Rigid Insulation Install Experience
We just finished installing 4″ rigid insulation in the rafter bays of a house we are building. I had read that this is the insulation choice of “last resort”. I now know this to be true.
We used it in 2×6 rafters for a vaulted ceiling. The material comes in 4×8, as well as longer lengths. There are absolutely no instructions available on-line, except for a few others that have posted their attempts at installing this stuff. Here is my experience.
We had to measure every bay, and cut-in for all sprinkler plumbing, wiring, and electrical boxes. We used a cordless saw, and then finished the cut with a fine tooth hand saw. This created a boat-load of dust. I have not had an opportunity to see if the material is carcinogenic and/or toxic to humans. Even though we wore full-on masks, the $40 ones, and protective eyewear, the dust got into every pore and crevice.
Then we foamed every joint with polyurethane foam, the kind for windows and doors. That stuff gets everywhere, on the floor, on you, on your tools. After the foam expands and sets you need to remove any that protrudes beyond the framing. Basically everywhere you foamed. We ended up using a sanding disc on a grinder, this tool also works well in shaping the foam for fitting in the bays. The best tool was my rotozip with the angle grinder attachment with the sanding pad. It was fast, light, and powerful. This introduced a whole other mess of dust and grit.
The architect designed the vaulted ceilings with 2×6’s, and in order to get an R-30 or thereabouts, he called for the 4″ rigid insulation, which delivers R-25. In the future I will ask him to change the design to 2×12’s so we can use R-30 fiberglass batt insulation.
Replies
Just FYI for next time (:)): Sometimes an electric carving knife works well with foam. (Depends on the specific knife and the specific foam, though.)
I use a sharp wall-board saw.
Another option would have been closed cell spray foam. The 2 lb. density has over R-6 per inch, and will seal tightly around the wiring and plumbing, cutting air infiltration to essentially zero.
Check that architect's license and continuing education units. R=38 is the norm here in the midwest for roofs. You're right that you'll need 2x12s next time.
spray urethene will do that in a 2x6Yohimbo, I can't imagine why you heard that comment. Mybe the labour involved in solid panels, but foam is the primary insulation insulation of choice from my POV
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I remeber insulating with foam panels well 4" thick stuff wasn't to bad just cut it with a band saw .
use 2 " foam all the time on interior of exterior walls cut it with a table saw .great thermal break .and don't have to worry about that double vapour barrier .here in OTTAWA ONTARIO CANADA
spray polyurathane foam on hard to insulate places and under basment slabs it is the ####'s
I love foam insulation
Actually we are faming a house now and we will spray the whole thing with foam
rafters are only 2x8"s not 2x12"s will post some pics soon intersting roof to frame
You can still add another layer of XPS below and perpendicular to your vault rafters (held in place with 1x3 strapping). Drywall is then attached to the strapping.
1-1/2" XPS will add R-7.5. I've done this several times. The additional material and labor costs are relatively minor. Electrical boxes would need extensions though, and of course you loose some headroom.
Here is a pic of this approach (used on a wall here):
we use 2" foam on interior walls same as your pics just 2 " .we just put the electrical boxes on the studs well screw them to the face of the studs works great