We are designing our one story single family residence in NM. The site elevation is 6950 feet, 5809 HDD, 11 inches annual precipitation. The slab will be insulated and exterior walls will be 6" nominal OSB/PUF/OSB SIPs. All the roof planes will have a 4/12 pitch, hip roof, wood truss construction.  Mechanical heat recovery ventilation will be sized and installed. The space will be heated by radiant floor, high velocity HVAC, or split units.

We are considering these three alternatives for insulating the top of the building envelope:
 
1) sprayed polyurethane to create a "hot" roof.
2) cellulose blown in above the ceiling.
3) capping the walls with PUF SIPs, followed by plates and the truss roof structure.

We have compiled our list of the advantages/disadvantages of these options and now want to tap into the expertise and experience of the folks on this forum.

For example, Option 1 minimizes air infiltration into the structure but means that the space above the ceiling would be conditioned by the HVAC system.

Option 2 can offer high R values but the roof trusses would require very high heels to accomodate the thickness of the insulation unless the wall heights were raised and a "pocket" roof created under the trusses. Carefully sealed vapor barrier and attic ventilation would be required.

Option 3 offers the opportunity for a tighter building envelope, uniform insulation properties and safer/easier construction of the roof structure by working from the flat deck of the SIPs. Building electric and mechanicals may require furring down the ceilings.

We appreciate your technical advice and feedback.

Bob
 

Bob;

we have done a couple of SIP projects with the ceiling panels in the flat and then topped off with a conventional rafter system  http://southernsips.com/job_lechenier.html

I have also done a project with the ceiling SIPS. Makes a very nice platform for working roof trusses. Electrical should be no problem, however, HVAC ducting could be an issue. Since you are slab on grade, either the ducting is above the ceiling in uninsulated attic space or you furr down under the ceiling. Neither would be my 'cup of tea'. I would seriously consider the 'hot' roof option. This gets your mechanicals out of the ceiling, but still in the insulated envelope.

With only a 4/12 pitch, you should definitely consider using SIPS for the roof unless the roof has a tremendous number of different planes that will result in waste.

Chris, Alton and Wes,

Thanks for the excellent feedback. It's great being able to bounce this stuff off others.

We have considered SIPs for the roof since that would get the shell up quickest. Our concern is getting high quality fit where the roof plane forming the hips meets the other two roof planes. We would not want to cut these on site. A SIPs manufacturer would need the capability to supply them ready to assemble.

If we adopt the ceiling SIPs assembly we are considering putting up the interior partition walls first as in conventional stick built construction. We believe the advantage is that we could have smaller SIPs that we could install without a crane (understanding that the SIPs walls would need suitable bracing to push the up the ceiling SIP). We have a concern about being able to properly seal the seams that would fall across or parallel to an interior partition top plate.

Is there a best practice for this?

What is your sense about this approach versus using full span ceiling SIPs which would be a maximum of 24 feet?

Thanks again,

Bob