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Community and Q&A

Installing Rigid Foam Over Sheathing

Robert Gemboleski | Posted in GBA Pro Help on

hi Martin
I’m finish interior carpenter
this time renovating my own house in Pennsylvania
I’m installing new roof  4:12 pitch
T&G boards are already installed. 7 1/4″x3/4″
1 board gets 1 or 2 nails? has 1 nail so far but can add
want to skip vapor barrier  (is it good idea?)
if any water gets past long screws it will evaporate through T&G gaps
Will seal around perimeter ( don’t know what exactly to use  caulk? double sided tape?)
just place 2   (or 3 ) layers  (6 0r 8 inches )foil faced EPS glued (foam glue) to wood boards and edges and between layers ( sure will stick on edges but Not sure if foam glue sticks to plastic foil)
on top of EPS foam layer of reinforced aluminum foil (to reflect heat or cold)
Do I need to install ice and water shield?
then horizontal purlins will be attached with long screws
is it better to install grid ( vertical and horizontal purlins)?
(do I need to hit rafters or just sheathing ?)
on top of purlins will be installed  embossed metal roofing (resembling old fashioned terracotta clay   tiles .
there will be vented ridge on top
Can’t find all answers in your blog

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Replies

  1. Walter Ahlgrim | | #1

    You may find this article interesting.

    https://www.buildingscience.com/documents/published-articles/pa-foam-shrinks/view

    Stagger the seams in the foam hit the structure when screwing and get the flashing in correctly.

    I don’t think you need /want glue.

    The T&G board will need an air barrier to keep the warm moist air away from the cold surfaces where it would condense and rain back on you.

    Walta

  2. Robert Gemboleski | | #2

    Hi Walta thank you for response
    In house from article it was 1 layer of foam not glued
    I think if I glue seams,
    sheating and layers it wont crack (thats the core of my idea)
    I just want to do as little layers as possible

  3. Walter Ahlgrim | | #3

    Please be sure to read to the end of the article Joe tells you how he corrected the problem when he replaced the foam and fix his mistakes.

    Walta

  4. Steve Knapp CZ 3A Georgia | | #4

    Robert,

    I think the general advice is that multiple, staggered layers are safer. Even if the foam layer doesn't move much, the wood framing it is attached is likely to move quite a bit. Layers help you avoid gaps.

    Just to be clear, the tongue and groove is installed as the roof sheathing, correct?

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