I'm converting my attic into a non-vented conditioned space. Should I use open cell or closed cell foam to insulate?


I'm converting my attic into a non-vented conditioned space. Should I use open cell or closed cell foam to insulate?

Asked by Gary Lichtenstein

I'm considering converting my vented attic into a non-vented conditioned space. I've been warned that closed cell foam can lead to sheathing rot from either roof leak or vapors from the house. If the foam is to be used on the roof, would open cell foam be the material of choice since it would allow water to escape in either direction?

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Steve Saunders's picture

The debate over closed cell vs. open cell insulation on the underside of roofs is an interesting one.

  • There are advocates and arguments promoting both types of home foam insulation.
  • The problem of sheathing rot is real.
  • However, most roofers working in a hot and rainy area will tell you that many roofs they repair have some rotten sheeting -- independent of any foam insulation applications.

Closed cell foam

Closed cell foam acts as a vapor barrier and will not absorb the moisture that comes through a leak in the roof.

Properly applied, it will prevent moisture from penetrating into the attic area. In this case, the water will pool on top of the closed cell foam and possibly next to the sheets of decking on the roof.

  • The water may or may not dry to the roof side.
  • If it dries in a timely fashion, there will probably be no rot in the sheeting. If it does not dry for a long period of time, rot will likely occur.

Unfortunately, there is no good way to predict when it will dry.

Open cell foam

Open cell foam, when properly installed with a vapor barrier, should operate very similarly to closed cell foam. That is, it too will prevent moisture from penetrating into the attic area. Any water that leaks through the roof will pool on top of the vapor barrier and possibly next to the sheets of decking on the roof.

However, when open cell foam is installed without a vapor barrier (which appears to occur often in real-world applications), the foam will absorb the water. As long as it remains wet, the open cell foam could potentially cause rot and mold.

The virtue of the open cell foam is that the water can soak through it and potentially dry to the outside as well as the inside.

Open attic vs. finished living space

Your stated goal is to turn your attic into a non-vented conditioned space. However, you do not describe the potential uses for that space, and that gives me pause as to how to properly evaluate the risks of your options.

If you planned to leave the open cell foam exposed to the attic, it would certainly dry to the inside much more easily. An open attic might be used for storage or to take heat load off attic-based HVAC equipment and duct-work.

But you may plan to finish the room for living space. This might entail encapsulating the foam with roofing on the top and sheetrock on the underside of the foam. If no vapor or air barrier were installed, the foam could still dry to the inside and the outside. Here the concern is roof integrity and how much water the open cell foam will be asked to manage. There are limits to the amount of bulk water the foam can absorb. In addition, the foam may absorb so much and hold so much that it may not be able to dry fast enough to avoid problems from rot or mold.

The best outcome

So far, the discussion of this question is about whether there is a "best product solution." However, the best product that is improperly applied is still likely to cause problems. For most consumers, the more strategic question is what process will generate the best outcome.

Get an assessment of your roof's condition

One of the first things you should do is get an assessment of your roof's condition. There are three basic types of professionals who evaluate roofs. They are roofers, insurance company property adjustors, and a much smaller cadre of professional engineers who work on both commercial and residential roof assessments.

  • Your insurance company may be able to give you a good recommendation on an adjustor in your area.
  • Alternatively, a roofer can be a great source for a free or low-cost assessment. I recommend offering to pay for an assessment rather than getting a free evaluation as part of a contractor's sales price. The cost of the assessment will depend on roof height, type of roof, the urgency of your situation, and other factors, but you should be able to get a low-end assessment for $300 to $500.
  • Using a professional engineer might cost more. As a general rule, these experienced professionals are on roofs every day and can quickly find the areas that show signs of deterioration and give you a reasonable estimate of the remaining life of your roof.

If the roof currently leaks, or if the roof has a limited remaining life, it would be prudent to fix or replace the roof as a first step. It is best to have a long-term roof in place prior to making an investment to convert your attic. Installing foam in your attic will be expensive, and the foam could be damaged or seriously compromised by water intrusion. If you don't want to have to replace the foam, then the current roofing solution needs to last a long time. For me, "a long time" would be at least 10 to 15 years.

Find the best foam installation company in the area

Second, I would focus on finding the best foam installation company in the area.

  • Check references and years of experience. Preferably, you want a company that has done foam for two or three years at a minimum.
  • What you are looking for is an organization that has done 50 or more installations and understands the range of issues that must be addressed for a quality installation.
  • You want an organization that has made the installation errors on someone else's home and will apply the lessons of experience in yours.

The State of Virginia's website includes a consumer reference guide to selecting a contractor called "What You Should Know Before Hiring A Contractor." That guide may give you some additional tools to help you evaluate possible contractors.

I would select a firm that sells both open and closed cell foam. This way you have the opportunity to understand comparative costs and tradeoffs. A company that sells both types of foam is less likely to be biased in one direction or another and, in general, will have sufficient experience to outline the pros and cons of each foam installation strategy.

The outcome remains with the home, so keep focused on quality and long-term value as you select among various companies and product pricing options.

For more information:

Check GreenHomeGuide'sbest attic insulation Q&A to see what other homeowners and contractors are saying about vapor barriers, open cell foam, closed cell foam, etc.

Visit Icynene's website for information on one of the open cell foam insulation manufacturing companies. Visit Thermoseal's website for information on one of the closed cell foam insulation manufacturing companies.