Hi, all. I'm considering SIPs for the roof and front wall of my new house, but my home designer is recommending stick construction instead. I'd like to describe the design (it's earth-sheltered and passive solar) and ask if you think SIPs would be a good choice, please. The house is a 2000 sq ft bungalow (including the garage), a trapezoid with the front width = 68', the back width = 52', the right wall (at 90 degrees) = 34' long, and the left wall (at 64 degrees) = 37' long. The roof is a single plane cathedral with 1/12 slope, 10'-6" at the front dropping to 7'-8" at the back. The house is like a walk-out basement -- it's slab on grade with poured-cement side and rear walls, with high-drainage soil bermed right up to the roof on the side and rear walls. The roof is a green one, continuing the berm up onto 2-4" of soil (over Bituthene, insulation, root barrier etc.). The front wall faces south with all the doors and windows (except one door and window on the right side, with retaining walls cut into the berm). It's a cold climate, central Ontario, much like Minnesota, so I'm aiming for an R40 front wall and R60 on the roof. Here are the SIP questions. Q1:are SIPs a good choice for the front wall, given that it's got windows and doors in a high percentage of its area? Q2: would SIPs require a much larger wall width between windows than the 4 1/2" that stick framing requires? Q3: I'm thinking of three big window holes, each 5-6' high and 10' wide; does that pose lintel problems supporting the roof for SIPs? Q4: would it be a better SIP fit to switch to many smaller window holes instead of three big ones? Q5: similarly, would SIPs be a good choice for the roof? It'll likely require holding a load of 80-100 psi (2"-4" wet soil + snowload). My designer feels the thermal bridging problems at the panel joints would be much worse than standard sheathing over engineered rafter-trusses with ship-lapped blueboard on top. Q6: And I've got one central kitchen/living/dining room, 30'w x 20' deep -- can that be spanned with engineered rafter-trusses under SIPs? On a different SIP issue, Q7: the front interior walls are concrete for thermal mass, but are there benefits of using SIPs instead of studs/drywall for the other interior walls? Thanks for your thoughts, ...Terry

Terry;
you are asking a SIP forum if we think they are better and stonger than stick frame, with less thermal bridging ? YES

Thanks, cmkavala, for that brief answer to my questions 1 & 5. Anyone willing to tackle the other questions, perhaps with more justification included? ...Terry

OK, I have no axe to grind, since I'm just a happy SIP customer not a builder, manufacturer or seller.  I'm not an architect or an SE either so these are just my opinions.

Q1 Yes. You don't get as much benefit from the SIPs but you should still have a lower cost build and better insulation.
Q2 Lost track of the design.  Probably (depends on the load).  You can also put the same jack/king there you ever would and just fill under the windows with SIP panels.
Q3/Q4 You're going to have some serious headers and supporting studs on 10' windows.  Many small windows would be way more cost effective in either structure.  The windows are lower cost and you'll spend much less on LVL's.  We have one 8' window with about $200 of LVLs.  The 6' patio door has 3 2x10's  - about $20 worth.  The design is about the same in either case.
Q5 Yes.  It's a better roof but will cost you more than trusses.  It's worth it if you want high, vaulted ceilings.  If you want attic space to store your junk, it might not be.
Q6 Not really my area of expertise.  The SIP guys will let you know if you can clear-span 20ft or whether you need additional supports.  One problem with big SIP roofs is that you may need some heavy equipment, crane, skyjack, something, to get the panels into place.

Unrelated.  I've been watching my neighbor down the street put up a SIP extension.  His reason to use SIP had nothing to do with energy efficiency or similar, local builder is familiar with SIPs and so the cost to build is about the same as stick and he wanted to get it finished before the spring.

