instructions on interior polyiso board?
user-901114
| Posted in General Questions on
Can somebody tell me how to install interior polyiso board? It’s new 2×6 construction. Might use wool batt insulation in the walls behind the polyiso. Thanks, Steve
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Replies
Stephen,
Your question is a little vague.
If you want to install interior polyiso, your thermal weak points will be partition intersections and rim joists, so you need to come up with a plan to address those areas.
Use only airtight electrical boxes on your exterior walls. Be sure that your electrical boxes are mounted correctly to account for the thickness of the polyiso.
Cut the panels to fit and tack them up with cap nails. Tape the seams for air tightness, and caulk or foam as necessary for airsealing at electrical boxes and penetrations.
When it comes to hanging drywall, you have two choices:
1. Strap the wall horizontally with 1x3s or 1x4s so that the drywallers are screwing the drywall to the strapping.
2. Use long screws to attach the drywall through the foam to the studs.
Thank you for the reponse Martin. I
-I'll use plywood extensions for my airtight electrical boxes on your exterior walls.
- How many layers do people typically use and how many inches per layer? Foil facing the inside I assume. Staggered joints too.
-Which brand(s) should I aim for? Does it matter?
- Please expalain what purpose the straping serves since I'm using long screws to attach the drywall to the studs. Airspace for more R value only? I fear that mice will get in there and build condos.
Steven,
Q. "How many layers do people typically use and how many inches per layer?"
A. One or two inches is fairly common, although there is no technical reason (other than issues swirling around the current "cold OSB" debate -- and if you are interested in that debate, see How Risky Is Cold OSB Wall Sheathing?) that you couldn't use thicker foam.
If you do choose to install more than one layer of foam, remember that it would probably be difficult to install drywall over more than 2 inches of foam unless you installed horizontal interior strapping.
Q. "Which brand(s) should I aim for?"
A. Any brand you want.
Q. "Please explain what purpose the strapping serves since I'm using long screws to attach the drywall to the studs."
A. Drywallers hate installing drywall over foam. It's hard to hit the studs. And in some cases the drywall can be a little spongy and you might have more screw pops.
I've used the long-screw-through-foam method, though, and I know that it works.
Q. "Air space for more R-value only?"
A. Not just for R-value -- although you will get another R-2 or R-3 with a foil-faced air space -- but for more secure and easier fastening of the drywall.
One final point: most energy experts prefer to see rigid foam installed on the outside of the wall rather than on the inside. But I assume you know that, and you have your own reasons for preferring to install foam on the interior.
Martin: "I've used the long-screw-through-foam method, though, and I know that it works."
Steve: So I'll plan on doing one layer of unfaced 2inch thick polyiso board with no strapping and the long screw method, with the only down side being, it's tricky to find studs
Martin: "cold OSB" debate"
Steve: I decided to spend the extra $700 and go with 100% plywood for my home.
Martin: "One final point: most energy experts prefer to see rigid foam installed on the outside of the wall rather than on the inside. But I assume you know that, and you have your own reasons for preferring to install foam on the interior."
Steve: I'm using air inlets and contant exausting in my home with airtight drywall. My sheething will be cold, but will dry to the outside. Hope my theories pan out..
Q: what do you think my R value will be using 2x6 filled with wool + 2 inches of Polyiso? Would foil facing buy me anything for this application?
Thanks!
Q. "What do you think my R-value will be using 2x6 filled with wool + 2 inches of polyiso?"
A. The wool insulation has an R-value of 3.9 per inch, so 5.5 inches would give you a center-of-the-stud-bay R-value of R-21.4. I'm guessing that thermal bridging through your studs will bring your R-value down to R-18 or R-19 -- just a guess -- and that the 2 inches of polyiso will bring the wall R-value up to about R-31 or R-32.
But your wall performance will be seriously degraded by your rim joists and partition intersections unless you come up with careful details at those locations -- which is why exterior rigid foam is preferred to interior rigid foam.
What kind of knife and tape should I use?& should I cut using an electric hot knife?
I cut foam on an old portable table saw, and do small cuts with a handsaw and or circle saw. Joints in foam need to be pushed tight or left loose and can foam sprayed tight along with taped. Lots of discussions to do with best long term tapes here at GBA. I think a top job requires acoustic sealant between frame and foam and dense packing insulation. You want to stop all air movement in wall assemblies and still keep one or both sides somewhat permeable at the same time.
Sheet foam with a hole in it is R 0.