JM Spider overhead
I was planning on using JM Spider on the underside of my structural roof deck. (There is a layer of peel and stick and 5.5″ of polyiso foam board above the deck so no worries about vapor permeable insulation inside). The installer is having problems with getting the Spider to stick overhead. The installer says JM changed the fiber recently and they are having difficulties with some wall installations too. While they are working with JM, I was wondering:
Has anyone here had JM Spider installed overhead recently?
If we can’t get this to work, can anyone suggest another insulation type (underside of a 3/12 roof with open web trusses 24″ on center).
Thanks
Eric
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Replies
Eric,
You definitely want to consult Johns Manville on this. For more information on Spider insulation, see Getting to Know Spider Insulation.
5.5" of polyiso derated for cool climates is only about R25-R27. The amount of fiber insulation you can safely install on the interior side with R27-ish exterior foam is highly climate dependent.
In US climate zone 6 you would be limited to about R27 on the interior side (a 50/50 split), but in US climate zone 5 you could install up to R40 (a 40/60 split) or so before the roof deck runs too cool in winter.
Spider would still need a class-III vapor retarder on the interior at those R-ratios. Alex Wilson did one better, and installed a "smart" vapor retarder on the interior side of his Spider-insulated roof, which is highly advisable even if you have a reasonable R-ratio for the climate:
https://www.greenbuildingadvisor.com/blogs/dept/energy-solutions/smart-vapor-retarders
Thanks gentlemen.
We have a call into Johns Manville, but they haven't been super responsive so far.
We are in climate zone 5B and were planning a maximum of about R35-40 on the inside (keeping in mind the 40/60 split). The gypsum board ceiling with latex paint should be suitable as a class-III vapor retarder.
My insulation contractor is offering to install an open-cell foam for cost which may be the way to go.
Thanks again for all the great articles and advice!
Eric