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Polyiso question

tennesseeken | Posted in Energy Efficiency and Durability on

Hi I’m new to this so bear with me , it’s a question on polyiso . I have 130 sheets of 1 3/4 polyiso , I’m going to use it on my exterior walls on a 14 x70 Mobile home then sheet with zip system . My question is on the roof , it’s a 4/12 and I’m using HD comp roofing with ice and snow barrier. The ceiling will be invented 2×10 . So the cavity will be 8.5 inches with rock wool filling the rafter bays . Can I put the polyiso on top of rafters before sheathing it I don’t want to sandwich it between 2 sheaths due to weight. It is mainly top help with the thermal bridging on roof . Can I do this from the inside and get the same effect. It’s a high ceiling so I wouldn’t loose head space on the inside . Is this still a effective use of the polyiso . Or might it be better used on the underside being it’s a mobile home . I’ve thought about wrapping the whole thing like my yeti cooler . I’m using the proper air and ventilation on the inside for air quality and efficiency. Thanks ken

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  1. Expert Member
    Akos | | #1

    You can't put the rigid directly over the rafters, they are not meant to handle that much contact pressure.

    If you want a compact roof, with the width you are proposing, I would drop the rafters down to minimum size for span/snow load, install T&G as roof deck leaving it exposed on the inside. This saves you the weight/time of finishing interior drywall. When I've done this, I find it is quicker to pre-finish the rafters and the T&G before installing.

    You roof would than be:
    peel and stick air/vapor barrier

    You further save some weight by going with a metal roof over 1x4 on flat and skipping the plywood over the foam.

    1. tennesseeken | | #2

      I’m spanning from one side to the other and it’s 15. .5 feet . So 2x10 is required on this one . I thought about t and g but I’ve got a ton of building materials like my zip sheathing and 2x10 and all this polyiso already . I don’t no if polyiso on the interior directly covering my rafters then doing Sheetrock over that would give the same break I’m wanting . It’s hanging so pressure on the polyiso would be minimal. With 8.5 inches I’m not sure if polyiso is really needed . At r5 a inch on rock wool that’s a r40 already . I’m thinking it might be better used on the under carriage of the mobile . And I’ve already saved 35 bundles of roofing also yup I’m a packrat. So my savings is great on this but I want a ultra efficient mobile when done . Polyiso would bring it to a r50 if I do it before Sheetrock .

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