Is Spray Foam Insulation with Fiberglass Batts a Good idea? | Foam University
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- เผยแพร่เมื่อ 17 ต.ค. 2024
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Spray foam insulation and fiberglass, isn't that the best of both worlds? Well, no, not exactly. Here's the low down on flash and batt insulation.
Flash and Batt
Energy Savings
Air Seal
Moisture Issues
Thermal Resistance
Building Envelope
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What do use to create better sound barrier? Closed cell foam is not a good sound barrier and can actually transmit sound in the same way wood does. My original thought was to use 3" closed cell foam then add Roxul to create a good sound barrier, but after seeing how other are having problems where they adding batt insulation and it getting soaked, the only solution to prevent that is go with the 3" closed cell on the outer wall, then add a secondary stud wall about an inch away from the outer wall and add the Roxul to the secondary wall. The main issue here is to create a really good sound barrier where I could build a work shop and not any issues with neighbors being disturb from any noise coming from a table saw, planner, ait compressor, etc., late at night.
If your main goal is to stop sound transmission you should be looking at some proprietary acoustic materials. Open cell spray foam can be a good sound barrier but that is NOT what it is designed for.
If I put foam boards or spray foam in a stud bay and then batt insulation on top, should I use faced or un-faced and why? I would use faced batt insulation if the foam board or spray foam wasn’t there. So why should I use un-faced if the foam board or spray foam is there?
Hey Todd, We recommend using faced with the paper facing the drywall during install. This is the best way to install the product easily and ensure stability. The paper will give it more structure and longevity. If you use unfaced, it will perform the same, it just might not hold up as long due to lack of structure. I hope this helps!
I live in a new house that has foam in the attic ceiling and I love it. I’m wondering if I could put it in my garage ceiling even though it’s not a/c and will it help when it’s hot and never cools off in the garage? Or should I have 38R glass blown in the attic ceiling floor?
Foam will definitely help keep the summer heat out of the garage. With that being said, on really hot days it will still get warm in there if there is no kind of AC
What about a double stud, super insulated wall system? Will it be cost effective to air seal in that system?
It's not cost effective to have a super thick layer of foam, or any insulation material. Once you have the air seal that foam creates, the hard part is done. Then you're just adding enough to fight thermal transfer.
What about applying fiberglass batt first on underside of roof then spraying foam on the fiberglass? Kind of reverse "flash & batt", it would be "batt & flash".
Honestly, the fiberglass would be a waste of money. Also, the foam needs a surface to adhere to, so spraying it onto the fiberglass wouldn't work.
The air seal created by the spray foam at the correct thickness means there wouldn't be a need for the fiberglass.
This js kind of misleading. Adding fiberglass to the assembly definitely adds R-value and efficiency. Any insulation pays for itself within a few years. R value is r value. Flash and batt is becoming very popular for 2x6 walls and rafter assemblies. There are calculations you have to make for the ratio, however. They usually want the r-value of the spray foam to be higher than the r value of the fiberglass. 2" of closed cell achieves air and vapor barrier.
In theory, more R-Value is better.
Unfortunately, in real world application there are many outside factors. One major factor with flash and batt is moisture and condensation.
We have seen many situations where a flash and batt system caused issues from the batts retaining moisture.
Additionally, your ROI will be vastly different depending on the building material. So, there are many situations where an insulation will not pay for itself within a few years.
Can you install faced batting over foam board installed on an attic floor between the trusses? I foamed the edge of the foam board with great stuff fire seal insulation. But I am wondering if water could still end up between the batting face and the foam board. Thanks
You can do this, but keep in mind it won't create an air seal, and moisture can build up between the foam board and fiberglass.
@@RetroFoamofMichigan isn't foam board a vapor barrier?
Moisture can form in the insulation from air movement through the materials. Essentially, when that outside air and inside air meet, condensation forms.
Is foam board against exterior wall sheathing a good idea? Is it not going to create mold issues?
Foam board is fine on the outside. Be sure to seal all joints and seams.
This is great information! I am in North Jersey and researching insulation options for renovating our home. I am considering to install Rockwool R-15 -> Zip R-3 -> 2 inch closed cell spray foam -> gypsum board on the walls. On the roof, 3 inch closed cell foam. Do you see any issue with this method?
My recommendation would be to stop at the 2-inches of closed cell in the wall because that creates the air seal. You would also be good with 3- to 5-inches of closed cell on the roof deck.
@@RetroFoamofMichigan thanks for your reply. Do you mean don’t do Rockwool and Z R-sheathing outside?
As far as the insulation goes, the rest wouldn't be needed as the 2-inches of closed cell creates an air seal.
Does this still hold true with Roxul? Roxul doesn’t mold or lose shape/r value when wet
Yes, same issues.
@@RetroFoamofMichigan not true. don't spread lies. where is your proof/data/research? foam is good but don't lie
man, I really liked this video explanation. I'm researching so much about this right now and there are so many opinions and nuances to the code and science. But can I ask you this, what if I spray foam headers to max at 3.7in with CC and then just the top 3 feet of concrete wall with 2.5 in. Then batt with FB or RW the entire wall and poly. So its a partial flash and batt method haha! The idea is to cut costs and just spray foam the coldest area of the concrete that typically reaches a dewpoint in winter.
Thanks Alex! The biggest problem for flash and batt is the potential to trap moisture. If you have enough foam to provide an air seal, you would be better off insulating with just foam to avoid any potential issues.
@@RetroFoamofMichigan So my plan now is, 2.5in spray foam (hardly a flash now) the whole wall, with FB batt overtop. NO poly vapor barrier anymore. I can't see moisture being trapped now?
@@alextirabasso2441 Any existing insulation over top of the foam have the possibilities to hold moisture. If the whole project is air-sealed, existing insulation isn't necessary since air can not pass through that foam.
@@alextirabasso2441 The best way to think about it is that you want your vapor barrier on the warm side of the wall. The foam is the vapor barrier. In a northern climate you want that to be on the inside of the wall. In a warm climate you want that on the outside of the wall.
So, if you batt over the foam, warm moist air will migrate outward and condense as it cools on the inside surface of the foam (inside your wall cavity).
I know people that have $500 power bills. Pretty sure that can pay for it over time
Definitely! The amount you save on energy bills can definitely help with the cost of getting foam done
Not a single word about condensation/dewpoints ? Wow.
At 2:10 moisture concerns are addressed.
@@RetroFoamofMichigan You are correct, I apologize. However will condensation not occur if 1" foam is sprayed and no batting added?
@@terrydietzler4096 Condensation still may occur on the surface of the foam, but with no fiberglass on top of 1" of CC it's less likely because moisture wouldn't be trapped.
@@RetroFoamofMichigan Then why would an R-7 ( one inch) even be considered ? I fail to understand why "flash & batt" is even an option in a home. I'm dont mean to be argumentative, I am trying to understand as my son is dealing with this right now and the foam company knew exactly his intention of adding batt and said nothing to the contrary.
@@terrydietzler4096 I am guessing, unfortunately, that it is because as long as they’ve quoted their profit margin on whatever thickness they spray, they don’t really care?
And to their credit, if they say “oh, no, don’t put batts over our foam, instead have us spray it thicker, they will look like they’re just trying to get more money from you.
It looks like you have a double vapor barrier in this video.
Fiberglass batts do not provide a vapor barrier.