New roof with existing spray foam attic in Texas
My house was built in 2017 just north of Austin, TX. It has open cell spray foam on the underside of the roof decking. The original shingles are Atlas Pinnacle. There is a whole home dehumidifier installed that I currently don’t use because the A/C does a good job itself.
Last May (’20), we had a massive hail storm come through, which my insurance approved a complete re-roof. I took my time interviewing contractors and doing research on shingle selection. I’ve now learned that most shingle manufacturers will not warranty their products with my roof configuration. The lone option is CertainTeed, which luckily is well regarded.
With my slow decision, poor planning by my contractor, and covid, the shingles I wanted are not being manufactured right now and CT has only given a vague response on when the line will resume production.
Of course that has given me more time to go down the research rabbit hole. I know understand that there is a risk with any shingle that my decking could rot due to a lack of ventilation. I originally thought the warranty issue was only heat related.
Now I’m trying to figure out what to do. We are planning on being in this house for 15+ more years until the kids are out.
1. Re-roof with shingles on existing deck and re-duct the dehumidifier to condition the attic.
2. Go standing seem metal roof using a ventilation mat between the deck and metal.
3. Add an air gap with a new deck using furring strips and purlins. Might as well do radiant foil too.
Any other options? #1 is most affordable but most risky for future rot. #3 most expensive.
Also, has anyone noticed degradation of spray foam adhesion on a re-roof with new nails being driven through and the vibration? Wondering if that’s another risk with #1.
Thanks in advanced.
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Replies
Eric,
Lots of people fixate on shingle warranties. Here is a 2011 comment from Martin Holliday that might be helpful for your situation:
"Frankly, asphalt shingle warranties from ALL companies contain so many limitations and exclusions (for example, labor exclusions) that roofing warranties aren't worth much. If you ever need a warranty, you're never going to get much from any manufacturer."
That said, you really want some supply going into your conditioned attic to manage moisture and mitigate risk. You also should monitor humidity (I used a SensorPush humidity sensor in a previous home.) and use the dehumidifier if levels stay too high.
If it were my house, I would avoid approaches that involved a lot of complexity and cost. (Maybe you'll stay in the house for 15 years, but maybe you won't.) When the damaged roof is removed, I would ask the contractor to inspect for any signs of rot. (I might even use a moisture probe to check the sheathing just to be on the safe side.)
That's my two cents. One of GBA's experts may chime in with a more informed opinion.
I’ll take a guess that if you have ocSPF under your deck, they installed 5.5” R20 only, and that’s 53% of the required R38 the codes require for CZ2-3 for thermal comfort. Since the laws of physics DO NOT CARE if your builder installed the insulation under the Performance code, or not, the chances of having moisture problems increase dramatically. As reference all you need to do is look at all the problems we had in TX last February.
I highly recommend to go with your option 3. If you have R20 under the roof, then install 3” R20 Polyiso on top of the roof decking to end up with proper insulation and condensation protection, even if its twice in 100 years. If you can swing the 1x4 purlins and metal roof, it would be best. The ¾” air gap is perfect for getting a cool roof. Don't forget to supply ventilation to your conditioned attic, and your dehumidifier. FYI, I recommend Kidd Roofing.
If you use a radiant barrier with option #3, make sure you get one of the perforated ones that is vapor permeable -- most radiant barriers are aluminized mylar (or similar materials), and are vapor barriers. A vent gap to help the sheathing dry won't help much if you cover the sheathing with a vapor barrier sort of radiant barrier...
Nails shouldn't be an issue for the spray foam, I wouldn't worry about that. If a nail poking into the spray foam is enough to cause the foam to seperate from the structure, then you have problems with the spray foam install and a whole new can of worms to deal with.
Bill
This video seems relevant. Risinger addressed this (open cell conditioned attic) in a recent video. He was highlighting the relevance of a vapor port (which might still be possible if you are re-roofing), but as an alternative (or complement) either a dehu or AC supply in the conditioned attic.
https://www.youtube.com/watch?v=zhkGcklWB_Q
I'll be daring soon on a Vapor Diffusion Port for the first time on a new house. I've heard Joe talk about it for years, but my concern was always about our labor pool doing it right, but in this case, the Builder is an experienced ZERH pro, and he's accepted the responsibility of getting it done right. I'm going to be specifying mineral wool or cellulose. You can use fiberglass as well, but I would double check if ocSPF is part of the VDP system.
Thanks for the replies. I'm certainly leaning toward #1 and the idea of using a humidity sensor is good. I do understand the limited value in the shingle warranty, but it does give credence to a looming issue that they don't want to deal with.
