Low-slope roof replacement questions
Thanks for this great resource! I am planning to replace a 30 year-old 3-tab shingled roof on a circa 1959 house with a low slope (~2/12) in zone 2A (Houston, TX). I have been reading through a number of the articles relating to cathedral ceilings and low-sloped roofs, but I still have some questions. More details on the roof:
- The roof has soffit venting but no ridge vents;
- The roof system consists of two gable-ended roofs (main house plus garage/utilty room) that meet in a dead valley over about a third of the length of the smaller roof;
- It does have a spinner vent in the attic space, but the attic only covers a small percentage of the house (only enough to contain the HVAC system and about half of the ducting, which was replaced in 2014 with R-8 flex duct);
- The rest of the ducting is an original box duct that is behind sheetrock (and which has retrofitted duct wrap insulation);
- The rest of the roof has original batting between the rafters and gypsum board ceiling attached to the rafters;
- The attic space has been retrofitted with fiberglass batting at some point in the past;
- The existing roof decking is ship-lapped boards;
- Given the dead valley, there is about 12 feet on one side of each roof that is non-ventilated currently.
I’m certainly not planning to replicate the existing 3-tab roof (amazing that it lasted this long given the slope). I’d like to improve the roof’s thermal characteristics, though I’m mainly concerned with cooling performance given my location.
Should I be converting this to a non-ventilated roof?
Should I be looking at rigid foam above the shiplap with air channels above that, before another layer of sheathing and whatever roof I end up installing (standing seam, SBS, etc.?)
What options do I have for improving the under-sheath insulation?
Again, thanks for such a great resource.
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Q. "Should I be converting this to a non-ventilated roof?"
A. Probably, although another possibility (after you install an adequately thick layer of rigid foam above the existing roof sheathing) is to include ventilation channels above the new rigid foam.
Q. "Should I be looking at rigid foam above the shiplap with air channels above that, before another layer of sheathing and whatever roof I end up installing (standing seam, SBS, etc)?"
A. Probably -- although the ventilation channels above the foam are optional. (The ventilation channels don't do much on a low-slope roof, and in your case you probably don't have to worry about ice dams considering your mild climate.)
Q. "What options do I have for improving the under-sheathing insulation?"
A. Considering the low slope of the roof, I'm guessing there are access difficulties. In your case, it probably makes the most sense to put all of the insulation above the roof sheathing in the form of rigid foam.
If you take this advice, remember to seal up the soffit vents (as well as any other obvious air leaks into the attic). You'll also need to insulate the gable walls in the attic.
Here are links to two relevant articles:
How to Install Rigid Foam On Top of Roof Sheathing
Insulating Low-Slope Residential Roofs
It's possible to take a substantial amount of the wallet-pain out of above-deck foam by using reclaimed roofing foam available from building materials salvage yards or specialized foam-reclamation companies. There are probably several smaller companies operating locally in the Houston area, but a quick scan of the local craigslist didn't turn them up (at least not this time). Repurposed Materials Incin the Dallas/ Ft. Worth area may still be worth a shot.
Using reclaimed goods is by far the greenest option, since no new polymer or blowing agents are being used.
Even if the pre-existing fiber insulation is a paltry R13-R19, adding 3" of polyiso above the roof deck would get you to better than IRC 2015 code-minimum performance on a U-factor basis (R33 "whole assembly".) If there were no fiber insulation it would take about 5".
On a low-slope roof in Texas it's worth considering using a CRRC rated "cool roof" roofing suitable for low-slope roofs. To get an aged solar reflective index north (SRI) of 60 would usually be white or off-white in color, but there are other colors that still have a reasonable solar reflective index.
GAF's Timberline Cool Series "Cool Barkwood" brown has an aged SRI of 28, which isn't bad for an asphalt shingle. There are gray or tan membrane roofs with aged SRIs north of 30.
The CRRC has a searchable database tool here: