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Community and Q&A

Fiberglass Insulation

WilliamC | Posted in General Questions on

Hi everyone! I’m an owner who is remodeling the wall and exterior of my house. I’m trying to learn as much as I can and do this well while being friendly to my budget. I would love to use Rockwool but it is 50% more expensive in my area than fiberglass is so I’m considering using fiberglass. This brings me to the fiberglass questions.

For context I have a 1978 split-foyer. Currently I’m planning on re-doing all the insulation in the exterior walls (it’s currently all some kind of spray foam that is crumbling and completely deteriorated), sheathing with Zip, putting furring strips on the zip for a rain screen and then using either vinyl siding or a fiber cement siding (still waiting on the quotes for those). I would love to use foam for continuous insulation but it drastically increase the overall price of the repair and don’t think it’s in the budget unfortunately.  Code in my area is R13 in the 2×4 walls. 

1) live in climate zone 4a right next to climate zone 3. If I go with fiberglass, should I use unfaced fiberglass in my walls in order to remove the vapor barrier? If I should use faced, which side should the face go on? from everything I’ve read I think the face should go towards the warm side, which would be towards the exterior. However sandwiching the OSB between the fiber’s vapor barrier and the WRB on the outside of the zip board seems like a bad idea.

2) How concerned should I be with the fiberglass getting significantly wet? The hydrophobic properties of rockwool are the main reason I prefer it (along with it being easier to install correctly). It adds a layer of protection if water does get in. Although I’m not sure how valid of a concern this is with zip on the outside. I have not noticed any leaks in the roof so, assuming I do the zip correctly, I presume no water should get to the fiber. 

3) I have a fireplace where the chimney is exterior and is brick. Would it be safe to use fiber in the wall cavity next to the chimney? I know that the brick on the chimney probably allows moisture in and am concerned that using fiber next to would result in the fiber getting wet.  

Thank you for any thoughts you might have. 


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  1. Expert Member
    BILL WICHERS | | #1

    The paper facer on fiberglass batts is a vapor RETARDER, not a vapor BARRIER. It won’t trap moisture. You want that facer on the “warm in winter” side, which in your climate zone should still put in on the interior side, behind the interior drywall.

    You want to make sure water doesn’t get into your wall regardless of how you insulate it. While mineral wool is a little more tolerant of a leak, fiberglass doesn’t degrade if it gets wet, so if it can dry out, you’re still ok. The big difference is that fiberglass will absorb moisture and “fill up”, a bit like a sponge sitting in a puddle. Mineral wool won’t do that. Either way though, you should be relaying on the detailing of the exterior of the wall to keep water out, and not be thinking about the insulation being able to deal with water that might get in.

    BTW, you might want to check if you can get high density fiberglass batts that are R15 in a 2x4 wall. High density fiberglass costs a bit more than the regular stuff, but is usually still cheaper than mineral wool, making it something of a happy medium. High density materials tend to be easier to install well since they hold their form better and help to avoid voids and squished spots when installing batts.


    1. WilliamC | | #2

      Bill, Thank you very much for the reply. That clears up a lot for me. I was under the impression that fiberglass would seriously fail if it got even a little moisture on it.

      I have a follow up question. I am installing the insulation from the exterior. Will it be possible to properly install the insulation with the facer toward the interior drywall since I will not be able to get the wings on the facer over the studs between the drywall and the studs?

      I have looked into the R15 fiber but did not consider that it would be easier to install correctly. That is a good point. It is about 25% more expensive than the R13 but is still far cheaper than mineral wool (which is actually about 80% more expensive than fiber here,
      I misspoke earlier.)

      1. Expert Member
        BILL WICHERS | | #3

        You won't be able to get the faced fiberglass installed as well from the exterior for the reason you indicate, but you can still do it. What you end up doing is making the facer "flat" (no wings, leave them folded tightly), then pressing it into the cavity, trying to get the facer as flat and tight against the interior as possible. That's about the best you can do when working from the exterior, but it should be good enough.

