Insulating a timber frame roof with spray foam insulation?
Hi All!
First time posting on this forum, but I have learned a lot from other threads here.
We built a 20×20 timber frame house in 2016. A local contractor cut out the timber frame kit, put it up, bolted sip panels on the sides/roof, and put on a metal roof.
A few months ago we noticed a water leak in our tongue and groove ceiling after a bad snow storm. Later in the spring we investigated and believe the water leak came in through the ridge vent. However the more concerning thing we found was large areas of the OSB on top of the SIP panels have extensive rotting.
After much research on this forum I think this is due to a low Perm rates underlayment (much less than 1) used directly on top of the SIP panels. Also venting above the SIP panels was minimal. Probably the roof SIP panels were not perfectly installed and allow heat/moisture to leak out.
We have a couple options moving forward. We can repair/replace some of the roof SIP panels, use a more permeable underlayment on top of the SIP panels, make a vented roof from vertical stringers, then decking on top, more underlayment and finally the metal roof to finish it off.
But recently we learned that many folks with timber frame houses in our area skip the SIP panel roofs (central Pennsylvania) and are using a closed cell spray foam on top of the T&G and building up the roof with vertical 2x6s on top of some horizontal stringers. There is room for a vented channel above the foam but below the decking. Many still use metal on top the decking.
We are a bummed that we are in this situation after just building the house in 2016 and really want to get this roof fixed so that we don’t have issues again for the life of the roofing.
My Questions:
1) clearly our roof SIP panels are leaking and moisture/heat is escaping to the low Perm underlayment and condensing. If we fix the SIP panels, use different underlayment and build a ‘cold roof’ will this stop the rot? My fear is the we go through all this time/expense just to find more rot in 10 years. It feels like the roof SIP panels are prone to rot at the seams/butt joints. I’ve read that attention to detail is key, but now that the panels are installed can we retrofit/fix this issue?
2) what problems should we be on the look out for with a spray foam roof system? The contractors that we will work with talk about it with a ‘it fixes everything’ mentality. But surely there are potential moisture problems and other significant issues we need to pay attention to.
3) the initial problem was wind blown snow getting under our ridge vent. It’s a painted corrugated 3 ft wide metal panel roof with exposed fasteners. The ridge vent is a standard pre-bent long piece of metal with foam in between it and the panels. It squishes and expands to fit the ridges and valleys. We live on a windy hill and the foam doesn’t seem strong enough to keep out the snow. We are considering a standing seam metal roof. The z-bar design ridge vent looks stronger, but does it do a better job keeping out wind blown winter snow? We have a 12/12 pitch roof.
Thanks in advance for any advice. We are overwhelmed with the possibilities and more than a little nervous of not getting the details right this time around.
-Ben
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Replies
Hi Ben,
Your experience with SIP panels isn't uncommon, because of the work of a few building scientists, problems have reduced over the years.
If you choose to go back to a SIP roof, I would make some changes from your original roof. You should remove all the panels, add an air barrier to the top side of the tongue and groove ceiling, then reinstall the panels. A product like VaporDry SA | Self-Adhered Vapor Permeable Roof Underlayment would work. The top side of the SIP would be strapped up the roof using a 2x furring strip. This will create a ventilation gap to help clear any moisture that should accumulate in the SIP. Add a second layer of sheathing to the top of the roof and clad with whatever roofing material you want.
If you decide to go with closed cell spray foam, I would still add an air barrier to the top side of the tongue and groove paneling. You will then need to install wood framing on the roof, I would go 2x10. You may need to consult a structural engineer for recommended spacing (16" or 24" on center) and a fastener schedule. I recommend spraying a minimum of 7 inches of closed cell spray foam (about R-50). The 2x10 will give you about a 2" gap for ventilating. Cover the roof with sheathing and install the roofing of your choice.
There can be issues with the closed cell spray foam option, spray foam needs to be adhered to the roof deck (in this case, the air barrier membrane) and the sides of the roof framing members. If applied when the roofing materials moisture content is too high, or ambient and building material temperatures are too low, this bond can fail. (The membrane added to the top of the tongue and groove can help if the is a delamination problem between the foam and roof framing.) Other issues can be improper ratios of the 2-part spray foam or spraying the foam in too large of a lift. If you decide to go with the spray foam option, hire a well vetted spray foam contractor.
As far as the snow entering the ridge vent, it's been my experience that this happens when the roof steel is too short and doesn't get close enough to the peak, or the ridge cap is too small they make different sizes, and many steel roofing suppliers will bend a custom size if needed.
Hope this helps,
Randy
Thank you Randy!
This is immensely helpful. We are really struggling with this and your advice really helps.
One follow up question: Would you put the same air barrier membrane (VaporDry SA | Self-Adhered Vapor Permeable Roof Underlayment) between the T&G and spray foam as you would used under the SIP panels?
Much appreciated!
The VaporDry SA is one of a few different products that are vapor permeable but airtight designed for roofs, and yes, it will work for both SIP and CCSF, it will also work for Michael's over-frame suggestion.
Thank you! 🙌
I have also dealt with rotting SIPs over timber frames and do not recommend using them. They can work well if everything is done perfectly and nothing ever changes, but it's the opposite of a resilient system. Spray foam can work but it's also vulnerable to mistakes.
I recommend over-framing the roof and insulating with a more resilient, permeable, natural material such as wood fiber or cellulose, and venting above the insulation.
Hi Mike!
Thank you for your recommendation. The SIP panel roof struggles are real! We are open to any options at this point.
Building up the roof and using something more resilient and natural sounds like a solid option. Any recommendations on articles I can read to get more info or which wood fiber/cellulose that you have had success with?
Thanks!
Ben
Ben, this has become a seminal article here on GBA: https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work.
I was the "guest expert" on this fairly relevant piece: https://www.greenbuildingadvisor.com/article/how-to-fix-ice-dams-when-re-roofing and the original Q+A it's drawn from (link in article) has good info too.
There are tons of articles and Q+As here that are relevant. The search bar works pretty well--try things like "SIPs problems," "low carbon insulation," "spray foaming roofs," etc..
Thank you for the article recommendations! Lots of good info. I appreciate all the help. 🙌
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I think you will find some helpful info in this article.
https://buildingscience.com/documents/insights/bsi-036-complex-three-dimensional-air-flow-networks?searchterm=Juneau
Walta