Housewrap, WRB, and Underlayment
A few questions on roof underlayment’s as I am a New England carpenter who has been trying to implement best practices to extend building longevity wherever possible. A few scenarios:
1. Standard vented attic stick framed or trussed with cold roof deck. Usually roof trusses, loose fill cellulose, soffit and ridge vents, CDX sheathing, metal roofing double strapped/battens setup for a nice air space between the underlayment and back side of metal. For this we have used one of the better non permeable synthetic roof felts such as RooftopGuard that has a perm rating of nearly 0. Would a permeable product such as Pro Clima Mento 3000 be a better choice as moisture would not be included to get trapped between the CDX and underlayment?
2. Closed cell spray foam roof assemblies. I see a handful of scenarios where we have limited rafter depth so spray foam makes sense. Usually for just small portions of the house or renovation. Sometimes these can be in valleys or areas that we would normally use a thick non permeable product such as Grace Ice and Water Shield HT. All of these projects would get a metal roof that is strapped to allow airflow below, but is a non permeable material a no go on top of a closed cell spray foamed roof deck? My thought is yes as if moisture cannot get out if it were ever to find its way into the assembly?
3. Do I dare say what about vinyl siding on this site? I run into it from time to time, so what is best practice for rain screens with typical vinyl clapboard? Is the horizontal air space enough to provide proper drying of the wall sheathing? Maybe just a dimpled house wrap then so water can drop easier if is does get behind?
Thanks!
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Replies
1. From the description it sounds like the roof deck should be able to dry to the inside. However, if you are investing time and effort to install strapped/battens, it would make a lot of sense to install a vapour open underlay over the roof deck. I&W shield (vapour barriers) may still be required at roof edges. If you don’t use a vapour open product on top of the roof deck, what is the purpose of creating an air gap under the metal roof panels?
2. How wide is a small area? I would say that a narrow but long strip of spray foam would not be the end of the world. If water wets the spot where the decking is between spray foam and Grace I&W shield the wood decking should be able to dry by releasing water through the adjacent surfaces. There is point where the area gets too large. I don’t have figures to share. You could confirm with Proclimat to confirm whether their vapour open products can be used on hips and in valleys. From what I read on their website, it looks like that is possible. Then have one vapour open face for the decking to dry out if it gets wet
3. There are now many products that work like the dimple material. The dimple mat is zero perm so unless your vapour barrier goes on the outside per your code, you should choose a product that is designed for rain screens that does the same thing.
Horizontal strapping and diagonal strapping is not permitted by the code where I live in Canada. That is because it does not allow for airflow up and down, the natural directions of hot air and cold air. Also horizontal strapping wood trap water horizontally. (Vertical strapping allows water to flow down easily. Siding like corrugated metal is different in terms of venting.)
sb1616ne,
Hugh has given you good advice. I'd just add:
2. As long as the roof sheathing is dry when it is sandwiched between the two impermeable layers, it will be fine. The trade-off is between the added protection an impermeable underlayment provides against roof leaks, against the drying a permeable one allows. The answer is seems to be pretty climate dependent. Here in the wet PNW, RDH concluded an impermeable one was a better choice, but the equation probably swings away from that in colder, or dryer climates.
3. Vinyl siding works well on its own as a rain-screen. A textured underlay would no doubt make it even more effective.
sb1616ne for the record, it is Malcolm who taught me about the venting requirements for corrugated metal!
Thanks for all the info crew!
The area I am working on with spray foamed roof framing cavities where ice and water shield will be used on top is only about 8 feet wide by 8 feet long of a total 20ft long rafter where the upper portion of the rafters will be an uninsulated as thee attic will insulated the space below if that makes sense.
I admit that I don't know the properties of CDX enough to speak with any level of certainty on something that is 8'x8'.
If you look at this GBA link, you are talking about assembly #4. The article mentions:
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
"Some builders avoid this approach, preferring assemblies that provide a way for the roof sheathing to dry in at least one direction in case the sheathing ever gets damp. For more information on this issue, see “Sandwiching Roof Sheathing Between Two Impermeable Layers.”
But then there is a link in the 1st article and I found this quote:
https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling
"As building scientist Bill Rose has shown, code requirements for roof venting were never based on research or scientific principles. In a well documented JLC article on roof venting (“Roof Ventilation Update”), Rose explained, “For the most part, the focus of codes, researchers, designers, and builders on roof ventilation is misplaced. Instead, the focus should be on building an airtight ceiling, which is far more important than roof ventilation in all climates and all seasons. … Once this is accomplished, roof ventilation becomes pretty much a nonissue.”
I think that these articles and the comments they contain would be a great way to better understand the risks involved in different assemblies and how to mitigate those risks. I have always read that you need venting. Now I am reading that a building scientist Bill Rose is saying that venting is not needed when the assembly is not really required if the assembly is properly sealed.