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Tiny attic / flat roof insulation options

Dave B | Posted in General Questions on

Hi GBA – I’m seeking some advice on best options for insulating my small 1100 sq ft single level home.   We have a modified bitumen flat roof that is over 30 years old.  It has a parapet wall with a height varying between 2-8″ (most places are 3-6″) depending on its distance to one of two side house drains.

We would like to have the new roof be SPF.   I’ve read that the old roof does not need to be removed, and I could consider polyiso or SIP as an underlayer to the SPF for even better R-value.   But I’m not sure that would work with my parapet wall height unless I build a taller parapet or somehow make arrangements for rain water to continue towards the gutter, and not over the side of the house onto neighboring property.

The “attic” space is about 20 inches in height with 2×6 joists 16 OC above the ceiling, and 2×6 32 OC below the roof – both running in the same direction.   There is existing blown-in cellulose that does not cover the ceiling joists, so I’d guess 3-4″ deep.
It’s a 1955 home in the SF Bay Area.  If we add an SPF roof, what should we do regarding the attic insulation?

– Leave the existing 4″ blown-in.
– Add additional blown-in on top of the existing.
– Remove existing blown-in and leave attic without insulation.
– Remove existing blown-in and add spray foam insulation to the underside of the roof.

The first option is of course the easiest/cheapest, but I don’t know what happens if I have 2 distinct building envelopes

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Replies

  1. Tim R | | #1

    You should strip the old roofing so you can repair any damage in the structure from termites & rot.
    What is SPF?

  2. GBA Editor
    Martin Holladay | | #2

    Dave,
    I'm assuming that the answer to Tim's question is, "SPF stands for spray polyurethane foam."

    Here is a link to an article about the use of spray polyurethane foam as roofing: "Roofing With Foam."

    You probably also want to read this article: "Insulating Low-Slope Residential Roofs."

    Note that if you install insulation above the existing roof sheathing -- either rigid foam or spray foam -- you'll need to (a) insulate the walls of your short attic, and (b) come up with a detail to ensure insulation continuity and air barrier continuity under your parapets.

  3. Dave B | | #3

    Martin,

    Correct about SPF. As you know, it's a common covering for flat/low-slope roofs.

    This recent article "Sandwiching roof sheathing" and the ensuing comments + rabbit holes has been very helpful. I'll need to re-read a few times to ingest all the info. https://www.greenbuildingadvisor.com/article/sandwiching-roof-sheathing-two-impermeable-layers

    It seems that in general, I should not be mixing attic-floor insulation with exterior roof insulation - which creates two distinct thermal envelopes. Except in the case of "You can install a more moderate thickness of closed-cell spray polyurethane foam on the underside of the roof sheathing, supplemented by a layer of air-permeable insulation below that."

    More than anything, I'm okay if this ends up being less than optimal (considering price/benefit options) - but I don't want to do it wrong (mold, etc)

  4. Expert Member
    Peter Yost | | #4

    As you work through this issue with the roof, keep in mind how you might connect the air sealing you accomplish with the roof to air sealing in exterior walls.

    Also, most parapets are neither completely "inside" or "outside" the thermal and air control layers; they are "confused" elements of the building. As you redo your roof, either pull the parapet walls complete "in" (run your thermal and air control layers up and over your parapet) or push the parapets completely "out" (reconstruct your parapet over air and thermal control layers at the roof perimeter).

    Useful discussion here: https://www.buildingscience.com/documents/insights/bsi-050-parapets-where-roofs-meet-walls.

    Peter

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