Should I Use Spray Foam Insulation in My Attic in Seattle?

Are you thinking about insulating your attic with spray foam because you've heard it's the best way to improve energy efficiency? Are you worried about making an expensive mistake that could actually trap moisture, cause hidden mold and rot, or create problems that are difficult and costly to fix later?

At RoofSmart, we've inspected thousands of attics throughout the Seattle area, and we've seen firsthand what happens when spray foam is used in the wrong place. While it has legitimate uses in certain roof assemblies, we've also had to tear out spray foam to repair severe condensation damage hidden beneath it. That's why we believe homeowners deserve to understand both the benefits and the risks before making a decision.

In this article, we'll explain when spray foam insulation makes sense, when it can create serious moisture problems in Seattle attics, how it compares to other insulation options, and which insulation systems we recommend for different types of roofs.

Spray Foam Insulation Risks, Uses, and Better Alternatives for Seattle Attics

What happens if I use spray foam insulation in my Seattle attic?

Spray foam is often misunderstood as a great vapor barrier for your attic because it is considered a Class II vapor barrier (Class I is the highest). However, using spray foam in your attic can lead to catastrophic failure.

Here are the risks of using spray foam in your Seattle attic:

• Trapped Moisture: Wood is porous and wicks moisture. Spray foam completely covers the wood in your attic, trapping any moisture that might’ve already been there. So, even though it might keep new moisture away from the roof, the wood underneath isn't guaranteed to be dry.
• Condensation: The trapped moisture can condense against the cold roof decking and cause severe rot or mold.

• Mess & Off-Gassing: So often, I don't recommend it because you will spend more money to get the spray foam machine in the attic. But it can make a huge mess, and you have to deal with all the off-gassing. Chemicals will release in the attic. Some homeowners are bothered by this, but others aren’t.
• Difficult removal: Spray foam is extremely difficult to remove for repairs, often requiring carving out rotten wood. Our crew has had to do this before. We had a customer who bought a house with spray foam insulation but no condensation barrier, so the wood in the attic was rotting. But the crew had to carve out the spray foam just to access the rotting wood underneath. In this picture, you can see the remnants of spray foam. But those empty spaces were all filled with spray foam:

• Insurance issues: Some insurance companies are denying coverage for people with spray foam because there have been problems with it getting glued to everything and being hard to get off.

Moisture and condensation are a huge concern for Seattle attics, so it is crucial that you understand which types of insulation will protect your roof from water damage. Unfortunately, many homeowners don’t know that spray foam will fail at keeping moisture out.

When is spray foam a good fit?

The best way to use spray foam in a roof assembly in Seattle is if the spray foam is not being used as the vapor barrier. If you're relying on it to keep moisture out of the assembly, it will fail. It’s much better as an air barrier.

Here are some examples of situations where spray foam is an effective and ideal solution:

• In Crawl Spaces: It will be effective as a vapor barrier on walls and floors of your crawl space because even if moisture somehow makes it through the floor, it's just getting into the house. It doesn't hurt anything. It's okay to have a little humidity in your house.
• In Hybrid Systems: If you're using it as part of a hybrid approach where you're going to spray foam the bottom of maybe a rooftop deck, and then you're going to put rigid foam on top of it, that'll work.
• When There Is Limited Space: It is the highest r-value per inch, so spray foam is useful where high R-value per inch is required. For example, when you only have 3 inches to work with. However, it can't be your condensation control layer, so you need to account for that as well.
• When You Need an Air Barrier: It works really well as an air barrier, but it's expensive. The cost is probably going to be around $1–2 per inch per sq ft. So, if you wanted 6 inches of spray foam as an air barrier, you’d be spending around $10/sq ft.

Since spray foam encases whatever it’s touching as it dries, it is good at keeping air out (which is why I sometimes recommend it for air sealing your attic). However, there’s always the risk of moisture being trapped in and getting through to the roof decking, which is the last thing that you want.

So, when you’re deciding whether spray foam is a good fit for your attic, it’s important to consider what you’re trying to accomplish. It won’t cut it as a vapor barrier, but it still has its uses.

How do you know which type of insulation is right for your roof?

There are many different types of insulation that you can choose from for your attic. Many Seattle homeowners gravitate towards spray foam because it’s an easy and relatively cheap option.

But choosing the right type of insulation for your attic depends on the type of attic you have.

If you have a flat roof or a roof with a complex layout, you’ll want foam board insulation that sits on top of the roof deck to protect it from condensation.

If you have a standard sloped roof, then fiberglass insulation in the attic is a classic go-to that works well. If needed, your roofer might suggest other options like cellulose or rockwool.

What is the best roof insulation for your Seattle home?

Spray foam isn't a bad product; it just isn't the right solution for every attic. In Seattle's cool, damp climate, preventing condensation requires more than simply choosing insulation with a high R-value.

Before investing in any insulation upgrade, make sure you're choosing a system that's appropriate for your specific roof. A vented attic, flat roof, vaulted ceiling, and rooftop deck all have different insulation requirements, and using the wrong approach can lead to expensive repairs down the road.

Read our article "Which Type of Roof Insulation Is Best for My Seattle Home?" to learn where each insulation type performs best, where it falls short, and how to choose the right solution to improve energy efficiency while protecting your roof from moisture damage.