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.
This approach would probably be most useful for a lower slope roof (so the batt made contact with ceiling of the floor below and the roof sheathing, although one could stack batts for a higher slope roof I guess. A benefit of this approach is that it 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.
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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.
Hi Martin,
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?
Thanks.
Rob,
The R-value per inch of a mineral wool batt will certainly be higher than the R-value of blown-in fiberglass.
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,
You can't install blown-in fiberglass without some type of a dam. A Roxul batt is better than nothing, but it won't "minimize" wind-washing. If you want to minimize wind-washing, choose a material that's an air barrier.