Open-cell SPF in an attic with no vapor barrier
Will Oberman|发布了能源效率和耐用性在
Hi,
我住在匹兹堡，我正在做一个旧家的翻新。在我父亲的帮助下，我自己正在做很多工作。与此同时，我一直在与建筑师合作弄清楚整体项目。在我的两个球域之间有分歧，关于如何隔离阁楼。
The (hopefully) key points: it’s currently a vented attic, I’m going to be putting HVAC and ductwork in that space.
我父亲建议我把the attic as vented, put down batting on the attic floor, and do various things to ensure the HVAC unit is leak free and insulated
建筑师建议在椽子之间喷涂开放电池泡沫并使阁楼成为未经常用的绝缘空间。
The biggest concern about the open cell SPF (other than it’s a lot more expensive) I have at the moment is the fact that’s it’s not a vapor barrier. But, the architects seem to think this isn’t a problem. How could moisture not condense in the foam? If it does, why doesn’t that matter?
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答复
Will,
如果您打算在您的阁楼内安装HVAC设备和管道，我同意将倾斜的屋顶隔热，使阁楼变成一个不经常的条件空间。
为此目的使用开孔喷雾泡沫是完美的。在像你这样的寒冷气候中，最好在冬季避免内部气旋阻滞剂，以避免屋顶护套中的水分积累。通常的方法是用气旋涂料喷涂固化的泡沫。这是完全充足的。
In many jurisdictions, you may also need to install a thermal barrier like gypsum drywall so that the spray foam insulation is not exposed. You should consult your local building department to determine what is expected in your jurisdiction.
For more information, see创造一个有条件的阁楼.
Thanks for the answer! I've been relying on the architects on all of the complicated and messy code issues.
My concern is that everyone I've talked to that is "pro-foam" uses fairly wishy washy language when it comes to vapor retarders/barriers (you "could" do this or that), making it unclear to me what is required vs. optional.
I found this from the SPFA:
http://www.sprayfoam.org/downloads/pdf/ay%20118.pdf.
The key line:
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If night or winter temperatures are cool enough, the water vapor normally diffusing harmlessly through the foam may condense. It is important to provide a vapor transmitting covering system (high perm rating) to the exterior surface of the polyurethane foam to prevent the build up of humidity within the foam and, thus, avoiding condensation.
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So it sounds to me like a high perm rating covering is non-optional, if the spray foam industry says it's important.
I'm not sure if this matters, but the plan was to completely fill my 2x8 rafters and even cover the ends. I believe the claim (from the first bidder I talked to) was that at an 8" inch thickness, the open cell foam itself counts as a high perm rating barrier. But, from my understanding, all that would mean is that "not a lot" of water is condensing in the foam. And, from what I've been reading, water and open cell foams aren't very friendly.
But overall what I'm hearing is: given the HVAC/ducts as a requirement, my best option is open cell foam with a layer of vapor-retarder paint on the interior.
再次感谢你的反馈！
Bill,
You are in climate zone 5. The rule of thumb is that open cell foam must be protected with a vapor retarder in zones 5 and higher. Regardless of thickness, open cell foam is very vapor open. That said, if you keep your home dry in the winter, you would probably be okay without it - but why risk it? My advice is to coat the open cell or better yet, go with a denser foam with a sufficiently high perm rating. The trouble with paint is that it adds a step (and hence cost) to your project. Additionally, open cell foam is tough to paint properly because of the uneven surface. We work in Cleveland, which obviously has climate close to yours, and over the years we have installed many open cell roofs without any problems. We switched to higher density foam a couple of years ago, in part because of the issue we are discussing here, and a very happy with our choice. Feel free to contact me if you would like to discuss this over the phone. I'll be happy to give you my two cents worth.