Exterior foam board insulation & metal or Hardie siding
Maybe I just need to find an energy efficiency expert. But..just got done reading a series of articles & threads on two topics: metal siding (specifically vertically oriented corrugated) and exterior foam board.
I’m in zone 4, off the coast but on the milder side of Oregon. Winters are mild and very wet. Our summers are very dry and the threat of wildfire increases each year. My house will be in the woods and has firebreaks but I’m interested in either metal or hardie board siding for the fire resistance.
I insulated my last building with rockwool insulation between the 2x6s, and am happy with the thermal performance as a result. However, some articles I read tempt me to go further and add exterior foam board insulation in addition to whatever is placed in the wall cavity.
1. Difficulty. I’m a fairly inexperienced builder (2 cabins so far). I do wonder if getting all the details right with the foam board around my windows will be incredibly time consuming. The other two buildings I did just had cavity insulation. I’m a teacher so I only have a summer to get this thing ready for weather. Foundation has been poured.
2. How much foam? I read that if I do add foam, adding just a little could be more problematic than skipping it (moisture reasons). How much foam would I need to significantly reduce thermal bridging and reduce heating cost/effort (heating with wood). I have my Oregon Residential Code book. R23 meets code for the wall insulation, and I have achieved that with the rockwool. So this would be about going above the requirements.
edit: Lstiburek seems to indicate here that the ratio of continuous to cavity insulation in zone 4 is 0.2. I interpret that to mean that my original R23 would have about R5 exterior. That’s 1 inch of foam board, or else 1.5 or so inches of mineral fiber board.
3. Sprayed Foam? Putting sprayed in foam between studs would be simpler than attaching foam (or other) outside (and add significantly to performance), but what are the greenhouse gases/blowing agents (just asked about this in another thread)? If horrible, then perhaps not. Also it’s very expensive.
4. Mineral Fiber Insulation board for continuous insulation might be a good alternative to foam board…reading a bunch of Lstiburek articles.
5. Heavy siding sagging on screws through exterior foam board. If I go with the exterior foam board, I am concerned about hanging heavy hardie board siding 2-4 inches away from the studs. As a result I’m looking at metal siding as an option. (I guess depending on the gauge I select it could be fairly heavy also.)
6. Drainage with corrugated. If I go with lap siding I know how to install a rain screen detail under it. If I went with corrugated metal, would the rain screen be a simpler detail as it’s basically built-in? So could I possibly add exterior foam, then either tape it or add housewrap, then furring strips (horizontal??) and then fasten on the corrugated metal? It seems like horizontal furring strips would catch falling moisture, but there would still be open air channels for drying to the exterior.
Or–If I were skipping foam board, would the corrugated metal just be screwed directly through house wrap and sheathing into studs (no furring strips required)?
7. Perfect Seam Seal?
Also with corrugated metal (assuming I find some that I like the look of), do I glue between panels and attempt to make it pretty air tight except for at top & bottom of walls, and below & above windows, where some sort of mesh for insects might be a needed detail? If the install is imperfect, then there might be less of a seal between panels, as I discovered recently when putting corrugated metal on a woodshed.
Well I just put a lot out there so any input is appreciated. Thank you!
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