Soffit Dams and R-Values for Blown-In Insulation
I would like to hear comments about the idea of using Roxul batts to serve as retrofitted soffit dams. The idea is to place, say, R-30 batts between the ceiling joists so they rest over the wall plate and squeeze up against the underside of the roof sheathing and ventilation baffles. Some trimming of the upper side of the batts may be necessary to avoid crushing the baffles.
这种方法可能对下坡屋顶最有用（因此BATT与下面的地板的天花板和屋顶护套接触，尽管我猜的是堆叠较高的斜坡屋顶。这种方法的好处是它allows one to place higher R-value insulation above the plates (relative to blown-in insulation) thereby helping maximize insulation over an area that may not permit ‘full insulation depth’ because of the roof geometry. The rigid batts would provide protection against wind washing but, of course, would not be nearly as airtight as foamed in soffit dams.
Separately, the R-values I have seen for blown in cellulose typically range from 3.2 to 3.7 R/inch. The values/ranges I have seen for blown in fiberglass are much more variable. For instance, a quick survey revealed values/ranges of 3.2-4.2, 2.2-4.2, 2.2-4.3, 2.2-2.7, 2.2, 2.1-2.7, and 2.2-4.3 R/inch. Setting aside air infiltration issues for a moment, these data suggest a product superior to or noticeably worse than cellulose, depending upon the values one adopts. Any opinions about what one should believe about the insulating properties of blown in fiberglass?
Thanks – Rob.
Replies
Rob,
A soffit dam -- also called a wind-washing dam -- needs to be made of a material that is an air barrier. Roxul batts are air-permeable, so you wouldn't want to try to use a Roxul batt to create a wind-washing dam.
Usual material choices include plywood, OSB, and rigid foam.
For more information, see this article:Site-Built Ventilation Baffles for Roofs.
嗨马丁，
Yes, the batts certainly wouldn't be air tight. I was just thinking that the density of the material is such that washing effects would be minimized. If they were kept in check sufficiently then the ability to realize a greater level of insulation in the (potentially) tight eave area might make the compromise worth it.
Do you have any insight into a reliable range of R values for blown in fiberglass?
谢谢。
Rob,
每英寸矿棉贝蒂的R值肯定会高于玻璃纤维吹入的R值。
The table below shows the R-value claims made by CertainTeed for its blown-in TrueComfort fiberglass insulation for attic floors. It comes out to R-2.4 per inch. That sounds about right, although some older types of blown-in fiberglass might be as low as R-2.1 per inch.
.
Rob,
没有某种类型的大坝，你无法安装闪烁的玻璃纤维。Roxul Batt总比没有好，但它不会“最小化”的风力洗涤。如果您想最大限度地减少风力清洗，请选择一个空气屏障的材料。