I want to put on a cold roof with 3″ rigid foam
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| Posted in Energy Efficiency and Durability on
I want to put on a cold roof with 3″ rigid foam, 1 1/2″ air space, 7/16″ OSB on air space 2 x 4s and standing seam roof on my Cape Cod.
It has about a 3′ x 8′ attic with insulation between rafters and vents on the gable ends. The roof joists are insulated.
Also, the current roof sheathing has 1″ x 10″ boards with 1/4 to 1/2″ gaps in between.
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Replies
And your question is?
The full message didn't come through. There is fiberglass insulation from the eaves to the attic.
The question: Do I leave the vents as they are or seal them? And will closing the vents cause moisture problems?
If you want to insulate above the existing roof deck then you need to not only seal the gable vents but insulate the gable walls so that the entire attic is within the thermal boundary.
And, yes, if there is still air leakage into the attic, there could be moisture problems, so you need to first seal all possible routes of air movement from the living space into the roof assembly and attic.
A clarification on the leakage issue. As I mentioned, there are gaps between the roof boards. Would the 2 layers of rigid foam be sufficient to seal those leaks, providing I tape each layer and stagger the seams between layers?
Horst,
Yes, as long as you come up with a detail to maintain air-barrier integrity at the roof perimeter, where the air barrier of the wall meets your roof. This is typically a weak link in many homes' air barrier system.
In regard to the #5 response, the "detail" could mean using an ice/water barrier material around the entire fascia and roof perimeter as a air barrier sealant?
Horst,
What you are describing in #6 sounds tricky. I don't know how well ice/water stick to foam and bringing over your fascia back to the exterior wall doesn't necessarily connect it back to the air barrier of your walls. (What is the air barrier of your walls?)
Are you familiar with the term "chainsaw retrofit"?
This method includes establishing an air barrier on the outside of the exterior wall sheathing to the outside of the exterior roof sheathing using ice/water shield but requires you the "chainsaw" off the roof over hang and tear off all your siding.
I was thinking through the possibility of establishing your air barrier on the underside of the roof rafters with the Air Tight Drywall method. If this was done well it could minimize exfiltration of warm air into your roof during the heating season, however air movement between the fiberglass insulation and new rigid insulation on the roof via the gaps between the original roof sheathing boards might undermine your R-Value. Maybe also if your going to air condition the attic humid air in the summer could condense on upper side of the drywall.
Sounds like a tough one.