I'm doing cost analysis for a future home. I shall use 6" SIP panels for the envelope. The house is rectangular and divided equally into 3rds. The 2 outside sections will have a ceiling but the center area will have a cathedral ceiling. The cost to increase the R value to R45+ in the ceiling using 8" SIP roof panels is $8,400. So to reduce the cost of a thicker SIP roof, I thought about putting fiberglass bats in the ceilinged portion of the house, and adding 2" of rigid insulation to the cathedral ceiling area and installing T&G planking over the rigid insulation. The walls that define the cathedral ceiling room go completely to the roof, and will be well insulated. My question is, what am I missing? Any input is appreciated. TY Dan

Posted By DAND on 24 Nov 2013 11:28 AM
I'm doing cost analysis for a future home. I shall use 6" SIP panels for the envelope. The house is rectangular and divided equally into 3rds. The 2 outside sections will have a ceiling but the center area will have a cathedral ceiling. The cost to increase the R value to R45+ in the ceiling using 8" SIP roof panels is $8,400. So to reduce the cost of a thicker SIP roof, I thought about putting fiberglass bats in the ceilinged portion of the house, and adding 2" of rigid insulation to the cathedral ceiling area and installing T&G planking over the rigid insulation. The walls that define the cathedral ceiling room go completely to the roof, and will be well insulated. My question is, what am I missing? Any input is appreciated. TY Dan

Are sound STC ratings important to you? A SIP has a STC rating of STC 28 in and of itself. Rigid insulation will help R-Value but will NOT help with sound control.

One way to increase STC ratings and increase R-Value is to install resilient sound isolation clips with hat channels. They decouple the SIP assembly which helps DRASTICALLY with sound control. 24" oc with the furring channels and 48" between IsoMax clips. Install the clips and 22 gauge furring channels (1.5") and this will give you 2" of space between SIP and drywall. As the IsoMax sound clip is 1"  but the furring channel sits 1/2" inside of it, therefore the 2" total depth.

Then instead of rigid insulation, install Roxul Mineral Wool Batts - At 2" they have an R-Value of R-8 and the STC ratings are a lot better than rigid insulation. Being 24" wide they will fit perfectly between the furred hat channels. They also provide excellent fireproofing to 2,000F and perform a lot better than fiberglass batts in terms of sound control and R-Value.

With the sound clips and mineral wool, and then finally 5/8" drywall or T&G planking. You should be around STC 40-45.

If you live in a noisy area, I highly recommend decoupling the SIP wall and ceilings. It depends on the area and the occupants of the home. Some would find a STC 28 rating of a SIP to be very noisy inside of the home while others might not notice much. For comparison, a typical wood framed home wall has an STC rating of STC 37 and an ICF wall has a STC of 51. The higher the number, the better sound control.

I'm a little surprised that you are doing a cost analysis and have decided on SIPS which, although they work well, are very expensive to other, simpler technologies and mehods to achieve the same goal.

I strongly advise against using fiberglass; the air movement within the cavities can short circuit your R value. Use dense pack cellulose instead. One way to increase the R value with SIPS is to build a framed roof insulated with cellulose and installing the SIPS on top of the framed assembly. Another way is to cap the SIPS with nail base insulation, or polyiso sheets since you don't need a 2nd layer of sheathing. A third way is to install polyiso sheets under the SIPS, strap though the foam and fasten your V groove to the strapping.

Be sure that your V groove is completely on the interior and does not go through the wall cavity to the exterior. I've seen lots of older installations where the V groove penatrates the walls and carries fresh air in and warm air out via the V's.

Posted By Lbear on 24 Nov 2013 03:48 PM

Posted By Bob I on 24 Nov 2013 01:46 PM
I'm a little surprised that you are doing a cost analysis and have decided on SIPS which, although they work well, are very expensive to other, simpler technologies and mehods to achieve the same goal.

It all depends on the design, especially the roof area. Some designs one cannot use a wood truss roof and it has to be stick framed which becomes labor intensive $$$. The other problem is that it can be very difficult to add insulation in a tight cathedral ceiling. This is where SIPS work great. SIPS will be easier, less expensive, insulate better, and are overall a better choice with a vaulted roof structure. It always comes down to the design. Some designs work better with SIPS while others don't. Vaulted ceilings and SIPS are a perfect match.

