Open cell spray foam on roof deck & vapor retarding primer
Hi,
我正在马萨诸塞州波士顿市购买房屋。我知道我在气候区5。当我决定购买房屋时，已经被构架了，屋顶甲板/after湾被喷洒了开放式牢房喷雾泡沫。我不知道屋顶底层使用的是什么，但我知道有沥青带状疱疹。after湾由2x12制成，完全充满泡沫。阁楼是一个步行路程，将完成居住空间，包括卧室，客厅和浴室。HVAC设备和管道位于墙壁后面，带有通道门和舱口供服务。我附上了一张图片。阁楼上有供应管道，但我不知道是否有返回管道，因为管道工作尚未完全完成。我需要与构建器和/或HVAC安装程序联系。
我读过很多关于需要蒸汽在这个地点上retarder in cold climates when using open cell foam on a roof deck. I’m under the impression that it would be needed for my installation. So my plan was to ask the builder to prime the drywall of the ceiling and walls of the attic with a vapor retarding priming sealer. However, I just read Joe Lstiburek’s article ‘Cool Hand Luke Meets Attics’ that seemed to imply I may not need the vapor retarding primer if the attic has supply/return ducts, although since those ducts are in the finished space, I have concerns about the area behind the walls where the HVAC equipment and ducts reside.
So my question is, what do the experts here recommend to make sure I don’t have moisture problems in the attic?
Thanks in advance,
-David
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这些图片看起来不像泡沫的完整填充物。它看起来（慷慨）接近〜8-10英寸的平均深度，而BTW不符合MA中的代码最小，有些斑点较薄，高达6英寸。
即使是完整的11.25”（铣削2x12 rafter湾的标称深度）开放式泡沫也无法满足最低R49的最低代码，并且不太可能您的框架分数足够低，无法以U因子为基础。代码以U因子为基础，必须以U0.026或更少的全组件平均值进行测量，这本质上是均匀的R38，而无需the绕after架的热桥。在8-10英寸的平均深度下，您将是幸运的是，如果它以R38平均甚至中心腔内进行，更不用说在计算热桥后的。
No matter what the above-deck layup is, unless it has about R20-R24 of rigid foam, the roof deck is at some risk of wintertime moisture accumulation. While vapor barrier latex on air-tight wallboard would work on paper, the long term air tightness is not guaranteed. A better solution would be to first bring the foam contractor back and fully fill the cavities, trimming it flush with the rafter edges. Then, install a layer of MemBrain detailed as an air barrier on the rafters, lapping & taping any seams between adjacent sheets ONLY at the rafters, and cauking the edges to the framing elsewhere. Then install 1.5" of UNfaced EPS (and not XPS) as a continuous layer on the underside of the rafters, and long-screw the ceiling gypsum through the foam to the rafters. That would bring you up to a center-cavity R of about R45-R46 or so, still shy of a code-min R49, but the R6 thermal break over the rafter edges brings the assembly reliably below U0.026.
At 1.5" Type-II EPS has a vapor permeance of about 1.8-2 perms, which might be sufficiently protective of the roof deck on it's own, but in combination with the MemBrain the wintertime moisture accumulation at the roof deck will be extremely slow, but the drying rate of the roof deck will be only limited by the 2-perm EPS. If you paint the ceiling gypsum, use a standard latex paint, not vapor barrier paint. Don't use vapor barrier paints on the ceilings below, or on any of the partition walls, etc. That way even the most modest amount of air exchange between the attic & the rest of the house will keep the humidity levels in the attic space tracking that of the rest of the house.
由于该装置似乎无法满足最低代码，因此城镇建筑检查员应该标记的东西吗？
Also do you think there are other viable options to address this issue? I want to be prepared for what the builder and/or foam installer might suggest as correct or incorrect solutions. For example, would adding closed cell foam over the open cell to fill the gaps and rafter bays completely to a full 11.25 depth be another viable option? Perhaps in the areas behind the walls the rafter bays could be foamed beyond the 11.25 depth since no gypsum would be attached to the rafters
