Replies

1. Expert Member
   BILL WICHERS | Nov 16, 2019 09:55pm | #1

   In the past, I’ve just glued together two pieces to make the L shape. I make the L ahead of time, and take it up for installation a few days later after the glue has fully cured (the foam board glue is kinda slow to cure). I haven’t found any factory made L shape foam pieces, except as a shipping material which I wouldn’t use for building insulation.

   Bill

2. 
   user-7572502 | Nov 16, 2019 10:34pm | #2

   Bill-

   Thanks. That was my first idea, still thought I would ask.

   Do you or others know where diminishing returns starts to play into things when blowing in insulation on non-energy heel roof systems? In other words, it doesn't make sense to blown in 20" in the field if you can only get so much depth into the short part over the plates. What is the "most" insulation an attic can take before adding more insulation is "pointless"?

   Thanks
   Aaron

3. 
   Jon_R | Nov 17, 2019 07:50am | #3

   A lack of insulation in one area has pretty much no effect on the heat loss in another area. So the right question is simply how much insulation is environmentally cost effective in any given situation.

1. 
   user-7572502 | Nov 17, 2019 03:33pm | #4

   Jon,
   yes, that might be a more succinct, more clear presentation of my question.

   So, if all you can get is say 7 inches of loose fill blown over your plates, how does one calculate the fall off of environmental cost effectiveness of blowing more (how much more) than 7" in the field?

   Thanks,

   Aaron

4. Expert Member
   Akos | Nov 17, 2019 03:50pm | #5

   Start here:

   https://www.energyvanguard.com/blog/76941/The-Diminishing-Returns-of-Adding-More-Insulation

   Depending on your fuel costs, for me in zone 5, its 12" of cellulose. Code calls for more, but the payback for the extra bit is very long.

5. 
   user-7572502 | Nov 17, 2019 05:57pm | #6

   Akos,
   thanks for the link, via that article I ended up at this link as well
   https://www.energyvanguard.com/blog/25547/Flat-or-Lumpy-How-Would-You-Like-Your-Insulation
   which really gets at the same thing I'm trying to get at with my ext. plate question.
   The second article points out that unevenly distributed, lumpy insulation will lead to lower than expected R and U values because heat loss will happen more quickly through areas that are less insulated relative to adjacent areas that are relatively more insulated.
   in other words, if you have a "mound" of insulation directly under a ridge, then progressively less deep areas as you approach the top plates, heat loss will occur more rapidly via the less deep areas.
   the second article points out that if you have lumpy insulation, simply raking it level might provide better performance than simply adding more insulation and leaving in the lumps.

   Without a raised heel truss, it is not possible to "rake out" a massive mound, if you blow in any more than the height from top plate to attic baffle height, you will always end up with a mound of some height.

   In a theoretical world, maybe one could use spray foam to get a higher r value/ inch around the ext. top plates, then blow in an equivalent r-value worth of loose fill in the field.

   Still thinking about how best to address my current scenario. This helps.

   Thanks,

   Aaron