With a SIP you get the structural part (OSB/Steel vs. wood truss), the sheathing part (OSB/Steel vs OSB), the insulation part (EPS vs. cellulose, spray foam, etc), the vapor barrier part (EPS), the soffit/overhang, all in one package. A good crew can install a SIP roof in 1 or 2 days and have a completed roof structure ready for drywall and roof finishing. Good luck getting that with a wood truss roof design in 2 days.

So while the SIP may seem like it costs more for the SIP itself, it can be less expensive than doing it the "old fashioned way." Personally, I priced it out and a steel SIP roof is a lot less expensive than doing it the traditional way.

Steel SIP:

• Sheathing
• Insulation (R-50)
  
• Vapor Barrier
• Structural Member
• 24" Finished Soffit/overhang
• 100% Airtight
• One crew installs it in a course of 2 days and the roof is done, ready for drywall and shingles/metal roof.
• No crane needed (3-4 guys can lift a 25' long by 3' wide panel)
• No roof/attic venting required
  

Traditional Roof:

• Wood Trusses
  
• Need to rent construction crane to install trusses
  
• Takes numerous days to install roof trusses
• Another crew comes to install OSB roof sheathing, more labor $$ and time $$
  
• Have to call in a spray foam or insulation company to come in and insulate the attic/roof area
  
• Not completely airtight, must be detailed very carefully to prevent air leaks/moisture
• Must call in another crew to frame out and install a 24" soffit area, which then must be vented, more parts & labor $$$
• The roof requires attic venting, so more penetrations and detailing required

I was under the impression that you generally need a structural ridge and horizontal structural support about every 10', although that will vary. That much structure can be excpensive and rime consuming. Do steel skinned SIPS have very different requirements?

Thanks everyone,
What about 2-3 inches of closed cell foam sprayed directly to the warm side of the SIP? (for the parts of house that will have a ceiling)
I'll be in the country so sound is not an issue.

Posted By Bob I on 25 Nov 2013 06:46 AM
I was under the impression that you generally need a structural ridge and horizontal structural support about every 10', although that will vary. That much structure can be excpensive and rime consuming. Do steel skinned SIPS have very different requirements?

Yes, a structural ridge beam is needed for a SIP vaulted ceiling. A steel SIP has a lot less "dead weight" than an OSB SIP. A steel SIP can span 25+ feet without support but that would NOT be possible with a wood SIP. A steel SIP does NOT require horizontal support every 10 feet. You can span 25 feet and only attach one end to a ridge beam and the other end can sit on the wall. A ridge beam is not that expensive and provides an aesthetic element to it which works in a vaulted ceiling design. People install "fake" ridge beams and while that is appalling to an architect, a real ridge beam would be both functional and aesthetic.

Steel SIPs offer other benefits that wood SIPs don't. They are lighter, stronger, termite proof, fire resistant, rot resistant and can span greater distances without additional support.

LBear you forgot, no splines! Steel SIPs have many advantages over the OSB type. Unfortunately when people think of SIPs they only think of OSB with all that supporting lumber.

I recently read somewhere that a steel roof sip needs to be cut (broken) at the wall to prevent cold migration from outside to inside. Does this not cause a structural problem for the roof sip?
Dan

Posted By DAND on 14 Dec 2013 05:43 PM
I recently read somewhere that a steel roof sip needs to be cut (broken) at the wall to prevent cold migration from outside to inside. Does this not cause a structural problem for the roof sip?
Dan

In certain climates, it can be benefical to cut only the bottom skin over the wall.  Cutting both top and bottom skins could greatly weaken the SIP.

Posted By DAND on 14 Dec 2013 05:43 PM
I recently read somewhere that a steel roof sip needs to be cut (broken) at the wall to prevent cold migration from outside to inside. Does this not cause a structural problem for the roof sip?
Dan

This would only be beneficial in a zone 5 or higher climate and like Alton mentioned, only cutting the bottom portion of the metal to create a thermal break. You do not want to cut anymore than that because it would weaken the SIP.

If there would be any thermal bridging on a steel SIP, it would come from the soffit overhang of the SIP on the bottom portion. Even so, we are talking minimal here because of the thin gauge metal (26 ga). A steel SIP is thermally broken on the ends since the ends are exposed EPS.