Exterior Roof Insulation – Rockwool vs PolyIso? Furring Strips?
We are designing a home in climate zone 3 (Los Angeles) and I’d appreciate some guidance on exterior roof insulation.
On the walls, we are currently planning to use a combination of Rockwool Comfortbatt and Comfortboard, along with a 3/4″ rain screen. Our house has 2′ eaves.
On the roof, our current design calls for the following (inside -> out)
– 3″ Comfortbatt (R-12)
– Vapor barrier
– 8″ Comfortbatt between rafters (R-32)
– Sheathing
– 4″ Comfortboard 80 (R-17)
– 2 1/2″ rainscreen
– Sheathing
– Asphalt shingles
The overall thickness of the roof is causing some issues between our first and second floor, where the roof around the perimeter of the first floor is encroaching on some of the window sills on the second floor (essentially the roof is rising too high). We are considering a few different mitigation strategies, but I’m also wondering if we need such a thick roof assembly.
My questions are:
1) Is there a significant benefit to using Rockwool on the roof as opposed to PolyIso? I can achieve a 17.4 LTTR with 3″ of PolyIso, thereby shaving 1″ off of the roof thickness
2) Do I need a rainscreen for either material and, if so, how thick does the rainscreen / air gap need to be? Our roof pitch is 5:12 and we are in a fairly temperate / hot climate; not too much rain.
I’ve generally preferred Rockwool for its fire resistance, bug resistance, and natural fiber attributes, but I’m now considering other materials for the roof if it will help me shave inches either in the insulation material itself or by needing a smaller (or no) rainscreen.
Also, in case it influences the answer:
– We have hipped roofs, as this will be a Prairie-style house
– We have one skylight, located near the peak of the 2nd story roof structure
– Due to the design, we do have a handful of valleys in our roof
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Replies
Calbears07,
I would suggest not using any exterior insulation. Between the batts in the rafter bays and those in the service cavity, you already have R-44, which is pretty high for zone 3. That will reduce the roof height, make the detailing a lot easier, (attaching the strapping, the overhangs, fascias, etc). One of the chief benefits of exterior roof insulation is to preclude the sheathing getting too cold and taking on condensation. You don't have that problem in LA.
Instead I would suggest using a sarking membrane (house-wrap) over the rafters, then 2"x4's to form the vent channel. This will allow continuous air-flow over the hips and valleys, and saves you one layer of largely redundant sheathing. https://www.greenbuildingadvisor.com/article/building-a-vaulted-high-performance-and-foam-free-roof-assembly
If you felt that you wanted more insulation, I'd use 2"x10"s rather than 2"x8"s as rafters. That's still easier and less expensive than putting foam or rockwool over the lower roof sheathing.
Thanks for the suggestion and certainly worth considering. We had gone with exterior insulation to have the “blanket” effect and eliminate thermal bridging. Is your suggestion that we need not worry about thermal bridging in our climate zone?
Calbearso7,
I think your service cavity will deal with thermal bridging pretty effectively.
From your description, you appear to be putting the 3" Comfortbatt under the rafters. If you install those batts this way, that will deal with thermal bridging just as well as exterior continuous insulation will. Continuous insulation works the same way in regards to helping with thermal bridging whether it's installed on the interior or the exterior. You just see continuous insulation more commonly in walls on the exterior is all.
I would put those Comfortbatts perpindicular to the rafters. Run perpindicular 2x4s to support any interior ceiling, that way you have no continuous framing from the exterior to the interior that would act as a thermal bridge.
Bill
Thanks for the suggestions. I think that will probably help a ton and will shave ~4 inches off the overall thickness.
If we did still go with external insulation, just out of curiosity, does the Rockwool need to be vented above the insulation? If so, how wide should that vent be?
(The thought is only if we decide we’d prefer to have the thermal break outside of the structure rather than inside of the structure, but I grant it’s likely a minimal difference in our CZ).
CalBear07,
Air permeable insulations (which rockwool is) require a vent cavity by code, unless you are in CZ 3 or less (which you are). You would be allowed to use a vapor diffusion port in lieu of venting which would also save on thickness: https://www.greenbuildingadvisor.com/article/vapor-diffusion-ports.
I agree with everything Bill and Malcom say above.
Josh Salinger
Malcolm left you an excellent suggestion. But do check to make sure there are no structural issues with separating the roof sheathing from the rafters with 2x4s. You are in seismic zone D and you'll be needing a very resilient and strong roof diaphragm. Run it by a structural engineer if possible.
It can be done. We are in seismic D here in Portland, OR where the video was shot. The tricky part is getting the code required venting while still allowing for roof sheathing nailing at the ridge and eaves. Definitely work with your structural engineer and refer to the article I wrote about the venting.
Josh Salinger