Replies

1. GBA Editor
   Martin Holladay | Feb 18, 2014 05:15pm | #1

   Gary,
   Q. "One contractor wants to add about 12" of blown-in fiberglass and the other one is recommending about 12" of blown-in cellulose. I believe the cellulose is the best option but worried about weight on the ceiling and the reduction in R value of the fiberglass once it is compressed by the cellulose. How much R value to you think I will lose in the fiberglass?"

   A. Cellulose is the best option, because it will reduce air leakage through the insulation layer, and will also reduce convective looping within the insulation layer. Even if the fiberglass is compressed, you will still end up with a better performing layer of insulation. Compressing a layer of fiberglass reduces the total R-value of the fiberglass but increases the R-value per inch.

   Q. "Should soffit dams be rolled up fiberglass or something like an AccuVent and the perimeter sealed with foam?"

   A. Here is a link to a video that describes the work you need to do: How to Ventilate Rafter Bays When Adding Insulation.

   Q. "Is spray foam applied to the rim joist in the basement area a health danger?"

   A. No, as long as your spray foam contractor doesn't make any installation errors. It's rare for jobs to have problems. If you want to read more about the type of problems that can occur, see Spray Foam Jobs With Lingering Odor Problems.

2. Expert Member
   Dana Dorsett | Feb 18, 2014 05:33pm | #2

   A foot of cellulose only adds about 1.25-1.35lbs per square foot, which is nowhere near enough to compromise even half-inch gypsum mounted on 16" o.c. joists. While it will compress low-density blown fiberglass a bit, the compressed fiberglass will then have a higher R/inch (though fewer inches), but importantly, will no longer lose performance to convection & infiltration currents flowing through it, due to the much higher air-retardency of the cellulose. Also, unlike fiberglass, cellulose is opaque to infra-red radiation, so you'll get the full-R performance in summer as well.

   The "right" thing to think about it is to focus on the total depth of the fiber, not the now thinner layer of fiberglass separately. A the compressed density the fiberglass and cellulose will have about the same performance per inch. So if you want an R75 attic, the initial blown depth needs to be at about 20-21", which will settle to about 18-19" in the first 5-10 years.

   The main criterion for soffit dams is that it be air-impermeable. Some people prefer to use cut up foam-board (any thickness), sealed in place with can-foam.

   Assuming the spray foam is closed cell or open cell polyurethane, the longer term health issues are pretty small, but the amount of outgassing is highly dependent upon the of the installer. An improper mix or installing at an improper temperature can end up with longer term outgassing. With open cell foam some lingering odor is normal, but that will drop considerably after a few days, though still be detectable with a scratch 'n' sniff at close range even a year later. There is some concern about the fire retardents used in all insulating foam, and people with other chemical sensitivities will sometimes react, though there is not widespread problems with it, only anecdotal evidence. The vast majority of installations have no health consequences to the occupants from the foam components, but occupational exposures to both the foam and the fire retardents is of at least some concern, and installers should always take precautions to limit exposure. See:

   http://www2.buildinggreen.com/blogs/epa-raises-health-concerns-spray-foam-insulation