CEI – combining different types of insulation
Hi—I need some direction on my wall assembly. I am building a Passiv type home in the mountains West of Denver – climate zone 5. The exterior wall is framed with 2 x 8’s and then from the inside to outside is 5/8″ drywall, 5 1/2″ of Rockwool batt, 2″ of spray foam, with 1/2″ OSB sheathing- with taped seams. For the exterior continuous insulation, I was originally going to use 6″ of Rockwool Comfortboard 80 with furring and rainscreen—but the rockwool is very pricey, so I was thinking of using 4″ thick polyiso and then 2″ of Rockwool Comfortboard on top of it to create the 6″ of CEI. The WRB is Commercial Tyvek-D applied to the OSB to prevent wicking and create a small drainage plane between the polyiso.——-one other detail–the Polyiso is a factory second and has the fiberglass paper on one side which I was planning to orient towards the outside of the wall. I have a few questions on this assembly:
1. Do you see any issues layering 2″ of Rockwool over 4″ of polyiso” –btw, I am using fiberglass girds so no issues on attaching the furring.
2. Do you see any issues for vapor drive or drying? I would think the 4″ polyiso kind of prevents the OSB from drying to the outside
3.For the 2″ of sprayfoam on the inside, would closed cell be a bad idea since it would prevent drying to the inside? and open cell would be a better choice?
4.Should I abandon this polyiso idea and just bite the bullet and use 6″ of Rockwool?
Thanks!!
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Replies
1- No, except for cost. I'd just use ALL polyiso, and no comfortboard. You'll get similar performance, maybe even a bit better, with all polyiso, but less cost. When you are using a layer of polyiso anyway, the higher vapor permeance of the Comfortboard is essentially cancelled out for the wall assembly, so you're not gaining anything by keeping some of the exterior insulation as mineral wool.
2- You will have limited drying to the outside with fiber faced polyiso, and minimal towards the interior due to the spray foam. I would skip the spray foam layer completely, since it's not really necassary, and air seal the exterior sheathing instead. Fill the entirety of the stud bays with mineral wool, or fiberglass batts if you want to save some money and/or have any issues getting thick enough mineral wool (and don't want to layer two sets of thinner batts to build up a thick layer). The spray foam layer here is probably for air sealing, but there are other ways to do that that are both cheaper and greener, by strategically using caulk and canned foam. I'm not a fan of thin layers of spray foam like this for two main reasons: first, the spray foam adds unnecassary cost to a project that could be better spent elsewhere (such as more exterior rigid foam), and two, the spray foam pretty much ALWAYS produces an uneven surface that the batts won't perfectly fill.
3- Open cell would be better for drying, but skipping the spray foam completely is really the best option in this assembly. There is NO NEED for spray foam here, it does nothing except add cost. Do air sealing the "old fashioned way", with a bead of sealant between the perimeter of the framing and the exterior sheathing, tape the seams in the sheathing, and use canned foam to seal any/all penetrations in the sheathing or framing.
4- Polyiso is fine. You're way beyond the minimum for your climate zone with 6" of rigid foam here, so your wall should be fine. If you want additional moisture protection, use a smart vapor retarder behind the interior drywall instead of using comfortboard on the exterior.
Bill
SkiHouse,
+1 on all Bill’s advice, especially skipping the spray foam, which adds nothing but risk.
Have you done any energy modelling of the walls? I would bet that In zone 5 the benefit of the extra 2” of exterior insulation will be next to nothing.
Thanks Bill and Malcolm - one slight correction - we are actually in a local Denver ski area which is climate zone 7. I did check the code tables - one strange thing, none of the tables include 2 x 8 walls - the thickest wall is 2 x 6. So I interpolated by adding 25% extra R value to their values - I am assuming this is ok? so per tables, it would be R23 + 15 for a 2 x 6 wall so I added 25% which gives me approximately R28 + 20. Which is why I went with 6" of CEI - Do you think I could get by with 4" of CEI? -------would be easier!
BTW, the reason I was using the 2" of Rockwool over the 4" of Polyiso was because we are in a high fire risk area, so I was hoping the 2" of Rockwool would help minimize the risk - is this valid vs using a full 6" of Rockwool?
One other complication - the available factory 2nd polyiso is only available as 4 x 4 sheets that are 4" thick - so I was planning on taping them and spray foam any obvious seams- is the 4 x 4 size a bad idea given I can't overlap the seams?
Lastly, I know some GBA experts sigh loudly when we propose new "perfect wall ideas" - but the 4 x 4 polyiso above sparked this idea - one obvious problem with polyiso is that it is not vapor open---what if I cut 2" thick Rockwool Comfortboard into 4" wide strips - then place them on their sides around the perimeter of the 4" thick polyiso - basically forming a "picture frame" of Rockwool around the polyiso which would then create breathing channels through the wall from the OSB to the rainscreen - not to difficult to do and creates a vapor-open wall assembly - sorry for causing any sigh's ------thanks again, Brian
The important thing with the exterior continuous insulation is that the RATIO of exterior to interior R values is correct for your climate zone. If you, for example, double the R value on the interior, you need to double the R value on the exterior too to maintain the same overall ratio of interior to exterior R values. As long as you extrapolated from the tables like that you should be ok. Refer to Martin’s classic article on “rethinking rigid foam” for more info.
Bill
Thanks Bill---so I also did the ratio as well - with 6" of CEI and 7" in my bay, I get 24/28 +24 = 0.46 which I believe is right in the range of the recommended 0.43 ratio. What did you think of the 4' x 4' x 4" thick polyiso surrounded by picture frame Rockwool idea? ---might be easier to just alternate rows of 4' x 4' polyiso with rows of 2" wide x 4" deep Rockwool above it. This would result in 4" thick wall of insulation, so I would then cover the whole thing with 2" of Rockwool for fire protection which gives me 6" of CEI - creating in theory, a somewhat vapor open
exterior wall. ---Brian
You didn't mention any vapor retarder, so you are likely planning to use latex paint on the drywall for the required class III vapor retarder.
In zone 7, use of a class III vapor retarder is allowed with at least R-15 continuous insulation over a 2x6 wall, so extrapolating to R-20 over 2x8 is reasonable and 4" of polyiso will provide that.