Insulation with ZIP walls
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I am having a new home built in MA. The exterior walls are the ZIP wall system which provides air sealing at the exterior. I have 2×6 interior wall studs. I was planning to use rigid polyiso foam for insulation getting me to ~R27+ @ 4.5 inches. Now I discovered that the polyiso insulation has foil facing. Can I use this product layered in the walls safely without causing a potential moisture problem? Can I face the foil to the exterior to repel summer heat and to the interior to help retain winter heating within the house.
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Replies
Not sure I understand what you're saying - you're planning on filling the bays with rips of polyiso? And by "interior wall studs" do you mean your exterior walls?
Have you started the project yet? Who designed the insulation system?
Ron,
It really makes no sense to cut perfectly good 4x8 sheets of polyiso into narrow pieces and stuff them between the studs. All that does is ensure that you have a thermal bridge every 16 inches or 24 inches.
It will take less labor to just attach all of your 4 1/2 inches of polyiso on the exterior of your wall, outside of the sheathing. Stagger the polyiso seams; tape each layer; and install vertical rainscreen strapping with long screws to hold the insulation in place.
Or come up with some other wall system. We build with double-walls in almost all our new construction projects, dense pack cellulose in bays. A lot cheaper than PIR, so screwing around with the siding.
If you haven't started the project yet, I strongly encourage you to stop and think through those details. It amazes me that there are still builders who are treating insulation as an afterthought, esp. in New England.
Martin,
Why don't you recommend a double stud wall like you would for affordable homes in Vermont?
I don't think that installing 4-1/2" of outside insulation is a piece of cake...or not-so-labor intensive.
....maybe if the house has no windows.
Also check the ZIP installation instructions ... I think a draining housewrap or "gap" is required between ZIP and vapor impermeable outside insulation.
John,
I was assuming that he had already purchased a pile of polyiso -- but upon re-reading his question, I see that it's possible that he hasn't bought his materials yet.
As you probably know, John, I usually advise anyone who asks about the best wall design to choose a double-stud wall filled with dense-packed cellulose. But it sounds like Ron has already framed his walls with 2x6s -- and my hunch may be right that there's a big pile of polyiso in his yard.
Martin,
Most likely this should be a new post ... but it leads right into your last response. Can you provide a link that shows the details of the double wall construction you're referring to above. I'm trying to decide between double wall vs. rigid foam outside OSB sheathing for exterior wall construction for zone 5 (central OH ... 5708 HDD, 797 CDD). Double wall would appear to provide for excellent insulation levels (R-40) as contrasted to R-23 (2x4 w/2" EPS) that one of the few local builders is spec'ing. The wall section would be helpful.
Having done some polyiso ripping to insulate a small area (about 30 feet of wall), my advice, if the polyiso is already on-site, is to go on Craigslist and sell it. Hopefully it's not on-site, or if you bought so much of it you got a good price and can come close to breaking even. It was a lot of messy work ripping it, many cans of spray foam to seal it, ugh. I thought it only came as thick as 4", so maybe the 4.5" is with three layers of 1.5"? In that case, you could use a layer on the exterior and get some use out of it - fill the 2x6 cavities with cellulose, and have ~R-27.
Hello everyone...
First, thank you for your responses... I have been out all day since I posted my above question.
Some more detail... I am a simple homeowner who designed our new single story 1927 sq/ft home. Had an architect draw up the plan and awarded the job to a local contractor/builder. The job started april 1st and now at the point of nearly ready for insulation. The original plan had an optional double wall to make the exterior wall come out at 10 inches. Planned to have dense pack cellulose blown in for ~ R34 wall with thermal bridging eliminated with the spacing and insulation between the two walls. However, after we got into the build I saw an interesting chart (www.icynene.com/assetts/images/builderremodeler/image002.jpg) on the Icynene web site that shows with an "air sealed wall" that high insulation factors are achieved with far less insulation than one might think. So with that I scrubbed the double wall build. Now I am at a point where rough elect is nearly done (can't consider double wall at this point) and am still trying to decide what to do for insulation, to get the best r value into the 6 inch exterior wall.. (spray foam is to costly)..
Any further suggestions..
thanks again
ron
Ron - This subject has been extensively discussed on GBA.
Have you read Martin's article?
https://www.greenbuildingadvisor.com/blogs/dept/musings/it-s-ok-skimp-insulation-icynene-says