Insulation does a commendable job of regulating the heat flow inside a building. If you are dedicated to ensuring your home is well insulated, you may be tempted to put fiberglass insulation over foam board.
Even though it’s practical and can help you realize plenty of benefits regarding heat regulation, there are some concerns you need to be aware of.
This article will explore some of these concerns and look deeper into foam boards and fiberglass insulation and whether it’s a good idea to bring both together.
What are the Concerns Associated with Putting Fiberglass Insulation Over Foam Board?
When renovating your home, putting fiberglass over foam board is labor-intensive. Other than that, you need to be aware of the following issues;
1. Adding fiberglass insulation to a foam board can lead to moisture trapping, leading to rot and mold development.
As beneficial as introducing fiberglass insulation can be, it has some drawbacks that most people usually aren’t aware of. Even if you make sure that the fiberglass insulation is moisture-free, there can be leaks in the future, resulting in rotting or mold growth.
As you ponder how to add fiberglass insulation over the foam board, you need to figure out what you will do about the moisture problem. A vapor barrier is always an excellent solution.
2. The other concern associated with putting fiberglass insulation over a foam board is that you may end up compressing the fiberglass batts, reducing the R-value and countering what you were trying to achieve in the first place.
R-value is used to grade the thermal resistance of insulation products. A decrease in fiberglass insulation R-value will lower its efficiency, and you may not get the benefits you were interested in.
To avoid this problem, we recommend installing the fiberglass carefully. The goal of doing this is to prevent compressing the batts and lowering its R-value.
These two are the main concerns about putting fiberglass insulation on a foam board. If you can avoid these problems, you can have a go at placing fiberglass insulation on a foam board.
What is a Fiberglass Insulation?
Fiberglass is a common material used to insulate most homes. It has been used in the construction industry for decades, and it’s why many builders prefer to use it. Fiberglass is available in loose-fill, rolls, and batts.
If you are interested in fiberglass insulation with the highest R-value, you should go for high-density batts. Fiberglass can be used in walls, ceilings, and floors.
The science behind how fiberglass works in insulating a home is pretty straightforward. It traps air pockets, thus limiting the amount of heat or cold that enters or leaves your home.
During winter, fiberglass ensures heat doesn’t dissipate through the walls, and during summer, it prevents heat from outside from getting inside. As you can see, the mechanism of how it works is pretty straightforward.
Moving on, fiberglass is quite affordable. Because the cost of insulating a home is always high, it can pay off to use fiberglass and cut down on costs. The installation process is pretty simple.
As great as fiberglass insulation is, it has some drawbacks, which is why many homeowners are switching to other insulation materials.
Fiberglass poses a health risk when the particles escape into the air. The shards of glass fibers can irritate your skin when you touch them. They can also irritate the organs when inhaled or gets into your eyes.
If there is a space in your walls, the fiberglass can leak and cause contamination. The other issue with fiberglass insulation is that it traps moisture, leading to mold growth.
You need to be aware of these issues before putting fiberglass on a foam board.
What is a Foam Board?
These are rigid insulation panels made using polyurethane, polyisocyanurate, and polystyrene. A foam board is available in varying thicknesses. It can be cut down to fit any project, which makes it so popular.
They do an excellent job of insulating your home. Foam boards are created differently. Always look for one with a vapor barrier since that will ensure it is resistant to moisture that can build up between the insulation and walls.
Like fiberglass insulation, a foam board prevents heat from being conducted from the house to the outside. Foam boards are generally poor conductors of heat, which makes them so effective.
Foam boards have their pros and cons. For instance, they are perfect for any DIY job since they can be cut according to your desired length.
Foam boards are also quite affordable. On the other side, you should also know that the joints of foam boards need to be caulked or taped to reduce heat flow. This calls for more work. Also, certain foam boards lose their insulating ability after some time.
Will Using Fiberglass and Foam Boards Increase the Insulation of Your Home?
Most people usually go with either a foam board or fiberglass insulation. If you are considering using both, then you must be very keen on insulating your home, which is a good thing.
Putting fiberglass insulation on a foam board is a great idea. But you must ensure that you deal with the concerns we have raised earlier. If you deal with that, you should enjoy maximum insulation from the fiberglass and the foam board.
What Should Go First – the Fiberglass or the Foam Board?
One of the challenges of using two types of insulation simultaneously is figuring out which should be close to the interior and vice versa for the exterior.
The foam board should be closer to the interior and the fiberglass should be further to the exterior to get the best results. That means you should put the fiberglass first.
Doing so will result in a completely sealed wall. But you have to be mindful of the effects of moisture. Having a foam board with a vapor barrier on the inside will prevent moisture from condensing in the middle of the insulation.
If you put fiberglass insulation over a foam board, make sure that the fiberglass comes first and then the foam board.