Thanks again,
-David
David,
喷雾泡沫安装程序至少犯了两个错误，而且是严重的错误。
1. As Dana explained, the installation does not meet minimum code requirements for total R-value.
2.建造了膝盖墙，并在安装了干墙和所需的蒸气延迟器之前安装了管道和HVAC设备。这违反了大多数开放式喷雾泡沫品牌的安装要求，包括ICYNENE。现在安装所需的干墙（或更好的是刚性泡沫）现在会很尴尬，可能需要临时去除膝盖和管道，然后重新安装。
如前所述，O.C.泡沫的完整填充量仍然使您距Code Min shy r10。因素基础，如果您可以保证在after边缘上有2英寸的深度），但它只会使您只能使用最尴尬的天花板石膏方法。
如果您在after边缘的平面上方没有水槽或空隙完成完整的填充物，请插入一个智能蒸气延迟器，详细介绍为空气屏障，并添加R6半渗透性刚性泡沫（未取代EPS的1.5英寸），它使代码制成代码在U因子上的最小值会产生相当平坦的天花板，并且不需要荒谬的石膏固定器。
I spoke with the foam installer. He informed me that because my town had adopted the 'Stretch Code' that the attic R-value required is only R-38. This doesn't quite seem right to me. The foam they used is R-3.9. He claimed the foam is 10" in or more in each bay and therefore the R value was met. He also indicated that gypsum with wallboard would cover the entire roof deck even behind the partition walls and that the gpysum would be primed and painted with a vapor retarding sealer/paint.
I am considering having the builder increase the foam to completely fill each bay to get the full11.25 depth which would be about R-43. Would this combined with the gypsum and vapor retarding paint be enough to address potential moisture issues?
Thanks,
-David
-dave
David,
Here isinformation on the Stretch Code: "The stretch code appendix offers a streamlined and cost-effective route to achieving approximately 20% better energy efficiency in new residential and commercial buildings than is required by the base energy code. This is largely achieved by moving to a performance-based code, where developers are required to design buildings so as to reduce energy use by a given percentage below base code, rather than being required to install specific efficiency measures."
这是对隔热承包商的问题：“由于您正在浏览阁楼绝缘材料，因此必须以很高的标准建造房屋的其他功能，以弥补阁楼中低R值的情况。这些非凡的其他功能是什么？“
Thanks Martin. I will speak to the builder about what else is being done. I guess my lingering concern still remains about moisture in the attic. Do we think the open cell foam at R-38 coupled with all of the rafters covered with gypsum and vapor retarding paint will be enough to prevent moisture on the roof deck?
David,
If the open-cell spray foam is upgraded to R-38, and if the contractor is really able to install drywall on the underside of the rafters -- a feat that will be difficult behind the kneewalls, where the ducts are -- and if vapor-retarder paint is applied to the drywall, you won't have any moisture problems.
For a vapor retarder to protect the roof deck it has to be completely air-tight. That's damned hard to achieve with an air-tight gypsum approach given where you're starting, and if the rafter bays are not completely filled you have large convection channels to amplify any air leakage. Air transported moisture is a HUGE risk here, even if you installed a broad sheet membrane vapor retarder like Intello or MemBrain.
即使他们拥有减脂数学，即使在阁楼的子代码R38中，从长期的水分弹性的角度来看，即使在整个房屋的代码中，它们都比整个房屋好20％，计划O.C.泡沫用after边缘的平面冲洗，并安装智能膜蒸气阻滞剂而不是油漆。但是，只要石膏和泡沫之间的气隙确实很小 - 尽可能紧密，而不是一个容易的对流频道，并且您虔诚地将石膏填补到框架上，您就可以使用蒸气阻滞剂的油漆方法可以。
David,
我不同意达娜（Dana）的意图，因为需要干墙是气密的。除非喷雾泡沫安装程序非常草率，否则8至10英寸的开孔喷雾泡沫构成了完全足够的空气屏障。
泡沫是伟大的8 - 10”空气屏障对正iltration, but not for stopping convection behind the drywall from drawing in air through leaks in the drywall layer. If the drywall layer is to be an effective vapor retarder, it has to be air tight:
At 8-10" of ocSPF in a 2x12 rafter bay it means you have 2-3" of convection channel between the drywall & foam, a channel that is subject to convection due to the temperature difference at the face of the foam relative to the room air. A 2-3" gap is convection channel bigger than is required for a vented roof(!), but in this case it's on the "wrong" side of the insulation. If they were only going for 8-10" of foam rather than a full fill it arguably should have been a vented roof, with the 2-3" channel on the roof deck side, not the gypsum side.
If the gypsum is not air tight it hardly matters what the vapor retardency of the paint is, since the leaking air can get behind it, and WILL due to the buoyancy forces of the denser colder air next to the foam falling to the bottom of the channel leaking into the conditioned space on the other side of the gypsum, drawing air into the channel higher up.