        Regarding mineral wool, keep in mind that there are several manufacturers, and the prices of the different manufacturers vary. In my area, there used to be three common mineral wool manufacturers: Rockwool, Johns Manville, and Owens Corning. I used to like using the Owens Corning product, because it was stiffest, but it's now hard to get and a LOT more expensive than it used to be. Rockwool and Johns Manville are my other two options, so I work out cost per square foot and buy the cheaper of the two. Right now that means Johns Manville, but last year it was Rockwool. They all have similar product lines.


        1. WilliamC | | #4

          That makes sense, I'll make sure to try to get facer tight agains the drywall without compressing it. Would it be worth using unfaced to make the installation easier or would that be a bad idea?

          Thanks for the information on the different manufacturers. I can't seem to find Johns Manville mineral wool here. I'll keep looking though. I'm willing to 40% more for mineral wool. Owens Corning's Thermafiber is far more expensive than Rockwool here. The Rockwool is still about 60-80% more expensive than fiber here depending on the price of the fiber.

          1. Expert Member
            BILL WICHERS | | #5

            I've found that sometimes stores list mineral wool as "mineral wool", and sometimes as "rock wool". Be sure to look under both if you're searching for products. If you have a Menards in your area, they carry the Johns Manville product. Johns Manville is a large manufacturer though, so you should be able to order their products pretty easily through commercial supply houses.

            You probably want the faced batts in your walls. Unfaced batts can be used if you have a seperate interior side vapor retarder, or if you have enough exterior rigid foam that an interior side vapor retarder is not necassary. It's usually safest to put in the interior side vapor retarder regardless. Note that I've also found faced batts are sometimes cheaper than unfaced, presumably because faced batts are probably sold in higher volumes in certain sizes (such as for walls).


  2. WilliamC | | #6

    I'll keep that in mind when searching. I don't have a Menards but I'm sure someone should be able to order it.

    I'll stick with the faced ones then. I just know vapor retarders can be tricky in the south. Thank you for the help!

  3. ericpanderson | | #7

    Having installed Rockwool on my house, I can say it's a lot easier to get a good install with the rockwool vs fiberglass. The best fiberglass install is still crap compared to what a careful rockwool install can do. You can easily cut around junction boxes and wires with rockwool and that is much more difficult with fiberglass. Also, Fiberglass can't be compressed or it loses R-value, while Rockwool is designed to be compressed into an opening (hence a 15.25" wide piece designed for a 14.5" opening). Also, if might want to consider R-zip. I'm in climate zone 2A and the r-zip (used R-3) plus rockwool has made my home so quiet it's hard to believe. I don't hear any road noise anymore and I live close to a multi-lane highway. That would not happen with fiberglass insulation, so something else to consider.

    1. Expert Member
      BILL WICHERS | | #8

      Fiberglass loses R value when compressed, but that's not a unique property of fiberglass: it applies to mineral wool too. If you take either a fiberglass or mineral wool batt that is made to fit a 3.5" deep cavity and compress it into a 3" deep cavity, either batt will give you less than rated R value (but a bit more R per inch).

      The "mineral wool is designed to be compressed" that you're talking about is because mineral wool is usually sold as an unfaced batt, so it has to stay in place by being a bit oversized so that it can "press fit" into a space. Faced fiberglass batts typically use the kraft facing for support. Mineral will still loses a bit of R value when compressed, you're not compressing it enough when fitting it into place to make much difference, and the depth of the batt remains pretty much unchanged here.


      1. ericpanderson | | #9

        Bill, I thought that if I put 4" of rockwool in a 3.5" cavity, I'd have R-15, but if I put 4" of fiberglass in a 3.5" cavity I'd have less than R-13, so this is not correct? This assumes typical R-15 rockwool and R-13 fiberglass insulation sold for normal 2x4 wall cavities.

      2. WilliamC | | #11

        I was under the impression that mineral wool was harder to compress as much as fiber and would recover its shape much more easily after being compressed. So compressing it didn’t hurt it as much.

        1. Expert Member
          BILL WICHERS | | #12

          It is more difficult to compress mineral wool, but that doesn't change that the R value is affected when you compress it. Typical batts are made to fit standard depth walls though (2x4, 2x6, etc.), so the batts that are made for a given depth wall will perform at the R value they're rated at.


