Is it feasible to do a sealed attic retrofit with 4/12 roof in Phoenix (Zone 2B)?
bimbomb
| Posted in General Questions on
I own a 2200 sf single level home in Phoenix built in the 1960s with the following features: slab foundation, uninsulated 8 in CMU exterior walls, ventilated attic, and truss roof with 4/12 pitch.
One of my goals for a good enough retrofit is to seal the attic and insulate the roof interior, bringing the attic into the conditioned space. Is that technically feasible using either spray foam or mineral wool for insulation in light of the fact that the space is pretty tight?
This would be accompanied by a new HVAC system with HRV and new siding that includes a layer of exterior rigid foam insulation.
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Replies
It probably does not make sense to do this, you would be better off sealing at ceiling line and if mechanical are in there, find a new space for it.
Thank you for the advice. I did consider the best practice of replacing the blown in insulation and keeping the attic vented. Unfortunately, I have a slab foundation, and the installed duct work runs through unconditioned space. It seemed more practical to expand the conditioned space than to build new soffits, but I didn't know whether anyone would be willing to spray foam in what felt like a tight attic space. I did find a reasonably experienced installer to remove the old cellulose and do a spray foam retrofit for a reasonable price, and I'll report back whether it ends in triumph or failure.
I also live in Phoenix so your structure style sounds very familiar.
In this case, especially because of the trusses, I think you'd be better served replacing the entire roof and using exterior insulation to encapsulate the attic. You'd have the opportunity to replace the old sheathing with radiant barrier, then build up layers of insulation to achieve code and your desired performance. This way, you'll eliminate all the thermal bridges of the trusses themselves. Working in between the trusses is a mess at the best of times, and once you introduce spray foam — which is by no means a perfect solution — you make it very hard to make any fixes and updates. (Plus you introduce all that polyurethane into your living space.)
It's a big project for sure, but you've got the right idea that exterior rigid insulation is the way to go.
I am sorry to be a kill joy but.
1 If you remove the ceiling drywall you will have to leave the bottom cord if the trusses or the roof will collapse.
2 In zone 2 the insulation requirement for the roof is R38. R38 spray foam may technically be possible but is total unaffordable and practically no one installs it. Getting 10 inches of mineral wool at the roof line is a lot.
3 In zone 2 ERV is silly the temperature differential from inside to outside is very low most days and peaks out at 35°. With so little energy to be recovered it would make more economic sense to crack open a window. It seems unlikely most 1960s home could ever be air sealed a point that ventilation would be necessary. Being block construction maybe you could but get a blower door test result under 1 ACH50 before you spend your money.
If you heart is set on a cathedral ceiling pick on of the five in this article.
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
If you are going to keep the ductwork in the vented attic, make every effort to be sure it has no leaks. Turn on the blower and hold a burning incense stick near every joint and fix the leaks. And burry the ducts under 10 inches on cheap fluffy insulation.
Note a cathedral ceiling will 25% more energy than a flat ceiling if they have the same R value due to its larger surface area.
Walta
Walta,
I don't see where the OP, or any subsequent poster, has suggested converting the house into a cathedral ceiling, or cutting the trusses.
Thank you for all the replies. I should have clarified that from budget and cost-effectiveness perspectives, my target is good-enough rather than ideal. If ideal, I totally agree that exterior foam all around would have been a good choice, but it would have added maybe $30k to a $12k re-roof project at a time when I didn't have that cash on hand but needed a roof. For less than that I was able to subsequently add a 14.4kW (36 panel x 400 watt) solar self-install 4 months later and R30 open cell spray foam and air seal at the roof. I know the open cell makes my sheathing vulnerable to rot and does not meet code requirements for my zone, but it is a marked improvement over the previous status quo and was maybe half the price of the mineral wool or closed cell equivalent. My existing attic-based ductwork is super leaky already in a very arid climate, so I'll monitor attic humidity with the expectation that it will remain dry enough to avoid rot (based on comments made by Dr. Joe Lstiburek about why rot was less a problem with this construction in the past).
I appreciate this community and the thoughtful replies. I'll definitely post an update if I end up with major regrets about the compromises I chose to make.