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Cut-and-cobble roof follow-up: Permeability / R-value relationship

qthisup | Posted in General Questions on

This is a follow-up to this post:

Flat roof insulation on ground floor apartment

Climate is London, UK, call it 4C for argument’s sake.

Context is a flat roof. 16″ centres, 8-10″ deep rafters TBC. Cut and Cobble technique as roof height is strictly limited. Roof covering EPDM.

I posted a comment at the end of this previous post, but I think I’ve answered my own question – the rigid foam layer can’t have foil facing in order to remain vapour semi-impermeable.

My question is now how thick can I/should I go with my rigid foam layer in order to maintain some modest seasonal drying with a cut and cobble method AND maximise my assembly R-Value?

I’ve found a PIR product (I’m UK based so apologies for the metric) that has a Lambda (W/m·K) of 0.018 which at 55mm has an R-value of 2.857 m²·K/W. These stats directly from manufacturer:

So I believe this is a quite a good PIR board as typically other PIR products seem to be about 0.020-0.0.22 W/m·K.

Now the permeability  “typically achieves a resistivity greater than 300 MN.s/g.m, when tested in accordance with BS EN 12086: 2013” and also another document says “For the purposes of assessing the risk of interstitial condensation, the insulation core vapour resistance may be

taken as approximately 18.5 MN·s·g–1″. Part of my challenge is I’m struggling to convert some of these units into US perms to figure out where I’m at as my understanding from previous posts is to try to achieve approx 0.5 US perms for some drying potential?

Essentially, based on your acknowledge, and assuming the above mentioned product is a suitable option, how thick of a PIR layer would you use? Perimeter of PIR will be spray foamed and balance of cavity filled with Rockwool ( which is 0.035 – 0.038 W/mK).

Thank you

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, if you want a formula to convert vapor permeance values between U.S. (English) units and European (metric or Systeme International), here it is:

    1 perm (U.S. or "English" unit) is 1 grain of water vapor per hour per square foot at a vapor pressure difference of 1 inch of mercury.

    1 perm (U.S. or "English" unit) = 60 ng/Pa s m2 (metric or S.I. perm units)

    Second: Using the cut-and-cobble approach for an unvented roof assembly (a) is risky because of possible air leakage and moisture accumulation, and (b) wouldn't require inward drying in any case, as long as the roof sheathing is dry on the day that the sheathing is encapsulated.

    My advice: Use closed-cell spray polyurethane foam.

  2. qthisup | | #2

    Hi Martin
    Thanks for the feedback. The problem is the spray foam contractors are not as common here and the couple quotes I got were very high. Indeed one insisted open cell foam was suitable and my understanding from your various articles is that this application needs to be closed cell to create the vapour barrier.
    I can however purchase a 2 part closed cell DIY kit which I was planning to use to fill the borders and any awkward areas.
    Kind regards

  3. GBA Editor
    Martin Holladay | | #3

    It's your call. You are choosing a risky approach to save money. In construction, that approach is usually penny wise and pound foolish -- but it's your decision.

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