Nailing into SIP
BSchalk
| Posted in Green Building Techniques on
I have completed a small 10×12 timber frame and have built SIPs for the roof. (4″ polystyrene sandwiched between two 1/2 CDX). I was planning to cover the SIP with 30# felt and nail cedar shingles directly to SIP. Is it possible to nail directly to SIPs or is it more appropriate to install furring strips and decking to create a breathing space and keep the roofing nails from penetrating the SIP? I’m near Indianapolis, zone 6. I do plan to carefully tape seams and spray foam insulate the joints. Any input would be appreciated.
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Replies
Brandon,
For many years, SIP manufacturers advised builders that so-called "hot roofs" -- roofs that lack a ventilation channel between the top of the insulation and the underside of the roofing -- work just fine. (Note, however, that most asphalt shingle manufacturers don't allow their products to be installed on hot roofs.)
In recent years, however, there have been reports of problems with SIP roofs that lack a ventilation channel. Most of the problems have occurred in cold climates. In cold climates, this type of SIP roof is associated with OSB rot, especially near the SIP seams. The mechanism for the moisture accumulation is usually air leakage at the seams. (Warm humid interior air leaks through the seams, and the moisture condenses on cold surfaces -- for example, cold asphalt felt.)
If you want your SIP roof to last a long time, you'll (a) Focus on carefully sealing the SIP seams -- meaning that you must have SIP tape on the interior of the seams as well as canned spray foam at the seam itself, and (b) Include a ventilation channel between the top of the SIPs and the underside of the roofing.
For more information, see Air sealing SIP seams.
Indianapolis is nowhere near zone 6- it's on the warm edge of zone 5 less than an hour's drive to zone 4.
https://www.greenbuildingadvisor.com/sites/default/files/images/DOE%20climate%20zone%20map.preview.jpg
4" of polystyrene is is about R17 (R20 if XPS, but that will decay to R17 over time), and well below IRC 2015 code-minimum performance for zone 5. To hit code minimum performance of U0.026 (max) with continuous insulation takes about 9" of EPS. You're not even half way there! Were you planning to add more insulation underneath the SIP?
Thanks for the replies. I will go ahead and batten the roof. Dana: I was not planning to insulate underneath. The building will be used occasionally as a small workshop for crafts. I was considering not insulating it at all. It will be heated by a small space heater in the winter and no a/c. I have cut a number of frames but have not closed any in.
Depending on what sort of crafts, and how often it will be used it may be worth installing a point terminal heat pump rather than a space heater for heating the place, and it would still leave you the option for air conditioning. A PTHP would have 1/3 or so the operating cost of an electric space heater.
Even a half-ton PTHP would likely cover the load, but it may be worth installing a 1-ton if you plan to head out there on a 0F morning and bring it up to temp in a reasonable amount of time. With the additional cost of the wall-sleeve a 1- ton Amana DigiSmart PTH123G35AXXX or LG LP126HD3B would still only run about $1K, only $50-100 more than the smallest of the line PTHPs (usually ~3/4 tons.)