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Can You Spray Foam Insulation Over Old Fiberglass Insulation?

Plenty of homeowners have decided to treat their home with spray foam insulation, even though they already use fiberglass.  So a typical question is whether or not you would need to remove the fiberglass insulation from your home, or if you can simply have foam insulation sprayed over it.


The short answer is that you can spray foam over fiberglass, but there are several reasons why you should seriously avoid doing it.


How your insulation deals with moisture in the home


The first thing to understand about insulation and moisture in your home is what a "dew point" is.  When you walk outside early in the morning, there's often fresh dew on the grass.  It forms because there is water vapor in the air, and when the temperature changes, the vapor turns into the liquid water that gets on the grass.  We call the temperature that the change happens at the "dew point."


If you were to spray only a thin layer of spray foam insulation over the fiberglass that is already in your home, you wouldn't be able to keep the top of your foam above the dew point. 


Okay, so what exactly does that mean for your home?  It means that warm and moist air that passes through the fiberglass will turn into dew on your spray foam insulation.  Then you've got moisture trapped inside of your walls or attic, and that can lead to mold and other problems.


When you remove fiberglass and treat your home exclusively with spray foam insulation, it keeps the warm and moist air from getting to the surface and turning into dew.


Spray foam insulation doesn't stick to fiberglass well


When you spray foam into walls or your attic, it expands and forms an airtight seal.  This keeps heat out of your house, and keeps moisture from forming.  But if you try to spray foam over fiberglass, over time the foam will not stick to the fiberglass.  And again, you have to deal with outside air and inside hair from your home mixing, which leads to moisture (and mold).


So why keep is keeping moisture out important?


When cold air from outside of your home mixes with the warm air inside, you get moisture.  Moisture can eventually turn into mold, in both your attic and your walls.  The New York times states that mold in your home can lead to plenty of health problems including:

  • Asthma attacks

  • Sneezing & coughing

  • Itchy eyes & allergic reactions

  • Permanent lung damage

If you get too much mold in your home, removing it can cost you thousands of dollars.  So if you want to keep your home sturdy and your family healthy, do what it takes to keep moisture from forming in your walls and attic.


Using only spray foam means keeping your home cooler


If you remove your fiberglass insulation, and only use spray foam, you will keep heat out thanks to a better R-Value.  What's an R-Value?  It's just a term that is used to tell how well an insulation keeps heat out.  The higher the R-Value of an insulation, the better it will keep heat out of your home.


The U.S. Department of Energy website tells us that fiberglass insulation is between R-2.9 and R-3.8 per inch, which is a bit low.  But spray foam insulation is as high as R-6.6 per inch.  As you can obviously see, spray foam works literally twice as hard to keep heat out of your home.


As everyone knows, if you run your air conditioner less, you're going to pay less in electric bills every month.  So if spray foam insulation used by itself keeps your home cooler than fiberglass, or fiberglass and only a bit of spray foam, you won't need to run the A/C as much.  Some home owners that had fiberglass removed, and spray foam added, saved as much as 50% on their monthly electric bills.


Spray foam insulation by itself can be a safer choice


There's another good reason to not spray foam over fiberglass, and it has to do with fumes.  Here's something that many people don't realize: fiberglass is often held together by something called a resin.  That resin can release formaldehyde fumes into your home. 


The problem with formaldehyde is that if someone breathes too much of it, they can experience problems such as asthma.  The EPA even states that the fumes may be a possible cause of cancer.


There's no need to panic, of course.  Having fiberglass in your home doesn't mean that there's necessarily an immediate threat to your health.  But the fact remains that most fiberglass resin releases formaldehyde fumes over time.  Spray foam insulation doesn't, though.  So when some people decide to make a choice between some fumes and no fumes, it's not surprising that they choose to only use spray foam as an insulation.


So should you spray foam over fiberglass or not?


Obviously, you can spray foam insulation over fiberglass.  But based on everything that you've just read, you should understand the benefits of using only spray foam, rather than trying to use both together.


We're Advanced Foam Insulation of Louisiana.  We specialize in treating homes with spray foam.  If you live in Louisiana and would like to get a free estimate, please call us today or click here to reach us by email.



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