The first thing to realize is both spray foams create an air seal, it really just comes down to what is a better fit for your project. In most cases, we recommend the open cell spray foam on the attic flat and roof deck, but there are instances where the closed cell is the better option.
RetroFoam of Michigan has been insulating attics since the beginning of time, or at least since we started this business close to 15 years ago. Every house is different, so there are times that we step out of what we normally recommend to make sure we are giving the best product for the job.
None of this answered your question, so in our continued efforts to educate homeowners like yourself, we will discuss what makes each material a good fit for the attic flat and roof deck.
Open Cell or Closed Cell Spray Foam in Attic
To spray open cell or closed cell foam in the attic, that is the question.
Like I said, both open cell and closed cell spray foam create the air seal you’re looking for in your attic. While they have this in common, there are some pretty significant differences that set them apart like their composition, expansion rate, moisture permeability, and blowing agents.
I think for the purpose of this article, the best way to determine which spray foam is best for your attic insulation is to just take a look at both and what makes them great.
Closed Cell Spray Foam in the Attic
There are a lot of people that will tell you closed cell spray foam is the only way to go for any project, but that’s not really the case.
Closed cell is very rigid, which makes it a great option for pole barns with exposed walls. It also has a low expansion rate, which also makes it perfect for projects like cargo vans.
So, why do so many people think it is the best option? Well, it comes down to numbers.
Closed cell spray foam has a higher R-Value, but you shouldn’t base your decision solely off of these numbers. R-Value doesn’t tell the whole story and should be more of a factor is you are considering traditional insulation, not spray foam.
If you’re considering closed cell because you want to add structural integrity to your roof, that’s not the way to go either.
This is another common misconception associated with closed cell spray foam. That being said, there are instances when it can add some structural integrity.
If you have a metal roof on your home, closed cell spray foam on the roof deck can add a little strength. This really comes into play if a tree branch or something hits your roof. The closed cell can keep the metal from bending from the weight.
Other things to keep in mind though is that if there is a leak, closed cell will hide it because it is water impermeable. That water will leak in, but have no place to go, making the leak unnoticeable to you until it is too late, according to Icynene. If a contractor needs to get into the attic and cut away the closed cell to fix the leak, it’s not pliable and cutting it can be very difficult because of its rigid composition.
The low expansion rate doesn’t work in your favor for the roof deck or flat, because it’s not going to fill the cavity the way you want without using more material and increasing the cost.
Another thing to be aware of is what you're roof deck is made of. In the case of closed cell spray foam, if the roof deck is made of oriented strand board, or OSB, there could be issues as the foam is curing.
The weight of closed cell is one factor for OSB, specifically if it is on the thinner side. The bigger issue is the curing process, as the closed cell will shrink a little bit, but when it does it pulls the OSB with it, causing structural damage.
If you have an OSB roof deck, you'll want to note how thick it is. If it's less than 1/2 inch thick in an area that's 24 inches on center in 2x4 rafters, then closed cell is not recommended.
With all of this being said, closed cell spray foam is a great material, but it isn’t always going to be a great fit for the attic.
Open Cell Spray Foam in the Attic
The expansion rate of open cell spray foam is a great reason to have it in your attic.
Open cell spray foam expands around 100 times its original size, which means it will fill all of the nooks and crannies. It is the cheaper option between the two and fills the cavity without having to add more material.
In the roof deck, there are joists and trusses where you’re going to find the smallest little gaps
If there is ever a leak in the roof into the attic, the open cell won’t conceal it like closed cell has the tendency to do. The water can move through it, without it retaining moisture like other insulation materials.
If the open cell needs to be cut to repair the leak in the roof, it’s very pliable and easy to work with.
The manufacturers we work with, as well as others in the industry, recommend open cell spray foam on the attic flat and roof deck because of its expansion rate and water permeability.
Choosing the Best Spray Foam for Your Attic
You can see through all of this compiled information why open cell is usually recommended and why there are instances where closed cell makes the most sense.
Whether open cell or closed cell spray foam best fit for your project, you’re going to get the air seal you’re looking for. It all comes down to that – which material is best for your home.
About Amanda Ringler
Amanda previously has worked as a breaking news and crime reporter, TV news producer, and editor in Flint and Detroit. Throughout her career as a journalist, she has won several awards from The Society of Professional Journalists - Detroit Chapter and the Michigan Press Association. As part of the RetroFoam of Michigan family, Amanda uses her experience as a journalist to write content that will help educate homeowners on the benefits of foam insulation. When Amanda isn’t writing, she’s spending time with her husband and rescued huskies. She also loves knitting, making art, cooking, and hosting dinner and a movie night for friends and family.