Thanks, stonecaveman, for your detailed reply -- good food for thought! ...Terry

I missed Q7.  I can see no reason for using SIP as interior walls.  There are some sound isolation properties, but whether SIP are considered good sound isolation or bad seems to depend on the individual, which suggests to me that it's frequency dependent and that some people are more sensitive to the frequencies that are not as well blocked by SIP.  Or that it depends on the nature of the outside sounds (traffic vs. kids playground?).

In any event, I think that sound isolation could be better achieved by other means and that if you just want more thermal mass then SIPs are not good.  If you're looking to (heat) insulate one interior space from another conditioned, I still think you'd do better with something else given that the quality of insulation is not so important when the temperature differences are low.  So to keep the bedrooms at 62 and the living room at 70 I don't think you need SIPs.

Terry.

In response.

Q1. Yes, Sips would outperform any stick system, regardless of windows/doors.
Q2. No,
Q3. No different than stick requirement.
Q4. Doesn't matter
Q5. Yes, a great choice if considering cathedral roof. There should be no thermal bridging. Panel joints are also SIP based.
Q6. Yes, done all the time.
Q7. Yes, common product for 1 sided gypsum SIP.


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Terrynew,

I’ve read your initial post several times, trying to get a good feel about what you’re trying to accomplish, and I’m seeing some problems. First, say your front wall is 60% windows and entry door. Now, if the walls are R-40, and the windows are R-4, that gives you an average R value of only about R-18 for the whole front wall. On top of that, if your windows are only 4-1/2” apart, I believe that you will have to stick build between them to support a header system to support the roof. If your front wall faces south, you can receive a fair amount of solar heat during the day, but lose a significant amount of heat during the night in the winter months. You could add some insulated window treatments to help control heat loss at night. You would probably need them for the summer to keep from over heating. Another suggestion would be to put 2” of blue board on the outside and inside of your concrete walls. Also, put blue board under the slab. If the ground temperature has an average of 55 degrees, who needs cold walls and floor. I understand that living in a basically underground house, bringing in light is a priority. You might consider smaller front windows and adding tubular skylights. These could bring sunlight to every room in the house. See http://www.nltubular.com/ I would definitely consider SIP’s for the roof. Good luck with your build, and keep us informed of your decisions and outcome.

Steve
GrandCountySIPs.com

http://www.guarchitecture.com/berm_house.htm

Sounds sort of similar to what you are describing?

Thanks, Steve, for that reply full of good ideas. You're absolutely right about the front wall's net insulation being pulled down by the windows. I'll likely be going with Fibertec triple-panes with R5 and SHGC=0.51. We'll use heavily-insulated roller blinds or shutters for nights, and the roof overhang will be calculated to shade the window in summer based on the sill height, the roof height, and the latitude (see a useful online calculator at http://www.susdesign.com/overhang/index.php). . I agree with you that a double 2x6 on either side of the picture window (between the picture window and each casement) is the best for lintel and high-load roof support. SIPs would still be a good choice for the rest of the front wall. . The earth-bermed walls on the side and rear will have 6" of blueboard on the outside (with a waterproofing membrane against the concrete), but none on the inside so the concrete can absorb the sun's heat by day and release it by night. . Right, I also want 6" of blueboard below the slab, including high-density ones under the footing for a complete heat wrap. . You're also right on the tubular skylights. The back of the house mostly has non-living rooms (storage, utility rm, laundry etc), but I'll use a couple of 10" dia. 'sun tunnels'. I'll keep the large front windows because they're my primary heat source, combined with the exposed concrete floors and walls inside as thermal mass that prevent daytime overheating and release the heat at night. . Thanks again, ...Terry

Good find, Como! That's quite similar to my plans, without the back upper entrance. I'll have the side and rear berms go right up to the roof line, and 4" of soil on the roof so that from the back the whole thing will look like a hill with a few vent stack protrusions! ... Thanks, ...Terry

I had this plan in the back of my mind for a future project.

From what I have read so far I was favouring ICF's. Your roof loading will be a lot higher. Maybe Steel I beams?

I have another small building to put up, SIPS for that one.