GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted Hamburger/Search Icon

Community and Q&A

Is rigid foam and fiberglass batt between studs a good Idea?

William Verbano | Posted in Green Building Techniques on

In my upcoming master bath remodel, I am considering stripping two exterior walls of drywall in order to make them 2×6 in thickness to allow for additional insulation. My plan was to install 1″ rigid foam board against the backside of the exterior plywood sheathing, with all gaps sealed with foam and then R-19 fiberglass batt insulation.
However, after reading about the flash and batt method in Fine Homebuilding, I am concerned with too much moisture being trapped in the walls.
Bad Idea?
Bill V.

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. Aj Builder, Upstate NY Zone 6a | | #1

    Bill, add the foam your way, then add strips of foam to studs to make your cavity deeper. Finish with airtight green board drywall. Seal penetrations well. Run your bath fan long enough via timer switch to control moisture.

    Disclaimer, if Straube says this is not a workable plan, then it will be edited.

  2. draginfly58 | | #2

    I don't know Mr. AJ but this doesn't seem like a very good way to do it to me. I think I would make the wall framing thicker and put the net stuff up and fill it tight up with that ground up celulose paper. Then put on the drywall. That fan sounds pretty good though.

  3. William Verbano | | #3

    Thanks for the reply. How about this:
    Batts in stud bays first, then 1" foam panels to the inside of the room, then drywall. