Question

I am thinking of supplementing the fiberglass insulation in my attic.

Asked by Jerry H.
Hopkins, SC

Is there any negatives to this product? how does it compare with fiberglass since it is denser is it a better insulator? How is it to work with compared to fiberglass?

Answer

Michael Holcomb

Answered by Michael Holcomb

Byron Center, MI

Alliance for Environmental Sustainability (Headquarters)

September 22, 2011

Jerry,

I can't answer your question because it doesn't contain information about the product you want to compare to fiberglass.

I am going to respond in general to improving attic insulation. If my comments don't touch on your concerns please resubmit your question with more detailed information.

  • Your climate zone is 3A indicating you are predominantly a cooling state.
  • I am working on the assumption that your summer energy bills are a bigger concern than winter heating bills.
  • Regardless, the solution to reducing energy bills associated with heat loss/gain between conditioned space and the attic is the same.

Summer heat transfer

Fibrous insulation is very ineffective at reducing heat transfer through convection. The lower the insulation density the greater the rate of convective heat transfer.

  • Fiberglass does not have the density to effectively reduce convection.
  • Adding any type of insulation over a fiberglass base of insulation will give you very little improvement.

Sealing thermal by-passes

Have you considered removing the existing fiberglass insulation? By doing so you will have access to the attic floor (ceiling of the conditioned space).

  • With the entire attic floor exposed your contractor can seal the thermal by-passes that exist in every frame structure.
  • Spray polyurethane foam insulation is an effective material to seal large thermal by-passes.
  • Areas of special concern are the tops of partition walls, the perimeter walls at the truss heels and all wire and pipe penetrations.

New insulation

Once the attic is sealed you can install new insulation with a density greater than loose fill fiberglass (14" of cellulose is a cost effective option). You'd now have an attic that significantly reduces both conduction and convection.

In a cold weather state this system could reduce your heating costs by 50% per year.

Radiant heat transfer

The only other heat gain issue you may want to deal with is radiant heat transfer (heat that travels on light).

  • Radiant barriers are often recommended to "bounce" heat away from a surface.
  • That's why you'll see many commercial buildings with white (reflective) roofs reflecting radiant heat back into the atmosphere to reduce the air conditioning load on a building with a flat roof.

Adding a radiant barrier to an existing home is a bit tricky.

  • You can't place a radiant blanket on top of the insulation because moisture has a tendency to build up vapor from the interior of the house.
  • Applying a radiant blanket under the insulation is totally ineffective. In order for a radiant barrier to work it must be exposed to light.
  • So.... installing a radiant barrier might not be effective.

Another strategy to remove heat build-up during the heat of the day would be to increase the attic ventilation. You may want to consider installing one or more solar operated power exhaust vent fans. When installing power exhaust vents make certain that you have adequate soffit (or gable end) ventilation to prevent pulling air from the interior of the house.

Hopefully you will find the response to the question that you meant to ask.

Good luck and don't hesitate to post a complete question to Green Home Guide if I totally missed your intended question.

Tagged In: attic insulation

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