A contractor recommended open cell spray foam for the attic of our 27 year old house in Atlanta, GA. What do you think?

Clark,

Thank you for submitting this question for review. As a proponent of spray polyurethane foam (SPF) insulation I always make sure that my clients participate in the costs, benefits and risks associated with this product in their new or renovated structures.

SPF can be applied as an open-cell or closed-cell product. Each type of material has its strengths and weaknesses.

Having inspected thousands of projects insulated with SPF I have developed a bias towards SPF based on the following questions.

• What is the experience of the installing contractor?
• Are you living in a mixed humidity climate?
• Do you use your attic space for storage or some other purpose?
• What type of roof covering do you have?

I will cover each of these concerns below to help you make your own informed decision.

Contractor's experience

The SPF market's growth has exploded in recent years. Unfortunately the increased numbers of contractors entering the SPF arena are not experienced in applying the material.

• Contractors that have sprayed closed-cell foam for years find it very difficult to properly spray open-cell.
• The same is true if they are used to spraying foam in a warm weather versus cold weather state.

So, before selecting a contractor make certain that they have been trained and are experienced. Go to www.spfa.org for a list of certified SPF contractors.

Climate

If you live in a mixed or high humidity region you should not spray foam directly against the underside of the roof sheathing. 

• Moisture from the living space can saturate open cell foam sprayed directly to a surface exposed to colder temperatures. 
• Closed cell foam can trap water in the roof structure resulting in structural decay.
• I have also had clients that have had serious health issues as a result of SPF applied directly to the underside of the roof with direct exposure to south or west sunlight.

If you are giving serious consideration to applying SPF to the underside of the roof you should have them install baffles that run the entire length of the truss cavity so any moisture or heat can vent. Open-cell should be at least 6-inches in depth and closed-cell 4-inches (or more of each depending on local code).

Attic use

If you do not use the attic for a habitable purpose you might want to keep your thermal barrier on the floor of the attic. 

• Removing the old insulation would allow you to air seal the ceiling/framing penetrations with open-cell foam before installing new cellulose insulation.
• The foam will act as a sealant to prevent hot air from finding a path into the home in the warm weather months or into the attic in the cold weather months.

This is cost effective and gives you the greatest return on investment.

Roof coverings

Roof coverings are best used to keep the attic dry and reflect UV rays in sunny states.

• All roofs should breathe so applying foam directly to the structure beneath the roofing is not in the best interest of the property owner.
• Hot roofs typically wear out faster than vented roofing material.

When the time comes that a roof replacement is necessary it is more costly if you have insulation adhering to the sheathing when sheathing needs to be replaced too. So, select a UV reflective roof and keep it vented.

Good luck,

The General

For more information:

Read "I'm converting my attic into a non-vented conditioned space. Should I use open cell or closed cell foam to insulate?", a Q&A answered by Steve Saunders.