Spray Foam Insulation for New Home Construction

February 4, 2011 | by Fred (email) |

Reader Question: My wife and I are going to be building a new home this spring and have been told that insulating our basement with closed cell spray foam and our exterior framed walls with open-cell foam was the way to go. Is this true or the best way and if so why? Is it because the basement needs a good vapor barrier and the framed walls need some air movement to allow things to dry so mold and mildew don’t form.  Also what would be the way to insulate the attic. We live in central Iowa so we get some pretty cold winters as well as some hot and humid summers. Any advice would be greatly appreciated. — Ben

Ben, thanks for the question. This is a complex subject and many builders disagree on the exact right approach. For some reasons why this question is so complicated, I recommend reading this article over at Building Science on vapor barriers and insulation and this article over at Green Building Advisors on open and closed cell foam. Remember, before acting on any advice, read our disclaimer.

Basement Insulation

The builder is recommending closed cell foam for the basement for a few reasons. First, as you said, closed cell foam acts as a vapor barrier, which is important in basement installations. In the summer, humid indoor air can travel through the interior wall and come into contact with the cold exterior basement wall, condense into liquid, get trapped in the wall, and ultimately foster a mold problem. A vapor barrier prevents this from happening. It also prevents other water exchanges between the indoors and outdoors (as described in that Building Science article). Most close cell foams form a vapor barrier at between 1.5 and 2.5 inches of application.

Second, since the basement is below grade, if there ever were a water leak on the inside of the house (e.g. burst pipe), closed cell foam will not absorb water, and thus is much more mold-resistant. Open cell foam will act like a sponge, which will absorb water and actually promote mold growth.  Note that if there is a water leak from the outside of the house coming in through the block (e.g. because you didn’t keep your rain gutters cleaned and the water floods right in front of the house), closed cell foam could mask a water problem on the block wall that might otherwise become obvious more quickly. However, I think the risk of this is low, and in all likelihood, if you have that kind of water problem you are going to be in trouble with whatever insulation you install. In this situation, spray foams (both open and closed) are just harder to deal with because they are sprayed in place and are difficult to remove.

Besides dealing with the walls, you may want to consider insulating the floor in the long run. Todd’s article on basement insulation gives some coverage on this topic.

Insulation for the First and Second Floors

The builder is likely recommending open cell foam for the upstairs because a water problem is much less likely on above-grade floors and open cell foam is cheaper to install (although it insulates at about 1/2 the R-value of open cell, per inch). I doubt that the concern is over drying as the builder is likely going to install a vapor barrier just behind the exterior siding or brick work anyway – potentially rigid foam board or polyethylene plastic. The article I cite above at Green Building Advisors has some thoughts on using open vs. closed cell foam that are worth a read on this; they also have some forums there where you can ask questions.

I have heard of more and more homes being insulated completely in closed cell foam because it provides a superior bi-directional vapor barrier and actually adds structural rigidity to the house without the danger of the insulation itself promoting mold, but it remains to be seen whether such approaches would ultimately result in other moisture problems that might actually lead to mold promotion. The exterior materials used to cover the house may have an impact on this decision – especially if those materials are water absorbers, like brick. You definitely want to make sure that if you have a brick exterior (even just a single wall facade) that appropriate plans are in place to keep water off of the interior walls and to ensure that water vapor isn’t trapped between the interior walls and the exterior facade in such a way that drying becomes a problem.

Insulation for the Attic

In terms of what insulation to put in an attic, there are a few competing theories. Some say to insulate the peak of the attic with closed cell foam and seal up all vents to make the attic more comfortable (and actually add strength to the roof). Others say to insulate the floor of the attic with closed cell foam and leave soffit and ridge vents to allow the attic to breath. Still others say to insulate the peak of the attic with open cell foam so that if there is a leak in the roof, the water can escape from the wood and won’t create wood rot in the beams and plywood. As you can see, there are a lot of opinions.

The Right Next Steps

My recommendation is to ask the builder questions about why they are recommending the products and methods they are.

If it were my home, I’d tend towards closed cell foam for the whole house. Why? Because closed cell foam offers 100% air sealing, has higher R-value per inch, and increases the structural rigidity of the house. Sure, you’ll pay more. But if you live in the house for 10 years, you’ll probably make up the cost in energy savings. The only drawback I would be concerned about is the breathing issue – and I would consult a pro to determine if the exterior materials would prevent the use of closed cell throughout.

We installed closed cell foam in our basement last year, and installed open cell in the rim joists. You can see the spray foam video we created during that project. If I could do-it-over, we would have installed closed cell everywhere.

Good luck with your house!

(Image Credit: Giles Douglas)