GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Insulation over and over…

seba83 | Posted in Energy Efficiency and Durability on

I also posted it under article titled “Monitoring Moisture Levels in Double-Stud Walls”, but figured some other people may read it here.

I have done quite a bit of reading around the Internet and I am still little confused. I own the house on uninsulated concrete slab (it will stay like this at least for some time, no basement). House had the second floor addition done in 2008, walls should be R15 and attic R30. Studs on both floors are 2×4,16″ apart. The problem is the first floor. It is 60 years old, and has poor rock wool insulation inside. Outside it has probably asbestos siding covered with vinyl siding. I do not think there is any rigid foam in between. I am in NY, Long Island, which can be pretty dump in summer time, zone 4A.

I would like to remove sheet rock (walls and ceiling) and insulate the walls, and ceilingrim joists (only where they “meet” outside wall). I have the following concerns (let’s say that money is not a concern in a sense that I would like to stay frugal, but I am willing to pay more for better and long standing solutioncomfort of my house):
1. First, I wanted to fill all stud cavities with closed cell foam. However, I am afraid that I will create vapor barrier, and as far as I understand reading on the Internet I should not do it, as the moisture will transfer to studs and can create mold problems. Is that, correct? Plus it is expensive and supposedly not feasible in economic sense as I would lose its R value do to studs…
2. Second, I was thinking to spray 1” or so of closed cell and fill the rest with open cell. Is that good option? Should I be careful again not to create vapor barrier by closed cell foam, so keep it thin? I also read that closed cell needs to be sprayed in at least 2″ layers…
3. Third option I was considering was this:
However, I would space horizontal 2×2 by 24” to further minimize thermal bridging. Can I, alike in #2, spray 1″ or 2″ closed cell foam first and then fill the rest with open cell foam (I think it would be to thick in this case and act as vapor barrier too), or rather cellulose (as pictured on the website)? My wall would be 2×6 only so I hope moisture problems related to extra thick walls will not apply? Do I need to calculate dew point here?
4. I am afraid of cellulose settling. Is it a big deal nowadays? It somehow makes me resistant to use it. Am I exaggerating? I saw cellulose being recommended here multiple times… so maybe I should just go with 2″ of closed cell and then cellulose…
5. In all cases I would NOT add vapor barrier on inside.
6. If none of the above is good solution, what would be the best way to achieve the highest possible R value for given circumstances (also keeping economy in mind)? I would like to avoid removing exterior siding.

I greatly appreciate any help and suggestions!

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    The fact that many types of spray foam and rigid foam are also vapor retarders or vapor barriers is not a problem. Because these insulation materials have a significant R-value and are air barriers, there are no opportunities for vapor to condense on cold surfaces.

    The Mooney Wall solution is fine, if you want to go that route.

    If you choose a competent insulation contractor who has experience with the dense-pack method, there is no reason to worry about cellulose settling.

    If you want the R-value of your wall to be higher, make your wall thicker. Most people compromise on R-value, rather than aiming for the "highest" R-value. There is no "highest."

  2. seba83 | | #2

    I appreciate your help. I think I have decided to go with flash and fill. I will expand my walls so they are 2x6 (money wall). I will do 2" of closed cell foam first which will be my vapor barrier, as there is only plywood and siding left towards outside of the house. Remaining, towards inside of the house, I would like to fill with either dense packed cellulose or open cell foam. Which one is better, and which one would you prefer if you were me in the given scenario? Once again, thank you for time and help!

  3. GBA Editor
    Martin Holladay | | #3

    Since this is a green building website, I would advise you, all other factors being equal, to choose cellulose over open-cell spray foam. Cellulose is more environmentally friendly than spray foam.

  4. seba83 | | #4

    The foam I want to use is 2# polyurethane spray foam, a Class III Vapor Retarder. Does it matter in my case? Will it become vapor barrier when sprayed 2" of it? Actually I think you have already answered that question Martin. I cannot delete the post... The topic is so confusing that right after I think I understood everything something new comes up. Anyway, I will stick to your first answer and will go with 2" of 2# polyurethane spray foam, a Class III Vapor Retarder, and will do the remaining with dense pack cellulose... Will double check what is under the siding and come back here in case of any surprises. Thanks!

  5. GBA Editor
    Martin Holladay | | #5

    You seem to be very concerned about vapor diffusion, vapor retarders, and vapor barriers. I'm not sure why.

    If you want to use closed-cell spray foam, go right ahead. Two inches of closed-cell spray foam will have an R-value of about R-12 or R-13 and a permeance of about 0.8 perm.

    As I wrote in my first response, "The fact that many types of spray foam and rigid foam are also vapor retarders or vapor barriers is not a problem. Because these insulation materials have a significant R-value and are air barriers, there are no opportunities for vapor to condense on cold surfaces."

  6. seba83 | | #6

    As I promised earlier I went and checked underneath of my siding. Turns out that my house has 1/2" polystyrene rigid insulation (extremely poorly installed, look at the gaps!!!) and potentially some sort of house wrap (look at pics)? I realize that siding should be removed and everything should be redone... However, there are more pressing things to do first. As a result, just wanted to confirm that my initial plan of 2" of closed cell and remaining of celluloseopen cell will work fine with 1/2" of rigid foam and potential house wrap? Hope foam will make the house comfortable and I will fix external layers later. Thank You for your help with my question.

  7. GBA Editor
    Martin Holladay | | #7

    As far as I can tell, there isn't any reason you can't go ahead with your plan. Of course, you always want to be on the lookout for any moisture damage when performing this type of work. If you see any water stains, rot, or mold, it's important to stop your project until you understand the source of the moisture and you have remedied all of the defects.

    If everything is dry, you can proceed, full steam ahead.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |