Any problems with rigid foam on interior of roof rafters?
I am building a new home in the upstate of South Carolina which is climate zone 3a. It is a 1 1/2 story house (please no complaints… it is what it is), and I am planning on using (2) layers of 3″ thick poly-iso on the interior side of the rafters. The plan is to offset the joints and tape the seams at least on the 2nd layer of poly-iso. It does have to work around a few studs/ceiling joists and I plan on spraying foam around these intersections where there would be gaps because of not perfect fit up. Some of the reasoning I like this system is that it allows the wood roof sheathing to have air movement and breathe (dryout when needed) because there will be eave and ridge vents with open rafters. Also, it produces a pretty clean insulation line that is continuous and closes in the attics and mechanical.
My questions are this:
1) Is there a possibility of moisture problems with this?
2) To be more specific, is there any chance of dew settling on the outside of the insulation in the rafter cavity like how dew can set on a roof in the morning? The worry here is that the water could run down the sloped insulation on right on top of my wood exterior wall and have long term problems.
3) Does taping the seams of both layers of poly-iso do anything for me?
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Replies
1) Is there a possibility of moisture problems with this?
>Not if everything is installed well.
2) To be more specific, is there any chance of dew settling on the outside of the insulation in the rafter cavity like how dew can set on a roof in the morning? The worry here is that the water could run down the sloped insulation on right on top of my wood exterior wall and have long term problems.
>The dewpoint will occur within the foam or, on rare occasions, in the vent space, where airflow will whisk condensation away. Dew on roofs is usually a result of night sky radiation cooling the roof surface below the air temperature; the vent space will be closer to the outdoor temperature most of the time.
3) Does taping the seams of both layers of poly-iso do anything for me?
>It's a second line of defense but probably overkill. Foam shrinks as it ages so I recommend using a plastic tape that can stretch, such as 3M 8067, rather than foil-faced tape, which isn't flexible.
I have to note that this assembly has a higher up-front carbon load than necessary. You could get a similar result with low-carbon, fluffy insulation in the rafter bays and a single layer of polyiso at the interior. I have done that on occasion here in Maine when the situation called for it.
I really appreciate the response. You answers were exactly what I was looking for.
Interested to know more on your last comment. I have thought about many different solutions and here are some reasons I decided against them. But maybe they are for good reasons... maybe they are bad.
When putting fluffy insulation between the rafters, I have assumed that I need something solid (meaning something that can stop air flow) between the fluffy insulation and the air gap underneath the sheathing. This is because I thought that if I didn't provide a good barrier, I would reduce my R-value of the fluffy insulation by some convection of air in and across the insulation. I have seen many solutions for putting different materials between the rafters while continuing to have a ventilation gap like rigid insulation, plywood, (potentially drywall/denseglass), or foam vent products. When I have investigated the cost and thought about the amount of labor required for each one of these, I have found them not as advantageous as just buying 2 - 3" thick sheets of poly-iso applied below the rafters. From memory, a drywall/dense glass solution is the cheapest, but requires a good bit of labor to cut it up, make the gap with some shims and then seal the edges. The pre-made foam vents made for rafter bays get expensive quite fast especially when you are looking to do a continuous run not just at a pinch point in a roof or just near the eave. These might be decent solutions in my mind for relatively small areas of roof, but I am looking at about a 2200 sf roof horizontal projection which ends up being over 3000sf of product on the slope.
There is always the consideration of closing off the rafter bays and not allowing the vent, but I work in the wood construction industry and I can't get over not letting the wood have ample drying ability. In my mind (personally), not letting it vent and putting fluffy insulation is more of an acceptable option than spraying foam directly on the underside which really limits breathability, much less has a much higher global warming potential. That is why I ruled out spray foam.
That is where I am coming from. How do you make up your assembly that you mentioned?
I did the same thing in Zone 4a with very good results for nearly 20 years. Increase the rafter size to get as much fluffy insulation in the rafters as you can. If you use vent channels (foam, plastic, or site-built), that will stop windwashing of the fluffy insulation and loss of R-value. There are lots of products on the market for this. If you use aluminum foil-faced polyiso for the interior insulation, you're pretty good in all climate zones. There is a very small risk of condensation on the top layer of foil in super hot/humid summer weather, but I never actually found that to happen, even with those rare temperatures over 100F. I did use foil tape for the seams and did not find any detached when doing some alterations after a decade or so. I cleaned the foil facers with alcohol prior to applying the tape and Wow! it sticks.
Biggest downside is sealing the penetrations, especially around those collar tie beams and of course, any light fixtures. We were very diligent about the sealing (canned foam, caulk and tape) and did pretty well on the blower door test, considering when we did this. There are better products on the market now than there were then.
Overall, your approach will work. Add more fluffy and lose a layer of polyiso and you will save money, carbon and time. Best of all worlds: find some recycled polyiso for the interior insulation.
I've used 1/4" waferboard, which is a cheaper, non-structural, version of OSB, to build air baffles in rafter bays to act as an air barrier before. It's easy to do, just cut the waferboard panels into strips that will fit between the rafters, then tack up some 1x2 furring strips to the inside edges of the rafters tight against the underside of the roof sheathing. A finish nailer makes fast work of this. Now nail the waferboard strips up against those furring strips using that same finish nailer. This is cheap, relatively fast and easy, and works fine. You end up with about a 1.5" vent channel, which is better than the 1" code minimum.
You can now put up batts, or whatever else you want, in the rafter bays. I've used polyiso on the interior face of the rafters before as an interior side air barrier, and for some vapor control, with seams taped (I usually use foil tape, but I like Michael's idea of using something with a bit of stretch instead in some cases).
if you go with just two 3" layers of polyiso, you have about R38 of R value. It wouldn't hurt to add some batts above, using the wafeboard to define a vent channel above the batts. You could also furr out the rafters to fit thicker batts, or something like cellulose with netting. You're probably right that polyiso alone would be the least labor option though.
Bill