The vapor retardency of ocSPF itself is usually over 5 perms @ 10", rarely under 2- the same range as standard latex paints and not sufficiently protective on it's own. So even fairly slow air leakage into the channel from the conditioned space side adds up to a real moisture burden potential. If the gypsum is air tight and painted with 0.5 perm paint, the potential is extremely low.
If the average depth of the channel shrinks to 1/16" or less it's not exactly an air barrier, but it's somewhat air-retardent. The rate of convection from those buoyancy forces can't be very high, and the amount of moisture getting by the vapor retarder is that much smaller, even with some leakage at the margins.
Given that 11-12" of ocSPF without interior vapor retarders has been demonstrated to deliver higher than safe sheathing moisture content even on walls with vented cladding that can dry toward the exterior, the problem is necessarily worse in roof decks with ~0.1 perm felt & shingle layups, which have inherently NO capacity to dry toward the exterior, and are subject to radiational nightime cooling, with lower average wintertime temps than wall sheathing:
//m.etiketa4.com/articles/dept/building-science/cold-sheathing-double-wall-construction-risk
http://www.buildingscience.com/documents/bareports/resolveuid/03990e0f18594c57b6643490d62bea0e
Dana,
If the rafter bays are completely filled with foam to remove the convection channel does that reduce most of your concern for moisture? I don't think I'm going to arrive at a perfect solution with the installer I'm just trying to reduce as much risk as possible given the foam is already installed.
-dave
David and Dana,
First of all, we all agree that it would be best to call back the spray foam contractor to finish the insulation job -- meaning to add enough open-cell spray foam to fill the rafter bays, and then to trim the insulation flush. This is the standard method for installing open-cell spray foam, which is easy to trim (unlike closed-cell spray foam, which is hard to trim).
However, I'd still like to resolve the issue that Dana raised -- the claim that there will be convection currents between the drywall and the cured spray foam that will undermine the performance of the assembly.
达娜是错误的，原因有几个。
First of all, it is simply untrue, as Dana asserts, that "If the drywall layer is to be an effective vapor retarder, it has to be air tight." A vapor retarder is designed to address vapor diffusion, not air leakage. Measuring the rate of vapor diffusion through a material has nothing to do with air leakage.
That said, there are many reasons why you want an air barrier. Fortunately, the installed open-cell spray foam is a very good air barrier.
第二:假设石膏板从未英斯达lled. (This is not a good idea, because you want to address vapor diffusion -- but I'm introducing a thought experiment.) On the interior side of the cured spray foam, the air is at interior conditions -- let's say it is at 70 degrees F. The spray foam is an air barrier.
Are there convection currents? Who cares?
Will the air in the attic degrade the performance of the insulation layer? Not really -- even if someone installs a ceiling fan up there to move the air around. The air in the attic is indoor air; the air on the exterior side of the roofing is outdoor air; and you have a good air barrier and a pretty good (not quite code compliant) layer of insulation separating the two. Sure, you'll get heat transfer through the roof assembly -- and that heat transfer rate is a function of the insulation's R-value. No air leakage, though -- so convection doesn't matter.
Now, let's add the drywall and some vapor-retarder paint. The air between the back of the drywall and the spray foam -- not much of a layer of air, by the way, especially if the insulation contractor came back to finish the job -- isn't moving much, because there aren't any air leaks to the exterior. It's basically still air.
即使空气稍微移动了一点，它也是内部空气，也不会从内部流向外部。您有一个好的空气障碍。
If room air gets by the vapor retarder, the vapor pressure across the vapor retarder is nil, and the vapor retardency pf the vapor retarder doesn't matter. A vapor barrier is only useful if it's sufficiently air tight that vapor diffusion is the primary mechanism of moisture transfer.
With the gypsum leaking air the vapor retardency of the FOAM then matters. When the vapor pressure at the interior surface of the foam it tracking that of the conditioned space, 11" of open cell foam is not sufficiently low permeance to protect the roof deck. It's not even reliable protection for the sheathing of a wall assembly with vented cladding, where the sheathing can dry toward the exterior, as demonstrated by the in-situ monitoring of Carter Scott's double studwall with 12" of open cell foam:
http://www.buildingscience.com/documents/bareports/ba-1501-monitor-double-stud-moisture-conditions-northeast
And in a roof assembly there is the aggravated circumstances of no drying toward the exterior, and colder sheathing temperatures.
So sure, the foam is a great air barrier, but it's not a good enough vapor retarder, thus the vapor retarder layer has to be sufficiently air-tight to make it's vapor retardency relevant.