We want to optimally insulate our attic, especially since it has our main HVAC supply ducts in it.


We want to optimally insulate our attic, especially since it has our main HVAC supply ducts in it.

Asked by Randal Smith

We live in Northern Kentucky and have an older 1.5 story cape cod style home that we built onto the back with a shed style roof. It is ventilated, from the eaves, up into the side attic behind the 2nd floor knee walls, up into the attic and then out. It has a metal roof with good underlayment. In the addition our roof slope is 3/12, with 2x6 roof rafters, and the area is packed with the new main HVAC ducting. The existing house has 9.5' celings and the addition only has 8' celings. We want to optimally insulate this area, especially since it has our main HVAC supply ducts in it. But between the limited space and the HVAC ducts our space is limited. Can we attached rigid foam insulation board under the roof rafters creating a ventilation path up into the side attic, and then fill the area under the rigid foam down to the ceiling with blown insulation? Could we also pressure fit say 2" to 4" of rigid foam between the rafters, then place say 1" sheet under the

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Scott Grieves, CPM, CEA's picture

First thing I would research: do I want a cold attic or a warm attic where my duct-work is located. I do not believe the foam board would be the best idea because of your weather/ climate.

Please consider close-cell spray foam or consult a local insulation company.

Consider what you need? Insulation Thermal Performance Factors!

  • A reduction in thermal bridging.
  • High or Low density and type of insulation. (recommend high-density polyurethane foam)
  • Reduce air leakage and gaps. (foam board will not stop this)
  • Area/ mass of the insulation assembly is restricted. (low roof pitch)
  • Duct work in un-conditioned space. (not a good idea and will add to increase energy cost's)
  • Convection-conduction-radiation-heat transfer should all be address when you are considering the best way to insulate your home.
  • No ice bridging/ damning on roofs.

I recommend: spray foam high-density which has an R-value R-6.0 to R-7.0 per inch. Great for warm or cold climates and limited space. In my opinion, this is one of the best ways to insulate the roof. It solves the infiltration and moisture problems. It also provides a warm attic, which is useful for storage and suitable for HVAC equipment and ductwork.

As with wall systems, it costs more, but it works.

Reference: Energy Star.gov, and Residential Building Insulation Systems for Cold Climates by: Joseph T. Kohler, Ph.D., P.E.

Regards, Scott

For more information:

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