in an old thread that was fairly on point but I am not sure many will see it, so I started this new Q?
I have tried to read as much of the information on this site as possible regarding insulation of 2×4 exterior walls and all of the problems that can arise with mold forming on the backside (interior facing) side of the OSB sheathing. So here is my question:
We live in southern Michigan and have a remodel of one room (20 year old house) where we had to remove the drywall and expose the studs. We want to install shiplap instead of drywall. The current insulation is the original kraft faced fiberglass batts. The contractor told us that in order to to make this work right, we have three options:
Option 1 (best) – Fill the stud cavities with closed cell spray foam and then install a thin layer of rigid foam insulation on the face of the studs and then attach the shiplap. This eliminates the ability of warm moist air to reach the cold OSB sheathing and also would help increase insulation value by creating a break in the thermal bridge of the studs.
Option 2 (2nd best) – Do option 1 and ignore the extra rigid foam insulation on the interior of the studs. He said if the room was comfortable (which it was) and our energy bills are fine (which they were) then the most important thing is to make sure you have an air and vapor barrier between the interior of the home and the OSB sheathing, and the closed cell spray foam will work great for that.
Option 3 (3rd best) – Leave the wall as-is and reinstall drywall, then install shiplap over the drywall. He said before spray foam, that is what he would have recommended but there is no reason to buy, hang and mud drywall (and maybe have to paint it to for vapor barrier?) just to cover it with shiplap now.
I actually think all three recommendations are in line with what the overall advice has been on this site, so I am at least hopeful that these recommendations indicate the contractor knows what he is doing.
Can anyone with expertise on this topic weigh in and let me know if I am right that all three options are ok, or if there is anything I am missing?
Thanks
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Replies
I'm sure an expert will weigh in on this. But if it were my home, I would caulk the insides of the stud bays. Instead of spray foam, I would opt for rock wool or high-density fiberglass. I would then install a smart membrane and drywall (taped but not finished). Then I would buy veneer style shiplap (unless regular boards were cheaper).
Thanks, Steve. That sounds a lot like the last option the contractor recommended but with a twist. Why smart membrane plus drywall? If we go that route, why not just use half inch XPS? Lighter than drywall, same thickness, and acts as a better air and vapor barrier plus a better thermal break for the studs, right?
You could use the XPS, but the blowing agents in regular XPS are hard on the environment. You could look for one of the newer products that has a lower global warming potential such as Sopra.
I'm not an expert, but I have done a lot of research on this in preparation to building our "Zone 7" home. Matt Risinger has a great YouTube channel and addresses this issue. The problem is that moist air from inside the house is getting in the wall cavity and condensing on cold winter days. Myron Ferguson also has some really good YouTube videos about airtight drywall methods. If you seal all areas where humid indoor air gets into the wall cavity, then the permeability of drywall will allow the minor amount of water vapor in the cavity to pass back into the house in summer. Moisture gets in in the winter, but mold grows in the summer.
It sounds like you are not going do deal with the outside of the wall. If you are, then adding some rigid EPS (R-Tech)between the sheathing and the siding would help to move the winter dewpoint outside the sheathing.