More Q&A Spotlight
In warmer parts of the country where basements aren’t common, the default location for the HVAC equipment is often the only space available—the attic. And unless the attic is conditioned, hot summers make it especially hard for the equipment to do its job. It’s sort of like trying to make ice in an oven.
“BurbankGreenGuy” (BGG, for short) has a 90-year-old home in a sunny part of Climate Zone 3C, and he writes here that he’s updating his attic and the HVAC mechanicals it houses. The 1000-sq.-ft. attic has two small gable vents, the AC/furnace system is 25 years old, the roof is a 6-7/12 pitch covered in clay tiles, and on hot days the attic temperature can reach 130°F.
BGG had an energy audit done, which informed his plan to remove the existing fiberglass batts, air-seal the attic, install a ducted heat pump system, then blow in cellulose insulation.
He asks if, while the attic is empty, it is a good time to perform other upgrades. He is particularly interested in whether mechanical ventilation should be added. His gable vents are small and the roof doesn’t have soffits, so adding ridge and soffit vents would be difficult.
He also wants to know how much difference added ventilation would make, and whether it is worth the effort and upfront costs. This is where we will pick up our Spotlight conversation.
Maybe ventilation isn’t the problem
One reader points out that the air circulating through the existing vents is too miniscule to have any effect on the attic temperature, and recommends burying the ductwork.
BGG responds with a quote from Joe Lstiburek: “In a hot climate, the primary purpose of ventilation is to expel solar-heated hot air from the attic or roof to reduce…
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12 Comments
Insulate roofline with R13 (R19 is required in new attics, but not triggered here. Still, should substantially reduce attic temps)
Air seal at ceiling plane. Retrofit new IC/AT lights at ceiling or place fireproof cover over existing lights to allow for insulation over them. Code requirement.
Insulate ceiling to a minimum R,49
Also code requirement see title 24 part 6 section 150.2(b)1J
In this scope of work, code requires ventilation to be updated to CRC806 (1/150th)
Air seal and insulate the attic floor...sure. But insulating the roof line, as well? Not so much. Fiberglass batt should never be in direct contact with a roof due to moisture concerns. And as presented in the article, keeping the attic cooler doesn't make much of a difference overall. The batt on the roof would also keep the attic warmer for longer, even if it does provide some reduction in heat gain. If BGG is set on having the HVAC in the attic, why not change the thermal and pressure boundary to the roof and use a closed cell spray foam to condition the attic space...?
You're right that f/g batts should not be in direct contact with roof sheathing without a vent space, except in climate zones 1-3 they can be, if there is a vapor diffusion port at the ridge.
Keeping the attic cooler will absolutely make everything perform better, if there is HVAC equipment up there.
Insulating the roofline slows heat flow in both directions. Adding R-13 will cut heat flow to 1/13 that of having no insulation.
Closed cell spray foam is a problem-solver when nothing else will work. Otherwise it's among the least-green materials we can use. It's terrible for the health of the installer, it contributes enormously to global warming (yes, even the ones with "low-GPW" blowing agents), and it doesn't always cure properly which is very difficult and expensive to remedy.
Quoting:
"In BGG’s case, it’s not so much a matter of whether the attic is properly ventilated but rather the fact that the air handler is in a hot attic. Ideally, it would be inside the building enclosure, but BGG has stated that is not an option—in which case, he should insulate and air-seal the air handler unit as much as possible."
I'm arguing that adding insulation at the roofline would substantially reduce peak attic temps to address this problem statement. Your solution of doing an encapsulated attic with ccSPF is also fine, and better. However it's expensive and sometimes easier said than done. I'm just articulating an alternative.
What moisture concerns do you have with air permeable insulation in contact with the roof deck in CZ3?
In my experience, this is done on most new construction. Indeed It's a code requirement- see table 150.1-A of the energy code
Luke,
I'm not that familiar with your codes, but I thought the only two conditions where you could have permeable insulation in contact with the roof sheathing were where you either also had insulation above the sheathing, or (in climate ones 1, 2 and 3) had a vapour-diffusion port?
Malcolm, you are correct. the IRC code section is long and complicated, but that's basically what it says: https://codes.iccsafe.org/content/IRC2021P1/chapter-8-roof-ceiling-construction#IRC2021P1_Pt03_Ch08_SecR806.1.
There is an apparent conflict with the detailing provided/intended in the energy code (part 6) and residential code (part 2.5)
It's all a bit... Err... Baffling
But in this case, it's academic.
Insulate the roofline and vent it consistent with 806.3. fine. It's going to be a MUCH cooler attic in the summertime peaks, which is the issue we're trying to address . Or is that not so, and that's why it wasn't included in the article?
The standard detail sans venting at roof deck insulation in a vented attic (from building science corporation) is drawn on slide 5 here
https://title24stakeholders.com/wp-content/uploads/2016/12/2019T24-Utlity-Stkldr-Mtg2-HPA.pdf
Josh Salinger did say that the best thing to do would be to insulate the roofline, but he assumed that wasn't something the poster would do, because he said he wouldn't.
Looking at the OP, I don't see any reason why adding insulation at roofline (batts) is infeasible or impractical. Someone is going to be up there to remove the existing batts. They can simply bring some baffles up, attach them to the underside of deck, then move the batts from the floor to the ceiling. Just compress them into the cavities, or if they're nasty, then install new ones. Airseal, cover for lights, Then install heat pump. Then blow in cellulose (minimum R49 as required by code)
Cooler attic, check.
Vented attic, check
As an academic exercise, how much airflow do you need to expel 100kbtu/hr of heat gain at a delta t of 40 degrees. Assuming 140 attic temps and ambient is 90. (100 kbtuh is a guesstimate of solar gain on the attic-- 100 BTU/hr-sq.ft. x 1000 SQ.ft.)
Back of the envelope (pun intended) is that passive ventilation is that it's a LOT more flow than you would imagine getting through 3x code required venting.
I agree, and if it were my project, I would insist on it. Having contributed to many of these Spotlight articles, though, I can say that there is always a lot to cover and we have to limit how far we stray from the original question. That's what comments are for!
Indeed
Well taken
The OP was focused on asking about venting and I felt a little sheepish about straying from that, but the comments and Josh's insights (rightfully I think) refocused on the underlying issues.
I did find it interesting that the intent of venting is in part to cool off attics... It's not nothing, but it seems like a really weak tactic all things considered
Why do you think that making attics cooler is a weak tactic?
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