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Rockwool Toprock instead of polyiso for low-slope roof insulation?

christophd | Posted in Green Products and Materials on


We are doing renovations on one-story our house in the East Bay of the San Francisco Bay Area. This question pertains to the roof. Specifically, can Rockwool Toprock DD be used instead of rigid foam insulation on the exterior of the roof?


The roof is a low-slope roof (about 3:12) with an unfinished attic underneath most of it and a portion of exposed rafters over the living room. The unfinished attic has fiberglass batt insulation over the ceiling. The roof is an asphalt shingle roof at the end of its useful life.

The lack of roof insulation leads to the living room heating up quite a bit even with just moderate sunlight. The prior owners had installed a house AC system to cool the air but we’d rather avoid it. We love the exposed rafters on the living room ceiling so adding interior insulation there is not an option for us. Since the roof needs to be replaced anyways, we are looking to add exterior insulation. Our roofing contractor suggested that adding exterior insulation over just the living room would not work, so they want to cover the entire roof with it. We are comfortable with this approach as it would also reduce the heat in the unfinished attic, allowing us to use that space for storage.

The roofer recommended 2″ of rigid foam insulation (with another 1/2 in sheathing on top into which the shingles will be nailed). We are concerned about the embedded carbon as well as flame retardant use in such insulation (EPS, XPS, Polyiso all have such issues according to our research). We identified Rockwool Toprock DD as a potential replacement with similar R value as rigid foam. However, we have not found a lot of documentation as to using Toprock as a replacement for rigid foam.

Question: Can Rockwool Toprock DD be used as a replacement for rigid foam in this low-slope roof insulation application?

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  1. Expert Member
    Akos | | #1

    You will have a hard time finding a residential contractor to install rigid mineral wool on a roof. Possible but a bit of a PIA to put plywood over it as it is still slightly squishy.

    Your roofers suggestion of 2" of polyiso is the way to go. This insulation is outside your house's air barrier, fire retardants from it have no way of getting in.

    If you want to avoid foam and keep the budget reasonable, would be simpler for the roofer to install 2x4 purlins on top of the roof with regular mineral wool insulation between. It would give you a much higher R value assembly than 2" of toprock DD and be way cheaper.

    No matter the choice, also make sure that there is a decent air barrier bellow the insulation. If you existing roof deck is plywood, have the seams taped or if deck boards or T&G, cover the whole roof with a peal and stick membrane.

  2. Peter Yost | | #2

    Well, I simply googled Rockwool Toprock DD and determined that it is designed specifically for low-slope roofs. At 3:12, you are technically not low-slope but pretty bloody close and for all intents and purposes for your project, the answer is yes.

    Just be very sure that you include in your new roof assembly a continuous air control layer, which you can't get from rigid mineral wool board but you could with other rigid foam boards (by simply taping the joints).


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