Unvented cathedral ceiling
prt236
| Posted in General Questions on
Hi everybody, at the cottage I have to replace the actual roof shingles and try to correct a problem with ice-dam, I want to install rigid Styrofoam board (1.25 inch) directly on the sheating then strapping over it and new sheet metal roofing. I wont be using a ridge vent since i’d like to make it unvented, also i’d like to leave the ceiling untouched if possible, right now the rafters are 2×8 there is only 3 inches of fiberglass bat in there whitout any vapor barrier. The pitch is a 3 to 12. So i’m wondering if I can do it by the roof only. About the actual soffits; should I seal them or let them as is ? Thanks for any advice.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Pierre,
It's hard to make recommendations without knowing your climate zone or location. In hot climates, R-30 roof insulation is adequate, but in cold climates, most building codes require a minimum of R-49 insulation. In-between climates can get away with R-38 roof insulation.
The 3 inches of insulation between your rafters has a nominal R-value of about R-10, but for several reasons (its age, and the fact that is doesn't fill the rafter cavities completely) it is basically worthless. So you should plan to put all of your insulation above your roof sheathing.
If you use EPS, you'll need about 9 or 10 inches of rigid foam to achieve R-38. If you use polyisocyanurate, you'll need about 6 or 7 inches of rigid foam. Of course, R-49 will require thicker foam than R-38. You'll need to buy some long screws to attach your strapping (furring strips or purlins).
Because there are practical problems with securing rigid foam that is thicker than 6 inches, you may want to consider a different approach -- for example, one that uses nailbase or SIPs.
Of course, for this type of unvented roof assembly to work, you will need to carefully seal any ventilation gaps at the soffit and ridge. The easiest way to do this is probably with a two-component spray foam kit.
For more information, see How to Build an Insulated Cathedral Ceiling.
Climate zone would be 7 but my main concern is to stop ice dam and since the shingles have to be replaced I was wondering if the 1.25 inch of rigid foam, strapping and metal roof (plus making sure to seal the soffit and ridge) would get rid of these ice dam ?
By the way thank you vey much for the answer, I wanna do the right thing but i'm retired and working on a tight budget, and will be doing it myself if this gets rid of my main concern I will be more than happy...
Pierre,
Q. "I was wondering if the 1.25 inch of rigid foam, strapping and metal roof (plus making sure to seal the soffit and ridge) would get rid of these ice dam?"
A. It's hard to predict, but it's safe to say that ice dams on metal roofs are less likely to cause water to enter a house than ice dams on asphalt shingle roofs. However, if your roof includes valleys, it's easy for ice dams to form, even on metal roofs, and for these ice dams to cause wet ceilings.
If you're going to bother to install rigid foam at all, I strongly urge you to do the job right and to try to install at least 6 inches of rigid foam. Your suggested foam thickness of 1.25 inch doesn't have much R-value -- about R-5 if you use EPS and about R-6.2 if you choose XPS. You need a much higher R-value to reduce the heat loss through your roof to reasonable levels.
Thanks again for your help and concern.
.
And you can buy recycled sheets of polyiso which has come off of commercial roofs; it's 1/3 to 1/2 the price of new, and works just fine. Google "recycled polyiso"