Add external rigid insulation to a roof over a cathedral ceiling?
Yes, I know there are many resources on this topic. I’ve done a lot of reading on Fine Homebuilding and Green Building Advisor. I just can’t put all the pieces together.
We have a 30’x33’ 1-1/2 story cape cod with first floor living space, 2nd floor unconditioned space with knee walls and 2 doghouse dormers along the front. 25 years ago we added an extensive low-pitch shed dormer at the back vented to the ridge vent and installed drywall cathedral ceilings in order to add 2 bedrooms and a bath upstairs. No skylights, no recessed lights. We are in southwestern PA, climate zone 5.
Before: 5-1/2” rafters, 1” board sheathing, tar paper, shingles
After: drywall, 5-1/2” rafters stuffed with 6-1/4” R-19 fiberglass batt insulation, 1” board sheathing except for the new shed dormer, 1-1/2” spacers to create air space, 1/2” sheathing, tar paper, shingles plus soffit vents and ridge vent. The knee wall space has insulation in the rafters but no drywall.
It’s now time to replace the shingles and we are looking for a cost effective and smart way to add rigid insulation above the current roof deck. Until we remove the current shingles and tar paper, the only problem we are aware of is along the edge of the shed dormer where we’ve had some ice damming. The insulation behind the knee wall is in good shape, so we are hoping the rest is performing as well. We will likely add exterior insulation to the walls next year which will intrude into the current soffit space.
Current plan is a vented assembly: remove shingles & tar paper; fix damaged sheathing; install underlayment (rain & shield along edges, tar paper (30#?) everywhere); install (2) 2″ sheets of XPS, staggered and taped; install spacers to create air space from soffit vents to ridge vent; add plywood sheathing, tar paper and shingles – addressing flashing, vents (soffit, ridge) and trim details appropriately.
Specific questions
1. What should we do with the current air space? Seal at the eaves? Eliminate by removing the spacers (painful!)? Use as the air vent instead of adding one above the rigid insulation?
2. Could we go for an unvented assembly? Could it be as easy as adding R20 in rigid above the current roof deck?
3. What will eliminate the ice damming at the edge of the shed roof?
4. For a vented assembly, is it most appropriate to to cut off the current eaves and rakes and tack on new ones?
5. My husband was hoping to get away with less insulation to simplify the project. I will get in trouble if I don’t ask the question – so: from a science perspective, what’s the minimum amount of XPS we can add and still do the job right?
6. Is all XPS better or worse than polyiso topped with EPS?
Thanks in advance.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Carolynn,
A lot of your questions are answered in this article: How to Install Rigid Foam On Top of Roof Sheathing.
Q. "What should we do with the current air space? Seal at the eaves?"
A. Yes. You should also seal the air space at the ridge. Canned spray foam is probably the best method.
Q. "Use as the air vent instead of adding one above the rigid insulation?"
A. No. You definitely don't want to invite exterior air into the middle of your insulation layers.
Q. "Could we go for an unvented assembly?"
A. Yes.
Q. "Could it be as easy as adding R-20 in rigid above the current roof deck?"
A. Not really. You would still need to seal the existing air channels at the soffit and ridge.
Q. "What will eliminate the ice damming at the edge of the shed roof?"
A. R-20 of rigid foam above the roof sheathing. Venting above the rigid foam is not strictly necessary but is definitely a good idea if you are worried about ice dams.
Q. "For a vented assembly, is it most appropriate to cut off the current eaves and rakes and tack on new ones?"
A. That isn't necessary.
Q. "My husband was hoping to get away with less insulation to simplify the project. I will get in trouble if I don't ask the question – so: from a science perspective, what’s the minimum amount of XPS we can add and still do the job right?"
A. You don't have to use any XPS, but the method you are talking about needs some type of rigid foam above the roof sheathing. You can use EPS if you want, or a combination of EPS and polyiso. The minimum R-value for this approach is R-20 of exterior rigid foam.
Q. "Is all XPS better or worse than polyiso topped with EPS?"
A. Green builders try to avoid the use of XPS because XPS is manufactured with a blowing agent that has a high global warming potential. Either EPS or polyiso is preferable to XPS from an environmental perspective. Polyiso doesn't perform well at cold temperatures, however.