Non foil faced polyiso
Building a new house in Southern California (climate zone 3). House is single story and about 5,000 sq. ft., so there is considerable wall square footage.
The walls will be:
1. airtight drywall (it probably won’t be perfect, but hopefully pretty good)
2. cellulose
3. 2×6
4. plywood or OSB (depending if it is a shearwall)
5. possibly 1″ polyiso (depends on the responses to this post)
6. Tyvek type material (still reading whether it’s better on top of plywood or on top of foam insulation). If i use foil and tape all the seams – I may not need Tyvek???
7. 1×3 to hold the polyiso and siding.
8. James Hardie type cement siding
I have looked at some used polyiso which is not foil-faced. This used polyiso (no foil) will be about $3,750 and the new (with foil) will be about $5500. The way I understand it the used polyiso will be have holes from the original screws/nails – is this an issue? I suspect in my area where we use A/C more than heat and it regularly gets to over 100 degrees in the summer the foil is important. Though I live in climate zone 3, I live in one of the hotter areas of the zone.
How important is the foil facing on the polyiso in climate zone 3?
For the amount of money the polyiso costs, is it even worth doing in zone 3?
Does anyone know of any building code which prohibits used polyiso? (This project is in the city of LA – strict codes)?
Thank you
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
What kind of facing does the used polyiso have?
I've heard that some big cities prohibit the use of exterior foam board in some applications. You'll need to research this for your assembly and jurisdiction.
Jack,
The only way a foil facing will provide you any thermal benefit is if the foil faces an airspace. If you install housewrap on top of the polyiso, the facing is irrelevant.
However, if you don't install housewrap, or if you install the housewrap between the polyiso and the OSB, then foil facing plus a 3/4-inch air space will give you an R-value between R-1 and R-2. That's something, but it's not a lot.
The R-value of polyiso with a foil face and polyiso without a foil face is exactly the same (assuming it isn't facing an air space).
The nail holes in the used polyiso don't matter much.
The climate in L.A. is fairly mild, so the reduction in energy cost associated with installing a layer of polyiso may be so low that it's hard to justify the added cost. You can use energy modeling software to determine how much the foam will save you annually.
To find out whether your local building department will accept your plan to install used foam, you should call up your local building department.
Martin, do you think spending that much money on polyiso is a waste for this climate? Is polyiso much more beneficial in a cold climate than a warm climate? Any other recommendations for the walls to reduce the cooling load?
Jack,
You have a good plan for your wall assembly. I like to specify the WRB between the sheathing and the rigid foam; it makes it easier to install windows, but either way is fine. In CZ3, thermal bridging is mitigated with 1/2" rigid foam, but 1" is much better, polyiso is a good choice, and not a waste.
Cheapest recommendation to reduce cooling loads is to design passive solar shading, with large overhangs, porches or canopies and large trees. The best recommendation is to seal the building envelope, and have your HVAC system designed, installed and commissioned the right way.
Other than that, make sure your moisture management is well detailed and mitigated.
Jack, first off you're building a million-dollar house so a decision on environmental performance probably shouldn't be decided on saving a couple of Ks - if you really want to save some upfront cost how about knocking a few hundred square feet off the plan rather than raiding the insulation budget.
Having said that, the polyiso/rainscreen approach is almost certainly overkill in your mild, mostly dry climate. Thermal bridging is only really a major concern in colder climates, and your 2 x 6 cellulose-packed walls will do a great job keeping the heat out, especially if you have decent roof overhangs. You do have decent roof overhangs, don't you? In any case heat gain through that 5,000 sf of roof is going to be a much bigger issue than whatever is coming through the walls. Pile on that attic cellulose to R60 or better and you won't regret it.
But going back to your point #1: airtight drywall that is "hopefully pretty good" is not good enough if you really want to want to optimize your a/c performance. Spend some of the polyiso $$$ on a really first-class air-sealing job and put the rest toward some rooftop PV. Done.
I posted my response before I'd seen Armando's so I wasn't setting out to deliberately contradict him. I do respectfully disagree with him though about the necessity for this wall assembly for a house in LA, and stand by my comments about that huge area of roof under the cloudless Southern California skies.
But the different perspectives do point up our natural preoccupation with the climate issues we each have to deal with in our local area of practice. Armando I think practices in the high desert Southwest where cold winters and thermal bridging are significant issues, and, in retrospect, I wonder if my emphasis on air-sealing comes from dealing with the warm humid weather North Carolina summers. Maybe pretty good air-sealing IS good enough in SoCal - anyone know? Noticeable that we don't seem to have any regular expert postings from the milder US climates. I wonder why?
