How does Icynene on the attic roofline/sillplate compare to old fashioned fiberglass or blown insulation?

Asked by Allyn McCormick
Rochester, NY

1946 ranch. no insulation except R-19 bats I put on attic floor. I have new windows. I would like to insulate and Icynene was recommended for sillplate and attic roofline, sealing soffit vents. Cellulose for walls. How does this new technology compare to old-fashioned bats or blown in insulation. If using Icynene, do the old fiberglass bats need to be removed?


Fiberglass batt insulation is made up glass fibers and voids which make up its insulating properties but does not protect the home from air infiltration and leakage which are the biggest forms of energy loss. 

Spray foam insulation such as Icynene (brand name) acts as an air barrier and seals gaps and crack within the walls and floors where the infiltration and leakage occur.

Spray foam installation

For proper installation of the spray foam insulation, it should be applied to the inside face of the exterior walls and surfaces.

  • If fiberglass insulation is present it will not allow for the spray foam insulation to be properly installed and could allow gaps with in the system where air can infiltrate or leak.
  • It is a good practice to install closed cell spray foam insulation on the sill plate and bond of the floor framing as well as the first one (1) foot of subfloor.

Covering soffit vents

Depending on the type of roof ventilation, it may not be a wise decision to cover the soffit vents.

Vented Attic Space: I would recommend removal of the existing batt in the ceiling and replace it with a heavy inch of closed cell foam to the attic side of the drywall/plaster ceiling followed by cellulose insulation.

  • At the proper levels you could easily achieve upwards of an R-50 (verify local code and energy codes for required R values).
  • When following this procedure you will want to make sure that roof baffles are installed to allow for attic ventilation provided by your existing system.

Non-vented Attic Space: For a non-vented attic space I would recommend the installation of open cell spray foam to underside of the roof deck to the full depth of the roof rafters.

  • Open cell foam will give you approximately R-4 per inch of depth of the roof rafters.
  • If the existing depth is not allowing for the minimum R value per code you may have to use closed cell foam which has an approximate R value of R-7 per inch of depth.
  • When and if you proceed with this method you will need to close in the existing soffit vents since the attic space will be considered as a conditioned space.


For more information:

Read "What is the safest and most effective insulation for our attic crawl space?" a Q&A answered by Ian MacLeod.

Tagged In: attic insulation, attic ventilation

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