Site-built SIP panel — What foam to use?
I previously posted a question about the wall section in an addition where the added guest bedroom and bathroom were going to double as storm rooms. But I’ll be danged if I can get back to that thread.
Anyway, on the exterior side of the studs, I was going to sheath with 3/4″ Advantech subflooring, then add 1-1/2″ of foam, then another layer of 3/4″ Advantech.
I seem to remember that extruded polystyrene (blue or pink foam) will shrink with age, and, in fact I have observed this in my crawl space.
So, what would the best sheet foam be to use? I would think that it would be best to use a foam that acted as a vapor barrier, so that the dew point would occur within the foam both winter and summer, so that would leave expanded polystyrene out, right?
Polyisocyanurate?
Thanks!
Bill
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Bill,
First of all, if you are interested in revisiting your earlier Q&A thread, it's here:
https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/23640/exterior-storm-room-wall-zone-4a3a
As far as I know, all types of rigid foam are dimensionally unstable over time. Some panels shrink, some expand, and some expand and contract with temperature changes. (To read an article I wrote in July 2000 for the Journal of Light Construction on shrinking polyisocyanurate, see "Shrinking Insulation Boards Plague Roofers." The article can be found near the end of the "Notebook" section I linked to.)
If the possibility of cracks between your foam panels bothers you, the best solution is to install the rigid foam in two 3/4-inch-thick layers, staggering the seams between the layers.
Resource efficiency should be a part of your equation as well, and using 3/4" sheathing maybe overkill.
Martin, thanks. Two layers of 3/4 is a good idea. The concern I had was cold spots and possibly condensation where the sheets butt. I'm pretty sure I'm OK as far as dew point / condensation goes here with a 1/2" min., so the double 3/4 should be overkill in that area.
Armando, the 3/4" sheathing is to resist windblown debris in a tornado A horizontal 2x4 shot at 100 mph is what FEMA sets as their standard. They spec a double layer of 3/4 plywood on their website. Also, will be installing high wind rated vinyl siding, which, in tests, usually fails with the nail pulling out of the sheathing. I'll use flat head screws with washers into the 3/4 Advanteck which should have a very high pullout strength.
Thanks guys,
Bill
Scratch polyisocyanurate. Exxtruded polystyrene in 2 layers sounds like the way to go.
Bill,
Before you conclude that extruded polystyrene (XPS) is more dimensionally stable than polyiso, you should read about XPS shrinkage at Timothy Lenahan's house. The story (and photos) can be found in this article: Using Rigid Foam As a Water-Resistive Barrier.
Thanks. So what do I use?
When I built the original house in the late 80s, I used either THermax or Tuff-R (polyisocyanurate with foil facing), taped the seams, and used a house wrap over that. When I did an addition in 99, I do not remember seeing that the tape had separated and don't remember seeing evidence of shrinkage.
Maybe a layer of each? Tape both layers and use a housewrap?
Bill
Bill,
Of the three common types of rigid foam -- XPS, EPS, and polyiso -- polyiso is the most environmentally friendly (due to the most benign blowing agent and the absence of noxious fire retardants). It also has the highest R-value per inch.
So that's the type I'd recommend.
Gotcha. Thanks.
Bill,
You may want ot check this info, in case you haven't already: http://www.buildingscience.com/documents/reports/rr-0603-impact-resistant-sheathing/view?searchterm=rr-0603
You also may want to contact Texas Tech. as they've done a lot of research on the subject