Replies

1. 
   matthew25 | Jun 01, 2023 08:43pm | #1

   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.

2. 
   Malcolm_Taylor | Jun 01, 2023 09:14pm | #2

   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

3. 
   tdbaugha | Jun 01, 2023 10:22pm | #3

   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.

1. 
   Malcolm_Taylor | Jun 01, 2023 11:26pm | #4

   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".

2. 
   matthew25 | Jun 02, 2023 07:07am | #5

   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?

1. 
   Malcolm_Taylor | Jun 02, 2023 11:50am | #6

   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

2. 
   tdbaugha | Jun 02, 2023 08:32pm | #7

   No HVAC in the attic.