Cellulose vs fiberglass
tdbaugha
| Posted in General Questions on
Hello,
I have read about windwashing causing loose fill fiberglass insulation to be derated in true R value whereas cellulose is not. Does anyone have a link to test data showing this? Or other information to share?
Does the same apply for a BIBS fiberglass vs wet sprayed or dense packed cellulose in the walls?
I’m just looking for a justification for spending more money for the same basic install cellulose vs fiberglass.
Thanks
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Replies
If you have windwashing in your wall cavities you have a much bigger problem on your hands. Everything should be air sealed.
There is no good justification for spending more on cellulose in the walls. Fiberglass has a slightly higher R-value, it is a little less environmentally friendly but still way better than any foam or mineral wool options. The blown-in blanket methods give you a nice install if the installer does a good job.
tdbaugha,
If you want to dive into actual numbers, see table 8 and the accompanying text in this link:
https://www.rdh.com/wp-content/uploads/2017/10/Van-Straaten-Windwashing.pdf
Mostly curious about attic windwashing. Although the link Malcom provided is kinda neat for comparing exterior insulation. Looks like rockwool and eps are both pretty solid.
I did further reading today and it seems like 20-30 yr old fiberglass was more prone to the reduced R value as compared to modern blends.
tdbaugha,
I haven't seen comparative numbers for attics, just general advice cautioning against the fiberglass, like this from Hammer and Hand's Best Practices Manual:
"Do not use blown-fiberglass due to light density / wind washing".
My advice to stick with cheaper fiberglass was specifically in reference to exterior walls. But, interestingly, even if you put fiberglass in the attic and it lost the full 0.7 R-value that RDS article stated, it would still be very close to the R-value of cellulose at R-3.5.
Edit: Actually I read the table incorrectly. The loss of R-0.7 is across the entire R-13 batt, it’s not an R-value per inch loss. So in that case the fiberglass batt matches the cellulose. It’s basically R-7.3 fiberglass vs R-7.25 cellulose for the same 3.5” stud bay. I would still vote for BIBS fiberglass, your effective R-value should be higher than a batt. Owen’s Corning BIBS system is marketed as R-4.2 per inch.
Are you including any HVAC equipment in your attic space?
matthew25,
What's not clear is how much wind-washing attic insulation experiences. The RDH research assumed fairly minor air-movement in the walls. Articles like the recent blog by Allison Bailes seem to show it can be quite high in attics. https://www.greenbuildingadvisor.com/article/the-impact-of-wind-washing-on-insulation-r-value
No HVAC in the attic.