SIPs ridging
An acquaintance has a 5 year old SIPS house with visible ridging happening on the (unvented) roof. The builder (not me) went back and taped the seams. The question is whether there are other measures that the homeowner should do at this point, other than eventually adding ventilation to the roof? And does anyone have ideas as to what exactly causes the ridging?
Yes. The edges of the OSB top layer is expanding due to moisture (and heat) exiting the house through the seams in the SIPS, hitting the moisture impermeable roofing, condensing on and essentially rotting the OSB. Seal the joint on the interior with a high quality acrylic peel & stick tape. You can also vent the roof, so all that moisture (and accompanying heat) is vented away and doesn’t damage the OSB. Or better, can seal the joints and cover the SIPS with enough sheet insulation to prevent condensation and keep the moisture and heat inside the house. This involves adding another layer of sheathing.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Best-guess on proximate cause (no surprise): Air leaks at the seams brings wintertime moisture to the outer OSB facers causing localized dimensional changes (and possible rot) to the OSB along the seam.
It may be worth doing some invesigatory surgery in some easy to repair spot to see if there is sufficient damage to be of concern or whether it's merely a cosmetic problem in it's current condition.
Bob,
This issue has been discussed on GBA several times. Here is a useful comment by Chris Koehn from a previous thread:
"I started installing SIPs over timber frame in 1988 in Wisconsin. For many years we simply sealed the double OSB splined joints well, from above, with expanding foam. We rarely encountered issues with this installation technique, but on a few occasions we saw, in time, "shingle ridging": composite shingles ridging up over panel joints in cold weather. Investigation found frost on the under-side of the shingles directly above SIP joints, indicating moisture moving through what appeared to be a well sealed joint.
In the late '90's, the SIP manufacturer I worked with, Insulspan, developed a method for sealing joints on the inside surface using seal tape: strips of peel-and-stick membrane. This certainly added a level of complexity to the installation, but it seems to make for a better and more durable job. Seal tape comes in a variety of widths, used in different locations. The trickiest locations are over timbers. Seal tape is draped over the timber, paper side up; the SIP is installed; the paper is then peeled back and adhered to the SIP skin from below. ...
"Seal tape seems to have cured shingle ridging, but many feel SIP roof decks should still be vented. Certainly with metal roofing I agree: metal tends to cause condensation to occur below it more readily than other roofing materials. There are many ways to accomplish this. Felt and strapping are perhaps the most common. Some municipalities require cross strapping (vertical then horizontal) so that any water that makes its way through the metal can drain to the eve unimpeded. Mat products are emerging, such as R-vent, and these appear to work also. They have the added advantage of keeping the roof profile a bit thinner in appearance."
Here is a link to an article with more information on this topic: Air-Sealing SIP Seams.
OSB ridging on a SIPs is like mentioned above, due to moisture issues and then the OSB swells. The SIPs should have been TAPED when installed, not years later, but it's better than not taping. Most likely condensation is occurring at the SIP joints and causing the OSB to take on water and swell.
A proper roof SIP should have T&G panels, gaskets at the joints, and finally tape the underneath of the SIP with a quality tape like SIGA. As Martin mentioned, SIP decks should be vented as an extra precaution. Is it mandatory? Depends on the climate but it is a belt & suspender approach to install a cold/roof venting above the SIP.
Martin, if one does vent the SIP roof with 2x's and then OSB sheathing, peel & stick and a metal roof on top. Where would water form on the metal roof at that point? It can't form underneath the metal roof between the metal roof panel and peel & stick membrane, can it?
Peter,
It sounds like your question refers to condensation that forms on the underside of the metal roofing. (If you see water on the top of the metal roofing, it's usually rain or dew.)
The amount of exterior air that circulates under metal roofing depends on the metal roofing profile. Traditional corrugated roofing allows lots of air flow under the roofing. Most types of standing-seam roofing, on the other hand, allow little or no air flow under the roofing.
If the metal roofing profile allows for air flow, then condensation can occur whenever the outdoor air is warm and humid and the roofing is cold. Roofing gets cold when there is snow on it, or due to nighttime radiational cooling.
Water takes up very little space, so better to assume that it will form between the metal and the membrane.
one other issue is that OSB is the worst sheathing for moisture absorption, so using it as roof sheathing is a gamble I no longer take; we use Huber Advantech instead which is treated to not absorb water. Huber ZIP is also used on roofs; note that it, too is standard OSB.