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Spray Foam Attic Floor or Rafters: Which is Best to Insulate?

attic insulation

Spray Foam Attic Floor or Rafters: Which is Best to Insulate? Blog Feature
Eric Garcia

By: Eric Garcia on February 14th, 2020

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Are you thinking about re-insulating your attic and wondering if it is better to spray foam the attic floor or rafters?

Either attic spray foam insulation system will work just fine, but the best way to insulate the attic really depends on your situation.

Being a residential spray foam insulation contractor since 2002, we’ve insulated our fair share of attics with spray foam, sometimes the attic floors and sometimes the attic rafters. Both systems work well, they are just different and have their own pros and cons.

In this article, I’ll discuss the reasons why you may want to insulate the attic floor or rafters so you can determine what is the best for your specific situation.

Spray Foam Attic Floor

Let's talk about the attic floor first. 

The first big pro to insulating the attic floor over the rafters is that if you measure out the square footage, you're going to have less almost every time when you're looking at the attic floor. 

Now, why is that important? 

Well, most of the time your cost is directly related to how much square footage is being foamed. So obviously the attic floor is going to be the most cost-efficient option, especially if you have a steep attic or a complex roof design with multiple peaks.

The biggest thing to keep in mind is if you're insulating the attic floor then you're going to need ventilation. Now, is that a pro or con? Sometimes that's up for debate, but keep in mind you will need ventilation usually in the form of soffit vents, baffles, and a ridge vent up top. 

It is also necessary to point out that insulating the attic floor cuts off the attic from the rest of the home, making it an unconditioned space that can experience vast temperature swings depending on the weather.

Spray Foam Attic Rafters

So let's move over and let's talk about insulating the attic rafters real quick, also referred to as the attic ceiling, roof deck, or a hot roof system

Insulating the attic rafters with spray foam creates a conditioned space where your attic is the same temperature as the rest of your home. This keeps the external weather and moisture outside of the attic space so you don't have to worry about any moisture problems or air infiltration. 

You also do not need roof vents when you insulate the attic rafters. Is that a pro or con? If you ask three contractors, you'll probably get four answers.

Generally, our rule of thumb is to seal everything up as tight as possible and rely on your mechanical ventilation to give your house the air exchanges that it needs. So when you insulate the attic rafters no soffits, baffles, or ridge vents are needed.

Another major difference is that if you insulate the attic rafters your floor is free. You may not have actual flooring on your attic floor, maybe it's just open trusses. This method creates the opportunity for you to lay a floor down and use that space for storage or potentially a living space if it's large enough.

Finally, if there are appliances in the attic such as an HVAC system or even just central air, which is common in southern climates, you almost always want to insulate the attic rafters in this situation. This is because you want those appliances inside the building envelope to help them run more efficiently.

Choosing the Best Way to Insulate Your Attic With Spray Foam

At the end of the day, both attic spray foam insulation systems work just fine. 

You just have to realize they are different systems with ventilation being the main difference. If you insulate the attic floor, you're going to need free air vents and make sure they are maintained properly over the long term. 

Suggested Reading

Vented vs. Unvented Attic: Which is Better?

Open Cell or Closed Cell Spray Foam: Which is Best for the Attic and Roof?

Why Does Old Insulation Need to be Removed from the Attic Before Installing Spray Foam?


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About Eric Garcia

Eric brings his knowledge and training in building science, training in spray and injection foams from the manufacturers, more than 8 years installing foam insulation, as well as selling and managing in the foam insulation industry. He is also BPI and Dale Carnegie certified and has taken several building science courses including air sealing and building envelope. Eric’s responsibilities include overseeing and giving support to all of the branches of the RetroFoam of Michigan company, office, estimates, and installs. He is also the Professor of Foam on our educational YouTube series Foam University. Even when Eric is off he is usually still “working” or thinking about work, but when he can get away he enjoys camping, hiking, hunting, and woodwork.