2×6 studs 24″ on center and spray foam or SIPs construction?
I’m planning a build a very basic 4 corner, single gable house around 1700-2000 sq ft. I’m wanting to have a very efficient home when finished as I am planning this to be a long term home (30+ years).
Originally I was going with a 2″x6″ w/ 24″ OC walls with open cell foam and an unvented roof system. Topped off with 1.5″ polyiso exterior foam board. I can accomplish this with SIPs as well. Going with a 10″ panel is appealing. It appears to simplify the process and cuts down the different subcontractors. I expect both systems to be expensive. Its a long term investment and one I’m not expecting to pay me back in energy savings.
Information on SIPs are all over the spectrum from great to disasters. I’m having a hard time finding recent perspectives from owners and builders not associated with the SIP association.
The advancement of CAD and automated manufacturing of custom panels should have fixed many of the fitment issues of 10 years ago, but I cannot find that perspective.
Any thoughts or personal experiences would be appreciated.
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First, where (what climate zone or postal code) is this house being built? That could be anywhere from more performance than would be necessary to hit Net Zero Energy, or it could be insufficient for dew point control at the sheathing, depending on your climate factors.
A 2x6/R20 24" o.c. wall with 1.5" (R8-9-ish in colder climates) has a whole-wall R of about R25, or a U-factor of about U0.040 affter factoring in the thermal bridging of the framing, the thermal performance of the wallboard, sheathing, & siding, interior & exterior air films, etc. A 10" EPS core SIP will usually outperform that, but actual performance is usually well below the advertised clear-wall R value of the SIP.
An open cell foam + polyiso solution would have a lower environmental impact at equivalent performance due to the lower net amount of polymer used. (The 0.8 x 5.5" of half pound density open cell polyurethane replaces 4" of 1.5lb density polystyrene SIP core from a net whole wall performance point of view)
Thanks for the response. I'm in Phoenix, zone 2b.
Other responses should note that Arizona has a lot of ants and termites. This would have to be addressed. Exterior foam is one issue. Using foam and OSB (SIPs) in a structural capacity in such an environment is another, and may be questionable (IMO).
Also, Arizona has big diurnal temperature swings from day to night, so mass may be useful in this climate...
Termites are an ever present issue. I don't think mass is as effective in AZ as modern insulation. It was best option when modern building materials weren't available. Thats because it can never release its heat in the summer's night to the point it can absorb more. AZ has low temps in the 90s during the summer. High mass walls would be constantly dumping heat into the conditioned space once they are heat soaked.
That's extreme overkill walls for a 2B climate on both of them. Download a copy of BA-1005 and see table 2, p.10:
http://buildingscience.com/file/5806/download?token=GouEIX9Y
Those are the approximate "whole assembly" R-values that would get you to Net Zero Energy with 15% efficiency PV on the roof with 2010 vintage heat pump efficiencies of about HSPF 10 / SEER 16. Typical home PV arrays in 2016 are around 20%, and even ducted mini-splits are hitting the HSPF 11+ / SEER 20 range. With a careful design you could hit net zero with a lower performing wall that that.
Per Table 2 you're looking at a whole-wall R of R15. A 2x6 /R23 rock wool 24" o.c. wall with OSB sheathing, half inch gypsum and vinyl siding comes in at about R17, no exterior foam. With half-pound open cell foam it's R16. If the temperature striping of the studs would bother you, a 2x4/R13 open cell 16" o.c. wall with 1" of exterior foil faced polyiso gets you to R16 too.
But Phoenix is also a climate where thermal mass in the walls can be your friend, and an area where WOOD in your wall can be a termite-banquet highway.
A 2" + 2" minimalist insulated concrete form (ICF) wall with an EIFS or hard stucco finish on the exterior comes in at about R17-R18, but would outperform an R18 whole-wall low mass wall.
An 6-8" concrete block wall with 2" of exterior polyiso with fiber cement siding hung on furring, with half-inch gypsum on interior furring would come in at about R16. That wall would outperform the 2x6/R23 wall despite the higher R-value of the latter.
I suspect it'll be easier to find more contractors capable of the CMU block walls with exterior foam approach than those familiar with SIPs or exterior foam framed construction. They may want to push you toward EPS instead of polyiso, which is fine, but you'd want at least 2" EPS if shooting for Net Zero, and they'll probably push for holding the line at 1-1.5". A CMU wall would meet code min with just 1" of exterior EPS, but you're probably looking for better than code min, even if Net Zero isn't a design goal. Time-wise ICF would be quicker, but materials-wise more expensive, and probably a smaller talent pool of contractors.
Per Table 2 you'd want a whole assembly R of R40 for the unvented roof. A 2x12 24" o.c. raftered roof with the bays completely filled damp-sprayed or dense packed cellulose and 1" of polyiso above the roof deck gets you there. Damp sprayed cellulose contains only borate fire retardents which are toxic to the gut flora of termites required by the host for digesting the wood, making it the preferred fiber insulation (even in walls, if you went with the 2x6 wall approach.)
If you spend the difference in wall costs on better windows, rooftop PV, & modulating heat pumps you'll be ahead of the game.
[edited to add] Phoenix AZ is one of the rare climates where thermal mass in walls actually does trump higher-R. At R30 whole-wall the thermal mass won't much matter, but you don't need anywhere near that performance to high high comfort and high efficiency.
Jeffrey,
I agree with Dana that in hot climates, you don't need a very high R-value in your walls.
