KISS- how to communicate additional exterior insulation needs to renovation contractors
Hi all,
We’re trying (again) to get help with upgraded wall insulation for our zone 5A Michigan home built in 1988. We currently have fairly poorly installed fiberglass bats in 2×4 walls with 0.5″ foil-faced polyiso under T1-11 siding. After a series of starts and stops with builders, we’ve gone from thinking “lets do an energy retrofit!!” to “let’s just make this better and keep the woodpeckers out.” Contractors in our area looked at us like we have 3 heads when we asked for air sealing with caulking and limited canned spray foam in our attic, so we’re also trying to keep it simple. Our energy modeler suggested we add R20 continuous exterior insulation and an air barrier/drainage plane. I think at this point we’d be quite happy with another R10-15 and a rain screen. We plan to replace siding (cedar, Hardie, or LP?), windows, and doors at the same time as the insulation installation. So- green building gurus- if this was your house how would you go forward to get the most bang for your buck while sticking to more widely understood building methods?
1. Would you leave the fiberglass in the stud bays? Would you replace it with mineral wool batts, which would also reduce busy street noise? Would you install dense pack cellulose? Is mineral wool easier than dense pack cellulose?
2. I assume we’re adding a layer of sheathing and then continuous exterior insulation. Could we reuse the foil-faced polyiso (in which layer?) or better to just remove it? Would you ask for continuous EPS with a taped WRB? I don’t think that’s commonly done here so I’m concerned we’ll get pushback like our attic project. How much EPS can you add before you need to start using longer hardware? Should we simply ask for insulated ZIP sheathing because that will make the window flashing more straightforward?
Thanks very much in advance,
Elizabeth
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Replies
I put 3" of XPS over the sheathing on my addition in two 1-1/2" layers. There were locations where I placed 2x blocking in lieu of one layer for deck and rafter attachment, and it worked well. On the renovation I used the same two layers, and left much of the interior as is.
After using two different WRBs, I'm convinced that water will never get past the top layer of insulation.
My builder was only comfortable quoting what he normally builds. But, after I installed the window and door bucks and flashing them, I think he saw how simple it was.
Switching from fiberglass to mineral wool isn't going to make much, if any, noticeable difference in street noise. Fibercement siding would probably actually help more here. I would probably replace the fiberglass batts with mineral wool though, since it will get you some extra R value, and likely a better install too, which will also help.
R20 worth of exterior rigid foam is quite a lot! That's just over 3" of polyiso, 4" of XPS, or almsot 5" of EPS. Most contractors won't be comfortable with that much. If you want to keep contractors happier, try 2" of polyiso, which is R13 -- still a good amount, but much easier to work with. Over 2" is usually considered to be "thick" exterior rigid foam, and that's when things start to get more difficult. If this were my own home though, I'd probably try for 2.5" or 3" of polyiso, ideally in two layers, which is better for air sealing, but more labor to install. You'll want to tape all the seams and put a bead of sealand down around the perimeter of the framing prior to putting up the polyiso sheets to get a good air barrier.
You probably don't want to try reusing the existing 1/2" polyiso, since it will probably get damaged during your renovation project. Using new material will simplify things. Note also that that T1-11 might be acting as your structural sheathing, in which case you need to be careful not to take it all off in one shot, and you need to put something structural back up (OSB or plywood). If you're worried about noise, using 3/4" OSB or plywood as your structural sheathing will help a little, and is thicker than required by code.
Zip does simpify things, and you can use Zip-R that integrates rigid insulation to save a step. Whether or not Zip-R is any cheaper compared to seperate materials is a different question though.
BTW, mineral wool batts will probably be easier for you than dense pack cellulose here, since any contractor will be familiar with installing batts -- dense packing is more specialized, so you're likely to get more pushback if you wanted to try that instead.
Bill