I’m currently struggling to explain why I want SIPs for my roof instead of trusses
mooney231lv
| Posted in Green Building Techniques on
My designer and I are designing a house as close to passivhaus standards as we can get. We have agreed to design it with SIPs but he thinks a truss roof with spray insulation would work just as well. I’m thinking a SIP roof eliminates potential for thermal bridging and is less costly to install versus the truss roof.
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Replies
Scott,
How about a roof framed with raised-heel trusses and deep cellulose insulation on the attic floor?
+1 on the deep celluose approach.
Foam insulation (any type) is a lot less green than cellulose, and at high R that is even more so. Foam is more expensive too, EPS and open cell foam are 3-4x more expensive than open blown cellulose, closed cell foam is more than 5x the cost. I've never seen a SIP come in at lower cost at high-R than a truss + deep cellulose solution.
With raised heel trusses you have very little thermal bridging, probably less than that of a SIP (which of-necessity has timber splines for structure. SIP roofs that have a ridges &/or valleys (as opposed to a monolithic shed roof) have both thermal bridging and high air leak potential at the seams too. Rot issues with this sort of pattering are unfortunately not rare:
http://www.buildingscience.com/documents/insights/content/bsi-036-complex-three-dimensional-air-flow-networks/images/bsi-036_figure_03.jpg
Seeing that the choices being discussed included SIPs, I'm wondering if the ceilings are sloped?
One issue with raised heel truss + cellulose versus SIP roof is that unless you have a shallow pitched roof you lose usable space in that attic. With the attic being part of the living space you can build a smaller house than you could with a raised heel truss.
SIPs vs Trusses:
1 - SIPs are stronger than a stick frame truss roof. No comparison. Ask a qualified engineer and they will fill you in on the details.
2 - Polyurethane SIPs are more fire resistant than a truss roof.
3 - Design: If your going for an open design with vaulted ceiling, SIPs are the way to go.
4 - Instant 24" overhangs & soffits with SIPs, just lay it over the wall, screw it down, your done.. Stick frame requires framing out a soffit overhang with 2x's, sheathing it, venting it, insulating it.
With SIPs you have to be extremely diligent in sealing the panel joints. T&G panel connections with gaskets, then using SIGA or SIP approved panel tape at the joints.
Trusses and cellulose have their place as does SIPs. Each has its pros and cons. SIPs appeal more if your design calls for vaulted ceilings and open ceiling designs with exposed beams.
Guy's,
SIPS may be self supporting but, it's far from fire resistant. It's disposable construction techniques. Conventional framing with dense pack cellulose is far more fire resistant than any modern methods of construction.
http://www.foamrun.com/wp-content/uploads/2009/10/foambook_ch5-fire_and_foam.pdf
http://www.greenmaltese.com/2014/08/13/looking-at-modern-construction-features-in-honor-of-lt-arnie-wolff/
http://www.firerescue1.com/fire-attack/articles/1041439-Fires-on-steroids-Dangers-of-disposable-structures/
This is a spray foam fire caused during installation.. http://www.rescue9photography.com/Fires-2014/Middlebury-structure-fire-7-3/
I'll second everything Peter L said plus:
For a shed roof, 12" EPS SIPs can be entirely self-supporting. Definitely easy to detail and air seal compared to attic construction.
R-52 is the highest I've seen for a SIP, which is just a bit higher than code minimum for zones 4-8. One way to bump that up is with a sprayed polyurethane foam roof. I like them for low slope roofs because they can live forever with good maintenance, but they are risky in hail-prone regions.
Richard,
You couldn't be more wrong if you tried (which apparently you did). "Disposable construction techniques" is lightweight dimensional wood (2x4's, wood floor trusses and roof trusses), also known as the standard residential home being built today. The articles you linked had NOTHING TO DO with SIPs. Absolutely nothing!!! Of the two articles you referenced they talked about spray foam and how it was incorrectly applied to a truss roof and the chemical reaction made it combustible as it cured. A SIP (polyurethane -factory applied) is not the same thing.
If you think polyurethane foam (cured) in a SIP is a fire danger than you need to brush up on your building science and stop throwing red herrings into the discussion.
The other two articles you referenced talked about how weak roof trusses are (which I agree). How weak dimensional lumber is in a fire (I agree) but nothing in those articles about SIPs. One article mentioned PU adhesive being a bad choice since mechanical attachments (screws) weren't used.
Best check (aka - read) your references before you post them.
So your comments that a polyurethane roof SIP is not fire resistant is a lie because they are rated as a Class A and Class 1 fire resistant material, the highest rating for a roof material. The other misinformation you gave is that SIPs are "disposable construction techniques", actually the opposite is true. You stated, "Conventional framing with dense pack cellulose is far more fire resistant than any modern methods of construction." and that is also a far-fetched statement. The articles you referenced actually stated the opposite and stated that wood trusses & conventional framing is "disposable construction" and poses a fire risk to firefighters. The complete opposite of what you stated.
Not to mention, ICF, SCIPs, AAC, etc, is far more fire resistant than modern conventional wood framing. You can get up to a 4 hour fire rating with ICF. Conventional residential framing will get you 30 minutes.
The bigger fire threat is from the contents. So from a fire standpoint it doesnt matter. If you want fire protection install sprinklers.
Polyurethane SIPs are something of an environmental disaster, until and unless they begin manufacturing them using low-global warming potential blowing agents such as HFO1234yf (at about 4x CO2 GWP) rather than the ubiquitous HFC245fa (at about 1000x CO2.)
In high-R assemblies the net lifecycle climate damage of using closed cell polyurethane as the primary insulation would be far higher than the CO2 it is offsetting by lower energy use.
Open cell polyurethane is blown with water, with very low GWP, but isn't structural the way closed cell polyurethane or EPS is, and has moderately high vapor permeance that needs to be addressed in the stackup (as does the low vapor permeance of closed cell polyurethane.)
EPS is blown with pentane, at about 7x CO2 GWP, making it much more climate-benign than closed cell polyurethane, but not as benign as cellulose.
From a fire hazard point of view polyurethane has a higher kindling temperature and chars in place rather than melting & dripping flaming liquids the way polystyrene does, but is pretty far from the fire-safety of rock wool, and not particularly safer than even low-density cellulose. The fire retardents used in polyurethane or polystyrene aren't nearly as benign from a human health point of view as the borates used in cellulose either.
Thank you for clarifying the point Dana. Peter may I suggest you review the links objectively before countering a comment as a lie. Is Larry Janesky's (founder of Dr. Energy Saver) home fire a lie to? Clearly it's more than what's in the house as it is very much what a house is insulated with as his fire illustrated in the link posted above. Had his home been insulated with cellulose, fiberglass or mineral wool the home would still be standing today. Unfortunately for him it's not. Just my opinion...