Here are links to three articles you might want to read:
Hot-Climate Design
All About Thermal Mass
How to Design a Wall
Thank you Dana for the write up. I really appreciate it. But now I'm all sorts of confused. I live in a new construction now with fiberglass 2 x 6 insulation and 1" exterior EPS and a tradition vented attic (thick cellulose bed) with a tile roof. I have solar/ solar water/ fresh air re-circulation. Solar is small scale producing about $50 a month worth (South facing, but 10 panels of a terrible Echo product). I also have solar screens on all my windows. None-the-less my AC runs 90-100% of the time (2 units), night and day in the summer here (115 highs/ 90 lows). I have $550 a month electric bills for a 2800 sq ft home. Semi-custom home and not cheaply built but also not to standards here in this forum. I attributed my constant AC to heat gain through my exterior envelope. Mainly my roof system. All my AC ducts travel through 160 degree air in my attic space. Even so, once my house reaches 76 degrees in runs constantly to keep it there.
As for termites, its common practice in PHX to have quarterly spraying for all pests. I use a company that concentrates a chemical from chrysanthemum flower as its pesticide. A vast majority of new construction over the last 30 years has been wood frames in the valley.
I was really interested in an ICF home, but I read here and other places that Phoenix is actually bad for thermal mass when you need insulation the most, the summer. 7 month a year you can live in a paper house and survive fine. But since the mass never has a chance to cool below interior temperatures at night it becomes heat soaked and loses all its efficiency from that point forward. Temperatures run from high to low and the interior space is always the low point in the summer. In a town of 5 million where housing is the number 1 economic engine, I've found 1 ICF company in PHX. From the article provided, "However, if the weather doesn’t follow this pattern, the walls stop working well. During the winter, if the weather is uncomfortably cool for 24 hours, the walls never get warm. And during the hottest months of summer, the walls never cool off. During those months, the occupants will probably wish the walls included a little bit of insulation."
Just very confused as to what method to most effective insulate my home. It will be a smaller home (1700-2000 sq ft) and I don't mind spending a significant amount more for a high performing product. My roof will be unvented in the sense that the sheathing is encapsulated but will be using a metal, reflective roof on vented furrings. I'm trying to maximize my retirement budget a couple decades away when this project is paid off. Keeping the monthly bill as low as I can helps. Also making the home capable of running off future solar is the plan.
If these were your goals what building techniques would you use? ICF, traditional framing with advanced air sealing, cave dwelling?
OK. I'm revisiting ICF's and it appears that I missed their insulating potential. Newer forms are able to have up to 8 inches of EPS foam to the exterior (and interior). That solves the thermal mass issue. Good thing I have 18 months of planning to get my small mind around all the decisions to be made.
I strongly urge you to consider insect-proof wall construction, given your location. It's a nightmare waiting to happen in heavy termite land. If it were me, I'd spec block walls with 3" of foil-faced rigid mineral wool or polyiso (acceptable if all exposed foam is covered with foil tape) with an air gap to make use of that radiant barrier on the outside. This wall will be amazing, and will put the mass of the blocks where it is actually useful for you--inside, protected by insulation. People are right that mass walls can become sun-soaked and radiate heat indoors after the sun goes down, but only it they are absurdly poorly-designed, with no insulation on the outside, no shading, and no nighttime ventilation. In Phoenix, shading is perhaps more important than wall insulation. Walls with shading, exterior insulation, and interior mass are exactly what you need.
Slab-on-grade with perimeter insulation (more mineral wool) but no under-slab insulation is also the perfect solution for your climate. Tile everywhere.
It's good that you're planning on an unvented roof with foil-faced polyiso insulation at the roofline (again, tape the exposed seams with more foil tape). That's a good choice. Install a white metal roof mounted on purlins for ventilation.
I live in neighboring New Mexico and can yammer on for hours about the effects of mass vs insulation through various seasons and periods of time and if it won't bore you to tears.
Thanks Nate. I'm going to have to look into mass wall systems further. Its seems exterior insulation will do the trick. I plan on 36" overhangs on the East, West, and North walls with a 8' covered porch on the South.
Desert trees aren't the best at providing shade, but I'll have a few of those as well.
OK, if I do go with a mass wall system like ICF's would SIPs on glulam beams be a good roofing system or would truss with an unvented system be preferred? It seems like ICF walls and SIPs roofs were made for each other.
There are plenty of great desert shade trees! Some that would be great in Phoenix are Mesquite, many Oaks (Live, Gray, Texas Red), Afghan/Eldarica/Mondell pine, Acacias, and Palo Verdes.
In a super hot climate I prefer an unvented roof, with insulation at the roofline and ventilation on the outside, between the roofing and the roof structure. In a typical vented attic, you run into "hot attic" problems that are endlessly controversial and difficult to resolve, and there are a lot of snake oil products out there purporting to fix them. SIPs are indeed a good match, but they need to be installed right! You can get metal SIPs that have tongue-and-groove joints that are easier to seal and more insect-proof.
Even better than ICFs would be pre-cast concrete panels/blocks that have foam insulation in the middle, sandwiched between concrete. These are great for your climate; the outer mass layer absorbs the sun and oscillates between the daily temperature extremes, while the insulation in the center keeps those extreme temperatures from the inner mass side that keeps you nice and cool. These products also simplify finishing on both sides.
Thanks again Nate. I'll have to check out the precast panels. And Im right with you when it comes to unvented roofs. My current roof/ attic gets up to 160 degrees in the summer and my ducting runs right through it. Makes no sense whatsoever.
I do like the afghan pines but after growing up in Missouri I have a different view of what a good shade tree is.