Whats the best exterior insulation for under siding
I’m renovating a 1960’s mid-century modern house and am planning to add 2″ of rigid foam insulation on the outside (under furring strips and sheet siding.) What is the best brand of insulation & technique for this application? How about tape for the seams? I was thinking that having T&G edges would be important, but most of my local stores don’t have lots of options. Thank you in advance for any advice on what brands will make for the easiest install and best results…
Also I was thinking to add a 2×4(cut down) at top, bottom and middle of this install. While it will make the insulation not continuous, it will also make for a very solid nailing surface for the siding and furring strips. Is this a mistake?
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There is little practical difference between most types of rigid foam insulation in an application like this. I’d just go with whichever brand of polyiso your local store carries and call it good. Most people will use foil faced polyiso in this application.
I would use regular 1x4 (or ripped 3/4” plywood, which is probably better) vertical furring strips here and not try to cut in 2x4s in the way you are describing. Cutting in 2x4s eliminates the advantage exterior rigid foam usually has of being a continuous air barrier, and all the cutting of the foam panels is a lot of work. 2x4s are also only 1-1/2” thick, so they would be recessed 1/2” into your rigid foam. Vertical furring strips on the outside of the rigid foam also gains you a rain screen which is a nice added bonus.
I haven’t ever bothered with tongue and groove rigid foam. Tongue and groove costs more than square edge foam, and it limits you in terms of how you can cut and piece panels together unless you’re willing to not have T and G joints everywhere. I would just use regular square edge foam and tape it with one of the tapes mentioned on this site (I’ve used a 3M flashing tape before, 3” wide to get a better grip on the foam).
Bill
Hi Bill - Thank you for your reply. Yes I was planning to use vertical furring strips on the exterior - I found 1x4's which are 5/8" thick so that would be my rainscreen. Where I was planing the 2x4s is top and bottom which I was thinking would both cap the end of the polyiso and keep insects out and provide strong nailing at the ends. I've been worried how I can make sure the full assembly is nicely attached to the sheathing - it's 3-1/4" thick!
Depending on details, the vapor permeability of unfaced EPS can be a significant improvement over foil faced polyiso.