New construction: Insulation options
SriniS
| Posted in General Questions on
Thank you for all the great articles! They are very informative for any one new(like me) to building a house as well as understanding various options and pros and cons of each of them.
We are planning to build a house in Dallas area(climate zone 3B). Would like to find out what would be the cost and energy efficient way to insulate the whole house using 2×6 walls. We need to be 2012 IRC/IECC compliant.
We are planning to use either cellulose or fiberglass dense insulation on walls and ceilings.
- On the exterior walls, planning to use stucco. would like to understand which one of the following options are are more energy and cost effective or any other alternatives?
- Drywall -> dense blown cellulose -> zip system -> 2 layers of WRB -> stucco
- Drywall – > dense blown cellulose -> Plywood or furring strips -> rigid foam -> OSB/Plywood sheathing -> 2 layers of WRB ->stucco
- On the roof, planning to use shingles.
- netted blown cellulose or fiberglass insulation -> Plywood/OSB sheathing -> rigid foam -> plywood -> underlayment -> shingles.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Here's an excerpt from on one Martin's articles on installing stucco properly:
"If you’re installing stucco over foam, here’s how to create a ventilated rainscreen gap:
Install 1×3 or 1×4 vertical strapping on top of the foam, screwed through the foam into the studs.
Fasten paper-backed metal lath to the strapping, and proceed with a standard installation of three-coat stucco."
Here's a link to the article:
https://www.greenbuildingadvisor.com/article/to-install-stucco-right-include-an-air-gap"
I assume that you are choosing to insulate between the rafters either because you want to have a cathedral ceiling or because you will have HVAC equipment in the attic. In either case, this should be helpful:
https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling
If it isn't treated wood, I'd expect stucco applied to wood furring to risk furring rot (excepting very dry areas). Perhaps something less permeable than asphalt paper between them would be wise.
For example, Rockwool recommends treated furring and their product allows far more backside drying than foam.
This is also worth reading (suggests that lath directly to furring isn't good for other reasons).
https://www.mtidry.com/uploads/PDF_Download_665538.pdf
Sounds like vented mat (no furring strips) is the recommended solution (continuous support for the lath).
In any case, don't do stucco without a gap.
For the walls I like option #1 with an added layer of foam over the zip. Insulation is great but if no one puts in the effort into sealing all the air leaks, it will not do its job.
Even with Zip sheathing you will need a lot of caulking and extra labor.
I do not understand your second question. I recommend you avoid cathedral ceilings and go with the flattest possible ceiling that will support lots of low cost cellulose insulation. It is very important that you find a way to keep the ductwork and HVAC equipment out of your attic, this will be very difficult in Texas as this bad idea is so common. It will be worth the effort as it is likely to lower your cooling costs by 20%.
Walta
Dallas/FW is zone 3A, not 3B, not that it makes a huge difference here.
IRC 2012 code-min walls can be be compliant with either 2x6/R20, 2x4R13+ R5 continuous insulation, or a U-factor of U0.057 (R17.5 "whole-wall". Any of your wall stackups make it, but be sure to back vent the stucco with weep screeds and top vents for maximum drying of the stucco. A good alternative would be dense packed 2x4 + OSB/#15 felt (1-layer) and 1" foil faced polyisocycanurate with a 3/4" gap to the stucc lath. The foil facer is a powerful vapor barrier against exterior moisture drives, and gives additional thermal performance, and the continuous foam thermally breaks the framing.
>"Drywall – > dense blown cellulose -> Plywood or furring strips -> rigid foam -> OSB/Plywood sheathing -"
^^^this one doesn't quite cut it cost/performance/protection-wise. Rather than a plywood (or furring) /foam/plywood sandwich, putting foam on the exterior side of the sheathing protects it from the moisture drive of the stucco. Installing only furring between the foam and studs may not be sufficiently structural without engineering the fasteners & spacing for the outer layer of plywood. Two layers of sheathing are expensive and not necessary.
The roof needs to be either R38, or U0.030 (R33 "whole-assembly). With rigid foam above the roof deck the foam-R needs to be a minimum of R5. Cellulose is heavier, but provides more moisture protection of the wood than fiberglass. The borate fire retardents in cellulose also offer some anti-termite & ant protection.
something like this? plywood/OSB -> foam -> underlayment -> Shingles.
My criticism in response #4 was your wall stackup. The roof will always need a nailer deck.
For the roof the main weather resistant barrier goes directly on the structural roof deck, under the foam. The nailer deck gets through-screwed to the structural roof deck, with timber screws (spacing depends on slope and wind resistance requirements), and the nailer deck gets a layer of #3o felt under the shingles.
In Dallas/FW there are multiple vendors of used roofing foam, which can make going with 3" or more of foam relatively cost effective (cheaper than deeper dense-pack.) With 5" of roofing polyiso it would make code-min on a U-factor basis.
Some of the local reclaimers advertise here:
https://dallas.craigslist.org/search/sss?query=rigid+insulation
In
Is it better to use polyiso or rockwool for exterior rigid foam? Was hearing that polyiso can contract much more than rockwall, so need 2 layers instead of 1 layer of rockwool.
Polyiso can shrink over time, but rock wool is highly vapor permeable whereas polyiso puts a powerful vapor retarder between the susceptible wood sheathing and the roofing, limiting the amount of moisture reaching the structural roof deck even with some minor roof leakage.
RECLAIMED polyiso will have already undergone most of it's lifecycle shrinkage. A 4' x 8' sheet may be 47.75" x 95.5" after 30 years of service, but won't shrink any further after that.
Thank you for all the replies!
All the ceilings are flat. Due to the HVAC, the roof is cathedral. Will check with HVAC contractor in moving the HVAC out of attic to second floor, so that roof can be more flat.
Have gone through the links shared, it seems making sure the gap is there when installing stucco (both from top and bottom for drying out) is one of the critical element.
Regarding roofing, Any recommended roof assemblies that work effectively both energy and cost perspective for the Dallas/FW(3A zone) ?
If your question is what insulation will pay for its self given your weather fuel cost and interest rate?
The best way to make that happen is to model your home with a computer program. I like BEopt it is no cost program paid for with tax dollars that has hundreds of variables for you to enter. Plan on investing over 30 man hours to watch the training videos enter and analyze the data.
https://beopt.nrel.gov/
As I said getting the HVAC out of the attic in Texas will be going against the grain. You any want to look at having a few mini split systems to eliminate the ducts.
It sounds like you want a conditioned attic. In my opinion a conditioned attic is an act of desperation that is only slightly better than nothing, and should be reserved for retro fit in existing homes where people have few options left to them. If this is new construction I see no reason to plan yourself into a conditioned attic.
Please read this article
https://www.greenbuildingadvisor.com/article/solutions-to-the-attic-duct-problem
When you say stucco are you talking about the real thing a cement based product or eifs stucco styrofoam covered in latex that looks like cement.
Walta
Thank you!
Looking for both roof and wall assemblies that are energy and cost effective for Dallas area. I am planning to use the real cement based stucco.