Attics with spray foam insulation are so cool. We mean that literally, by the way.

When you insulate the top of your attic with spray foam, you’re bringing your attic into your home’s envelope. As a result, you can reduce the temperature in your attic by a lot – anywhere from 120 degrees to around 95 during peak summer. If you want, you can even transform a spray-foamed attic into a finished space!

But in many spray foamed attics, it can be hard to manage humidity. During the spring, summer, and fall, relative humidity (RH) in a spray-foamed attic often exceeds the comfort threshold of 60%. When it exceeds that threshold by too much – and for too long – you could experience problems, like mold growth.

If you’ve got a spray-foamed attic and you’ve got high humidity, here’s how to fix it for good.

1. Make sure your attic is properly sealed.

As with pretty much all home improvement projects, your contractor might have done a great job spray-foaming your attic… or not.

In a proper spray foam job, a contractor doesn’t just spray foam all over the top of your attic and go home. They also need to seal off areas of air leakage to the outdoors. Otherwise, the humidity will quickly creep into your attic.

Gable vents and soffits are the big culprits here. So are other gaps and cracks that let in outdoor air. Check and see if all of those areas are sealed. If they aren’t, call your contractor back and tell them to finish the job.

2. Check for negative pressure.

If mechanical systems in your home create a negative pressure environment, you could experience spiked humidity levels in your attic. Examples include:

  • Duct leakage: If you’ve got leaky return ducts, they might be sucking air from your attic into your HVAC system. This situation increases air infiltration from the outdoors into your attic. The result? High attic humidity.
  • Powerful kitchen fans: A lot of kitchen fans (aka range hoods) move air at a rate of 1000 CFM or higher. If you’ve got one of these, it could be pulling air from your attic into your home every time it runs. What kind of air enters the attic to make up the difference? Humid outdoor air. That’s what kind.

There are a couple of ways to test whether this is happening. One way is to test for duct leakage. We use a duct blaster test for that. Another way is to perform a blower door test to get a sense of your home’s pressure balance. We do this as part of our whole home assessment.

After sealing up your ducts and achieving a proper pressure balance in your home, check the humidity levels in your attic. A simple humidistat can help here. Or, if you’re a data junkie like we are, you can install an RH sensor that reports humidity levels in the form of a graph. That way, you can see when (or if) humidity spikes at certain times of the day or during the times you engage in certain activities… like running that powerful kitchen fan.

3. Install a dehumidifier in your attic.

Still got high humidity. A dehumidifier can remove it.

After from sealing things up and fixing your pressure balance, this is usually the best solution to high humidity in a spray-foamed attic. Put a dehumidifier up there, plug it in, set it between 50 and 60% RH, and forget about it. Bye-bye, humidity!

Ok, it’s a little more complicated than that. You’ll have to dispose of the water the dehumidifier removes. Since you probably don’t want to go into your attic twice per day to empty a dehumidifier reservoir, you’ll want a unit that drains itself and a dedicated condensate drain just for the dehumidifier. It should drain just like your AC.

Oh, the dehumidifier also does the double duty of moving air around. The next time you’ve got work to do in your attic, it won’t be quite as stuffy up there.

You could run some air conditioning ducts to the attic, but…

There are downsides.

For starters, your AC will only dehumidify the attic while it’s running. So, if you don’t run your air conditioner much during the spring and fall, the humidity will continue to build up in your attic.

Then there’s the question of pressure. By blowing “new” air into your attic, you might positively pressurize the space.

Why is that a problem? Well, positive pressure could force attic air into your home. And as we all know, attic air isn’t exactly, um… super clean.

To be sure, a spray-foamed attic is typically less dusty than an attic with blown-in fiberglass or cellulose. However, positive pressure can force dusty, smelly attic air into your living space through unsealed air gaps. If anyone in your home is chemically sensitive, there’s a small chance that blowing “spray foam air” into your living space will cause discomfort. Not likely, but also not inconceivable.

And never mind the fact that you’re stealing the air from your AC to condition a space where nobody lives!

Adding air conditioning to a spray-foamed attic only makes the most sense if you’re planning to finish the attic, make it into a room, and spend time up there. Otherwise, a dehumidifier is the better option.

Humid spray foamed attics are pretty common.

It’s true! Even when a spray foam contractor does everything right, you might still have high humidity in your attic.

More often than not, all you need to do is verify that everything is sealed and, if needed, install a dehumidifier. After that, put a little humidistat in your attic and check it every once in a while, to make sure everything is gravy.

You’ll be a whole lot more comfortable (and drier!) the next time you open the hatch and ascend the fold-up ladder.

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