Insulating a house with 2×4 framing using exterior rigid foam
Planning extensive remodel on a 1961 home. I’m taking interior down to studs and replacing siding/roof/windows, so I also want to take the opportunity to make it as energy efficient as possible.
Current House:
-Rectangular ranch style with 2×4 trusses running the entire length of the house
-Vented attic with ~3 ft peak height so it’s lowish pitch and shallow eaves
-Exterior walls are 2×4
-Climate Zone 5. Calls for R20 walls and R38 roof but I’m all for going above it 🙂
Planned wall upgrade:
-Batts between 2×4 studs
-2-4” of XPS outside of sheathing
-Metal siding over XPS
-R23-R33 walls seem achievable depending on XPS thickness
Planned roof upgrade:
-Replacing 1/3rd of trusses with ridge beam and rafters for higher ceilings in living area, and leaving the rest as is. Roof decking on cathedral and truss portions will match and be level.
-Since part will be cathedral and part will be truss, my thought is to just make whole thing unvented and go with exterior rigid foam.
-That would mean: batts between the studs, 6-8” of XPS over deck, and vented gap under metal roofing for ice dam prevention (problematic in my hood).
-R43-R53 seems achievable but thick roof since most if above deck.
Both walls and roof keep more than 50% of insulation outside of sheathing.
Questions:
1) In everyone’s opinion, is striving for R33 walls (4″ of exterior foam) overkill if roof will only be R43-53?
2) The cathedral portion of roof will have plenty of cavity for insulation but I am concerned with existing 2×4 truss section. Is there something better than just putting 4″ between the studs so that I can get more R value inside and not increase the above deck thickness too much?
Then secondary, I am reading so many mixed messages on the order of assembly for this setup. Some sites say vapor barrier on inside, others say none needed because XPS is the barrier. I’m confused.
4) What would be correct order of wall from drywall to metal siding?
5) What would be correct order of roof from drywall to metal roofing surface?
Thanks to some great articles on this site, i’ve managed to find the right setup and now just trying to finalize the details. Thanks for the help.
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Replies
Jet,
The most noticeable fact about your plans is that you are specifying XPS, a type of foam that is shunned by green builders because it is manufactured with a blowing agent that has a high global warming potential. I urge you to switch to either EPS or polyiso. For more information, see this article: Choosing Rigid Foam.
Q. "Is striving for R-33 walls (4 inches of exterior foam) overkill if roof will only be R43-53?"
A. The R-value of the roof is irrelevant. Since you plan to install new windows and new siding, you have an excellent opportunity to install a continuous layer of exterior rigid foam, and I think it's wise to do so. If you want to save money or simplify your details, you can install as little as 1 inch of exterior polyiso and still meet the minimum foam-insulation-to-fluffy-insulation ratio to avoid moisture problems (although thicker foam will result in a wall that performs better than thinner foam, of course). For more information, see Calculating the Minimum Thickness of Rigid Foam Sheathing.
Q. "The cathedral portion of roof will have plenty of cavity for insulation but I am concerned with existing 2x4 truss section. Is there something better than just putting 4 inches between the studs so that I can get more R value inside and not increase the above deck thickness too much?"
A. In your climate zone, if you are insulating a roof assembly with a combination of rigid foam and fluffy insulation, at least 41% of the total R-value of your roof assembly needs to come from the rigid foam layer. (For more information on this issue, see Combining Exterior Rigid Foam With Fluffy Insulation.) If your local building code calls for a minimum of R-38 roof insulation, that means that at least R-16 has to come from the rigid foam layer. (That would be 3 inches of polyiso.) You mentioned two other goals: either R-43 or R-53. If you aim for R-43, then at least R-17 needs to come from the rigid foam layer. (Again, 3 inches of polyiso would work.) If you aim for R-53, then at least R-22 needs to come from the rigid foam layer. (That would be 4 inches of polyiso.)
The R-value of the fluffy layer can be calculated by subtraction. The R-38 roof needs R-20 of fluffy; the R-43 roof needs R-25 of fluffy; and the R-53 roof needs R-29 of fluffy.
You can't get R-20 to R-29 of fluffy insulation in a 3.5-inch-deep space, so you will either (a) need to install furring to deepen the truss chords, of (b) just install open-cell spray foam (which is vapor-permeable) on the interior side of your roof sheathing.
Q. "Some sites say vapor barrier on inside, others say none needed because XPS is the barrier. I'm confused."
A. Listen to GBA. You don't need an interior vapor barrier. You don't even need an interior vapor retarder (a less stringent layer than a vapor barrier) as long as your rigid foam layer is adequately thick. This is explained in an article I linked to earlier. Here is the link again: Calculating the Minimum Thickness of Rigid Foam Sheathing.
Q. "What would be correct order of wall from drywall to metal siding?"
A. Drywall installed with attention to airtightness / studs filled with fluffy insulation / OSB or plywood sheathing with taped seams to limit air leakage / a water-resistive barrier (WRB) like housewrap or asphalt felt / rigid foam with taped seams / furring strips to create a rainscreen gap / siding. Note that the location of the WRB depends on whether you want innie windows or outie windows; in some walls, the WRB is installed on the exterior side of the rigid foam. For more on this issue, see Where Does the Housewrap Go?
Q. "What would be correct order of roof from drywall to metal roofing surface?"
A. Drywall installed with attention to airtightness / rafters filled with fluffy insulation / OSB or plywood sheathing with taped seams to limit air leakage / some type of air barrier (either the taped sheathing or an airtight membrane) / rigid foam with taped seams / either a second layer of roof sheathing or purlins 24 inches on center / roofing underlayment (could go under the purlins) / roofing. For more information, see How to Install Rigid Foam On Top of Roof Sheathing.
Thanks so much for the detailed answer Martin. I will eliminate XPS and most likely go EPS since I read in one of your articles that polyiso loses effectiveness in colder climates (our winters).
The roof breakdown is very interesting and I was using 50% as rule of thumb for interior fluffy vs exterior rigid foam to lean on the side of caution. I did not think to fir out the truss framing or that spray would be cost effective vs more rigid foam above deck. I will evaluate the cost of those with contractor and possibly engineer.
Also thanks so much for the assembly orders. I can't believe how much conflicting data there is on this topic across different websites and diagrams.