    2. WilliamC | | #10

      Thanks for the info! The ease of installation is a good point. I definitely think rockwool is a much better product. I’m just having a hard time justifying double the price for a similar product. Although I would love to use it. I am going to keep looking to see if I can find some cheaper.

      I appreciate the suggestion to add foam. I have already purchased the zip although I can get some r3 Rmax boards or Foamular for not too much more. I just wasn’t sure if only r3 would be worth the extra money. I was looking at r10 boards but they are outrageously expensive here. If R3 is a substantial difference though I may consider that.

      1. Expert Member
        BILL WICHERS | | #13

        In your CZ 3, R3 of exterior rigid foam is safe according to the tables. You can go as much thicker as you want, you just can't go below the minimum -- and R3 is already a little above the minimum.


        1. WilliamC | | #16

          Thank you thats good to know. My CZ is 4a. But 3 is about 30 to 45 minutes south of my so it’s close. What table can I look at to determine minimum R value on foam continuous insulation?

          1. Expert Member
            BILL WICHERS | | #17

            Somehow I was thinking CZ3... CZ4 is pretty close though, and you're still good according to the tables. You can reference the below (and the associated article on GBA too):



        2. WilliamC | | #18

          Thank for the link and the information. It looks like Marine Zone 4 is 2.5 for 2x4 walls So I would definitely be over that. I noticed there is no guidance for 4a or 3. Should I interpret that to mean that it's warm enough in my climate zone that foam thickness is not a concern or just that research has not been done in my CZ?

  4. MartinHolladay | | #14

    Keep in mind that most vinyl siding manufacturers don't allow their product to be installed over furring strips; they usually require a solid substrate like OSB or plywood.

    1. WilliamC | | #15

      Thank you for the heads up. From what I have read in this site it seems that the foam under the vinyl siding does not require furring strips(rain screen) if the foam is an inch of less since vinyl already inherently ventilates behind it. Is this accurate?

  5. WilliamC | | #19

    Thank you all for the help! I actually found some Rockwool about 2 hours away for the same price as fiberglass around here. So that has made the decision easier for me and I will be going with the Rockwool.

    Now I just need to decide if R5 foam CI is something that I'm going to do. I was under the impression that it would be beneficial for energy conservation and comfort but the responses to another question I have posted here have lead me to question what the purpose/benefit of CI is.

    1. Expert Member
      BILL WICHERS | | #20

      The general rule is the exterior continuous R value is more helpful than in-cavity R value, and is also safer for the assembly. That means that R5 on the outside will perform better than R5 on the inside, and the higher amount of the total R value is on the exterior, the safer the assembly will be, since the interior of the wall will stay warmer.

      Exterior CI isn't affected by thermal bridging of the studs, which is why it performs better. You still get into diminishing returns though as you put more and more R value, regardless of where it goes. For moisture safety, go with AT LEAST the R value recommended in the tables, for better performance go with more than the recommended minimum. Up to about 2" isn't too bad to work with, over 2" of exterior rigid foam can really complicate trim details. 1" and under is pretty simple to deal with, and requires very little changes to anything else.

      I personally am a big fan of exterior rigid foam, and have about 3" going into some of my projects at my own home, putting more R value on the exterior than the interior. I keep costs manageable by using reclaimed and factory second polyiso for the material, and I do my own installation work. This puts my walls up around R34 or so, which is in superinsulated territory, and probably well past the best bang for the buck spot on the cost/benefit line.

      I'd put up AT LEAST the minimum, but probably in your area, I'd go with 1", which would be R6 if using polyiso. Higher levels of exterior R value are of more benefit in the more northern climate zones (I'm on the northern edge of CZ5, almost in CZ6).


      1. WilliamC | | #21

        Hey Bill, thank you for that information. It makes alot of sense that CI would be very beneficial. Especially in a 2x4 16 OC situation.

        I haven't decided if I'm using polyiso or Greenguard's XPS LG. polyiso is quite a bit more expensive than GG's LG XPS here. I of course was trying to avoid XPS for the GWP but it looks like GreenGaurd's LG XPS has reduced their GWP quite a bit.

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