James,
Q. "Noticeable that we don't seem to have any regular expert postings from the milder US climates. I wonder why?"
A. My guess: when your energy bills are very low, you don't have much interest in energy details.
No explanation needed, James; just differences of opinion. It is true that thermal bridging happens more visibly in northern climates, however, it does happen in southern climates for longer periods of time, and is easily justifiable. It is true that most of my designs have been in NM and TX, but CA, AZ & NV are closed behind, and looking at the HERS Ratings of those homes, outsulation makes a big difference in the thermal performance and air sealing, even in SoCal’s mild climate. I’m working on a 3,800 Huntington Beach house, with 1-2ACH50 and 3 tons of AC, with 1" outsulation and 5.5" cellulose, same as Jack's. If all is done well, Jack should have no more than 1-4 ton or 2-2 ton AC units in his house.
In addition to the points made before, Jack must have all HVAC system in the conditioned space, with all metal ducts in dedicated chases bellow the ceiling or a conditioned attic, and sealed with mastic.
I do like your suggestion of PV, with systems at $3 per watt; it’s a no-brainer nowadays. SoCal has pretty high electricity, and its not getting cheaper.
Armando,with all due respect, Huntington Beach is a completely different animal than the west san fernando valley where this house is located. The beach climates of so cal are wonderfully comfortable year round. The west valley sees constant summer temps in the high 90s and many days over 100. We also see high 80s during much of the year. We do have many nice days in the winter in the 70s with probably 20 - 40 or so nights in the 30s. Compared to other parts of the country it is mild. Climate zone 3 is interesting b/c you have such a disparity in temps between areas like the beaches and the valley. I don't think you can make a reasonable comparison between huntington beach and the west san fernando valley.
I don't think there is any way a 4 ton unit will cool this house in the summer heat. There are no houses in the west valley that i know of with such a small ac unit for a house this size. But the lack of air sealing by contractors may be the factor which keeps us putting in a ton+ per 1k sq ft.
James what do you consider good overhangs?
The notion that our energy bills are low is not entirely accurate. In the valley areas our electric bills in the summer are significant from a/c. Natural gas bills in the winter are reasonable.
All that said, when i talk to other contractors about the idea of rigid foam on the exterior, they look at me like I'm from another planet. I am unsure if old habits die hard and it hasn't caught on over here or it provides negligible returns on investment.
It is also standard practice to put hvac and ductwork in vented attics over here. Any suggestion to the contrary gets funny looks.
Thanks for all the feedback
Jack - what's a good overhang? Depends on the height and orientation of the wall it protects. Two feet and up, more on east/west elevations. Deep porches can help a lot on western exposures.
Put ductwork in the attic if you must but make sure it's plenty tight and heap plenty of cellulose on top of it. The air handler should never be in a vented attic for at least two good reasons: you can't enclose and insulate it like you can the ducts and it requires service access which will compromise both insulation and air-sealing. If there's no other option, build an insulated box around it within the attic large enough to accommodate all service work.
Jack,
A house in the valley has more reason to deal with thermal bridging, as a house in Las Vegas, Phoenix or SW Texas do with hot and dry wheather. Delta temperatures between outside and inside are quite high for long periods of time from Spring to Fall; besides, another job for AC units is to remove humidity, and you don’t have much of it, so it should be easier to cool your house.
FYI, I’ve designed many houses in really hot and dry climates at 1,200-1,500 sf/ton; and Allison Bailes, one of our “famous” GBA bloggers is working on a 7,000 sf house in Atlanta that I believe they are using 2,000 sf/ton; and that is a lot of humidity to deal with.
I believe you, as most homeowners and folks in the industry, maybe underestimating how small AC units get when you have a combination of a good building envelope, a good HVAC system design and installation in the conditioned space, and passive solar. Having said that, 1 ton/ 1K sf is not bad, better than most. At the end of the day, I guess is what you are comfortable with.
Martin - it was a rhetorical question.
In a 5000' single-story the wall-R represents a miniscule fraction of the cooling load. And in SoCal the infiltration loads are pretty low too (due to the very low latent loads in that climate), but air sealing still counts.
Using a high SRI cool-roof finish and R50+ cellulose in the attic can take a big chunk off the peak & average loas, as can minimizing the total square footage of WEST facing glass (that isn't easily protected from solar gains by overhangs due to the low sun angle).
Ducts in vented attics are an abomination, even if they're legal (and common.) Under CA Title 24 they would be at least better sealed and insulated than in most of the US, but even if you have to frame a taller wall to build in a service cavity in the ceiling below the attic insulation it's worth it in reduced load and fewer holes in the pressure boundary of your house.