Exterior roof insulation assembly in Zone 2A
Background: I live in a 1980s home in Houston (Climate Zone 2A). It is just about time for a roof replacement and I am considering a standing seam metal roof. It is a hip roof with 2×6 rafters. At this time, the attic will remain vented though I would like to prepare for the possibility of transitioning to an unvented attic in the future. Because of this, I intend to have the standing seam panels installed on battens or over Delta-Trela to create a drainage and ventilation plane. Some sort of fully adhered roofing membrane will be used, probably Sharkskin SA. The roof is approximately 1400 sq ft. I would like to add some exterior insulation to the roof deck but it has been difficult to find a roofing contractor that was up for this challenge.
Question 1: Given zone 2A, if the attic was unvented, the underside of the roof deck insulated (probably closed-cell foam), the deck well air sealed using panel tapes and impermeable membrane, do I still need to have exterior insulation to control for condensation concerns? I know that exterior insulation would be expected in colder climates, but intuitively it seems that condensation on the underside of the roof deck would be unlikely given the climate here.
Question 2: The exterior insulated roof assemblies I see tend to have the polyiso sandwiched between two layers of roof decking. If battens were screwed through the polyiso, all the way down through the bottom layer of decking and into the rafters, is the decking layer on top of the polyiso really necessary? I suppose the issue would be identifying a membrane that could be applied directly to the polyiso.
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Replies
Hi Trey.
In terms of condensation control, you would not have a problem filling your rafter bays with closed-cell spray foam. Whether you could meet code minimums is another question and depends on what your local codes call for. I believe the 2018 IRC calls for R-38 in Climate Zone 2. And when installing rigid foam above the roof sheathing, you do not necessarily need another layer of sheathing. That depends on the type of insulation used, type of roofing, if there is to be a ventilation channel, etc. You may find these two articles helpful: How to Build an Insulated Cathedral Ceiling and How to Install Rigid Foam on Top of Roof Sheathing.
Keep in mind that closed-cell spray foam is among the more environmentally harmful insulation types and options like a combination of exterior (reclaimed) rigid foam and fibrous insualtion like cellulose in the rafter bays are also possibilities for your roof.
Thank you for the links, Brian. I share your concern regarding the spray foam. I considered it an option when not going with exterior insulation. Ideally, I would like to use two layers (1.5-2" each) of reclaimed polyiso, staggered seams on the exterior and then a fibrous insulation in the rafter bays.
What's the advantage of the "hybrid" approach of exterior foam plus interior fibrous over an all-exterior-foam option? Is it simply cost? Or would putting 6+ inches of exterior foam to get to R38 be logistically or structurally unwise?
Quite frankly I had not considered this. I was thinking of it from the perspective of augmenting what could fit in a 2x6 rafter bay and controlling for condensation. After reading the second article referenced in Brian's response, it appears that a minimum of 7 inches (dependent on material) would be required outside of the sheathing to meet code requirements.
Is your HVAC equipment and duct work in the attic and you are wanting to bring the attic into the conditioned space? This is the only good reason to insulate a roof and it is only slightly better than not because the roof has 50% more surface area and is unlikely to get insulated to the same R value as an attic floor due to the cost.
Abandoning the attic HVAC and installing mini splits with R60 on the attic floor is likely to cost less in the long run.
Walta
You make a valid point, I suppose it would ultimately come down to cost differential.
Yes, the HVAC equipment is in the attic. I like the idea of mini-splits, but I have not had any success convincing the other half of the voting population in the house to approve the "eye sore". Additionally, since we do not have a basement it would present an opportunity to use the space in the future for storage or maybe a small office.