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Is it more cost-effective to insulate a roof plane or use SIPs?

Pascalli2 | Posted in Green Building Techniques on


I am planning to condition our attic and use it as a bonus office/studio space in the house I am designing. The house will be a basic 44×30 rectangle with a full gable roof. Despite the known issues with skylights, we will probably install a couple of skylights on the north side of the roof to bring light into the space. I am building in Zone 6, and would like to have an R-Value of at least 45-50. I do not want to use spray-foam and would need to figure out a solution that involves some combination of rigid foam (probably exterior) and cellulose or rock wool insulation of one sort or another.

What I am trying to figure out, is whether it would be more cost-effective to use 12″ SIP panels for the roof or to insulate the roof planes. It is of note that I will not be doing the work myself. There is also a possibility that the whole thing either way would be too expensive and we will just have to settle for a non-conditioned attic (with no vents or pipes running in it).

Can anybody chime in on a) whether they think SIPs or Insulation would be more cost effective and b) Any geusstimates on cost of having a conditioned attic vs. unconditioned ($5,000, $10,000, $20,000, etc).

One final thought – the SIP dealer mentioned that SIPs won’t require as much truss support as a standard roof, which might reduce the cost a bit, as well, though floor joists would be required at that point.

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  1. GBA Editor
    Martin Holladay | | #1

    You wrote, "I will not be doing the work myself." Since that is the case, the obvious person to whom you should be addressing your question is your builder.

    SIP costs vary, depending especially on the distance between the SIP assembly plant and your job site. Labor costs also vary. Because of these variations, there is no single answer to your question that applies to all of North America.

    In general, though, I think that a combination of some rigid foam above the roof sheathing, supplemented by fluffy insulation between the rafters, usually costs less than SIPs.

    For more information on your options, see How to Build an Insulated Cathedral Ceiling.

  2. jackofalltrades777 | | #2

    Skylights and SIPs don't mix. You will have moisture problems if you do SIPs and install numerous skylights. By default you have to do roof trusses. Get rid of the skylights and SIPs can be factored back in.

    At R-45 for your roof you will NOT meet code for your area unless you have some exemption. You are Zone6 and R-45 is pretty low. R-49 is requires in Zone4 and I went with R-60. You are Zone 6 and you should be looking to do around R-60 or better.

    May I ask why do you want skylights? They are just R2 holes in your roof and create huge nightmare problems with moisture control. Plus you are in Zone 6 and those skylights will be drawing that warm conditioned air right out of your home and out the literal skylight window. Your energy bills will be bad during winter and the efficiency of the home will suffer for the next 50-100 years or however long the home will be standing.

    Design it correct from the start: Dump the skylights and go for a minimum R-60 roof.

  3. fitchplate | | #3

    Dump the SIPS, they require a committed, experience builder who can value and understand the technology and time to properly air-seal the panels and its difficult to make the insulation of soffit and eave assemblies work in cold climates. Its rarely done right. Go with dense packed cellulose. Exceed 60 R easily with rafters made of wood-I's or truss beams of 16".

  4. ohioandy | | #4

    I have just built a house with a SIP roof. It's an option with some enticing advantages, but Flitch Plate is correct: you need the services of an experienced builder and engineer to do this right. I submit, though, that for a conditioned attic you need these same professionals whether you build with SIPs or I-joists or trusses.

    Skylights are always a bad idea, but if you insist on penetrations the engineer will specify some LVLs inserted in the SIP on either side. I had three small dormers done this way. [Yeah, it's an air-sealing train wreck.] Beyond that, the SIPs are the structure, and and my roof has no other support--not even a ridge beam. The structural capacity of a SIP increases with thickness, so roof spans can be impressive..

    But my gut tells me that Martin's also right: it's likely that conventional methods can achieve the same result at less cost. I still encourage you to consider SIPs, if you can find an experienced and reputable contractor.

  5. Pascalli2 | | #5

    Thanks everyone - this is some good food for thought. I know the troubles with skylights, but if we are going to be using a room up there, it would be really nice to have the light. It sounds like Flitch might be on to the best route, cost-wise. I don't know how much it would cost doing it that way, but the local SIP dealer quoted about 25-27,000 for SIPs, material only, and 12" EPS can only get up to about R-45, I believe.

    I might have to do the work and spec out the cost of an insulated roof with a bit of rigid foam on the outside and dense pack cellulose on the inside if I want to make an informed decision here.

  6. jackofalltrades777 | | #6


    There are pros/cons to each and every building material. If you go with a 8" polyurethane core SIP you will end up with R-50. Plus it is a Class A and Class 1 roof assembly, if fire prevention matters for you. It does out here. A vented roof/attic/soffit in my area is an invitation to have your house burn down in the next wildfire.

    Give ThermoCore SIPs a call out in Indiana and get them to bid you out a SIP roof. The price includes the actual install by their trained and professional SIP crew. You will know it's done right as they have been installing SIP roofs for years.

    It's worth the few minutes to get a bid and if you decide on stick frame trusses, at least you made an informed decision. Know all the alternatives, the pros/cons of each, and make an educated choice.

  7. Pascalli2 | | #7

    Hi Peter,

    I did look into urethane SIPs, actually, and was quite excited about the idea. I had to give up on them, though, for a couple of reasons. 1) the cost of shipping to British Columbia was prohibitive, as the closest manufacturer is in Colorado (US), and the only Canadian manufacturers I found were on the East coast and 2) there is some question as to R-Value dropping over time, though I believe the EPS SIP manufacturers like to exaggerate this.

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