Fiberglass Insulation vs. Foam Boards
Even though fiberglass insulation can complement the purpose of foam board on your walls, it’s essential to know what each brings to the table. So here is a quick comparison of the different foam boards and fiberglass features.
• Construction
A rigid foam board comes in 4 x 8-foot sheets. The sheets have a thickness ranging from ¼ to 2 inches. You can choose from expanded polystyrene or high-quality green polyisocyanurate.
These boards respond well to moisture and will resist cracking and splitting. On the other hand, fiberglass is sold in batts or long rolls. Unlike rigid foam boards, fiberglass is softer.
A critical difference between fiberglass and foam boards is that the former shouldn’t be used in the presence of moisture. Fiberglass is known to trap moisture, which can lead to mold growth on your walls, leading to more severe problems.
You can take comfort in knowing that fiberglass batts come in a thickness of 3.5 inches to 12 inches. That’s pretty thick, and you are assured of enhancing the R-value of your overall home insulation.
• Applications
A rigid foam board can be used on basements, walls, rim joists, and other locations. Fiberglass can also be used on walls. Fiberglass, however, works best as insulation for homes that are still being built.
If a house has already been erected and you want to insulate it, you should use spray foam. It can quickly get in-between spaces because it’s less invasive.
Fiberglass has more applications because it can be used in the attic and floors. The same can’t be said for foam boards because of their structure.
• R-value
The other key difference between these two insulating materials is the R-value. A foam board has a higher R-value per cubic inch than fiberglass.
Therefore, if you desire to conserve energy, go for rigid foam. But in this case, since you want to use both, perfectly combining them will ensure you get a better overall R-value.
• Price
The last difference we are going to look at is the cost. Fiberglass costs way less than rigid foam. However, this won’t matter because you intend to use both.
Nonetheless, it’s essential to know that fiberglass insulation is much cheaper. If you install both, fiberglass will have less waste since it doesn’t have to be cut, as with foam boards.
Is It Worth Putting Fiberglass Over Foam Board?
Before embarking on such a project, you need to determine if it’s a worthy investment. After all, you will use plenty of resources and time.
Adding fiberglass to a foam board is a project that will need money and even labor if you are working on a big area. Provided everything is done correctly.
The moisture is dealt with, and the fiberglass batts are not compressed. This is a project that shouldn’t lead to any issues. On the contrary, your home’s insulation will be significantly improved.
This information doesn’t make a lot of sense to me. Not trying to be argumentative but to understand and outline some other ways of approaching the foam plus batts set-up.
The foam board is a vapor barrier in and of itself, right? (1″ of foam should not allow much if any moisture to get through the foam). So if foam board is placed on the inside of an exterior wall (say glued to the cinderblock wall in a basement) why is it a problem to then build out a frame wall and fill with standard insulation? Personally I’d prefer rockwool to fiberglass batts but the same concept should apply, right?
About 10 years ago I did a lot of research before insulating a finished basement wall that had no insulation at all installed before it was framed & drywalled. The original wall had a horrible moisture problem and mold on drywall caused by plastic sheeting the original owner installed for vapor barrier next to the concrete. Moisture would run down the plastic, hit the base framing, soak the lumber and create a nice mold growth on the drywall.
After removing everything to the concrete wall, I glued foam board directly to the cinderblock, sealed all the seams, then installed my stud wall overtop, filled the stud cavities with batt insulation, then drywalled over it; in a decade I’ve observed no issues although admittedly I haven’t ripped out the wall to inspect.
If I had done the reverse as the article above suggests, (put my batts right next to the exterior wall, then put foam board insulation over top, followed by drywall), any moisture from the outside would I think have penetrated the wall, absorbed in the batts and been trapped since the foam board would prevent escape. This would have lead to a mess inside the walls I think (a perfect breeding ground for mold). If however, batts are closest to the living space, if any trace moisture gets through the foam board, it should in theory be able to “breathe out” through the drywall.
Again, happy to be educated but adding my thoughts for consideration.
Hi,
Thanks for sharing your thoughts and experiences—it’s always valuable to hear different perspectives on insulation methods.
You’re right that foam board acts as a vapor barrier, especially when properly sealed. Placing it directly against the cinderblock wall, as you did, can help prevent moisture from entering the living space. Filling the stud cavities with insulation like rockwool can then improve the overall R-value of the wall.
The concern with placing batt insulation against the exterior wall and then covering it with foam board is indeed about moisture potentially getting trapped in the batts. By installing foam board against the exterior wall first, you’re creating a continuous barrier that limits moisture ingress.
Building science can be complex, and the best approach often depends on specific factors like climate, the condition of the exterior wall, and local building codes. It sounds like your method has worked well for you over the past decade, which is great to hear.
Thank you for contributing to the discussion—it’s always helpful to consider different methods and experiences!
So,@Still_improving, did you use a vapor barrier then between the drywall and fiberglass insulation then? I am thinking of doing the same.