Armando is correct, the open cell just fills the 2x6 rafter cavity. I have a 4 ton dual stage A/C and a propane direct vent furnace for 3000sqft. My electric use max is 2000kwh in July/Aug and I run through 400gal of propane every 10 months. The February artic blast didn't seem that bad even when we didn't have power for 20hrs straight.
That's a lot to say that I feel good about where the house is efficiency-wise and would be hard to justify the cost of eeking out a little more.
Just wanted to follow-up on this topic....
1) Roof was replaced. When they pulled the underlayment up, there were no signs of rot thankfully. Roofer provided pics throughout the job.
2) Installed a temp/humidity sensor about halfway up underneath the peak of the attic, which is probably 12-15ft high. It's connected to my home automation hub, so I can monitor remotely.
Humidity in Austin has been terrible throughout June. Was reading 80% in the attic the first day I started logging. Yikes!
3) Modified the 90 pint dehumidifier so that it no longer is being fed house air. I installed a rigid 8in duct to pull air near the peak of the attic into the dehumidifier, which then blows the dry air back into attic. Unit is set to 52%, and I'm seeing readings from 50-60% on the sensor throughout the day.
4) When I plot out the humidity readings, I'm baffled that peak humidity occurs early afternoon and the low is at night. When I look at the outdoor humidity readings, it's the exact opposite, max humid around 6am and min around 5pm.
Also a little surprised that the attic temp is only ~5deg cooler than the outdoor temp. Eg. 94deg outside and 88deg in the attic this past Friday. Could the dehumidifier be warming up the attic that much?
For what it's worth, I've got open cell in my zone 4A attic (hot/humid similar to your area), and my performance matches yours, and from other reports, my understanding is that this is normal. The post below explains the humidity fluctuations. As to temp, yeah, my attic will also be about 5 degree below outdoor ambient on hot days. So on a 95 degree day, my attic might peak around 90F. Think about it this way though: without the spray foam, that 90 figure would probably be more like 110-120. Also, if you opened up even a small supply a/c duct into your attic, you could probably get that 90 down to about 80.
One other random observation: In the winter, my attic stays about 30 F warmer than the lowest outdoor ambient temps. Last winter I don't think my attic ever dropped below about 62 or 63 F, even though outdoor temps routinely drop below freezing at night, and occasionally down into the upper teens. Not sure why winter performance seems better than summer.
In any event, that was a long, rambling reply but my primary point is that I believe the performance you are seeing is pretty normal.
Thanks for the reference point. It's pretty easy to go down a rabbit hole trying to make the whole system (insulation, HVAC, etc) more efficient.
RE: “I’m baffled that peak humidity occurs early afternoon and is low at night”
Joe Lstiburek might have the answer why:
https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-016-ping-pong-water-and-chemical-engineer
“Moisture in the indoor air migrates into the attic spaces, passes through the open cell low density spray foam and is stored in the wood sheathing. It then gets driven out by temperature changes – largely driven by solar radiation – and accumulates at the upper portions of attics.”
Thanks. I remember reading that article before, and now it makes more sense.
I would expect that the amplitude of the peaks should deminish as the decking dries out, but maybe that's just wishful thinking.
Maybe I am missing something but however it was built originally it seemed to work well in that nothing was rotting/ molding and the shingles seemed like they would last there normal life span. I have to ask why change anything?
Do you think you are taking change the air flow thru the attic is a big risk?
I guess I should stop reading the posts from TX, they make me crazy with the silly stuff that everyone does there and calls it normal.
Let’s stick all the HVAC equipment in the attic because that is just how we do it in TX. What it is hot in the attic with all the leaky ducts and the AC has to work really hard to cool itself and the ductwork before it can start cooling the house. Maybe we should move the tiny amount of insulation we will install up against the roof because the roof only has 2 or 3 time the surface area as the ceiling and we will use spray foam for insulation because is glues itself in place. We don’t care if spray foam insulation costs the more per R than anything else. What you can’t afford the bid for R60 spray foam? I am shocked all you have in the budget is enough for R15 so that will be good enough says the guy selling the foam. No that is not a conditioned attic we encapsulated the attic it not connected to the conditioned space or the outdoors it magically isolated onto itself no need to remove the heat from the sun baking the black shingles we got spray foam to keep that out. Really all we need in the attic is a dehumidifier because we said it was encapsulated. We don’t care if the attic 98° that heat knows better than to come thru the drywall ceiling and into the house. Rant over.
As far as I can tell if you insist on having an attic full of HVAC equipment conditioning the attic is just slightly better than a vented attic but only slightly and only if the R values are the same and the attic has enough HVAC vents or leaks so the attic is more or less the same temperature as the rest of the house. All bets are off when you call it an encapsulated attic that is more or less the same temperature as the outdoor air.
Walta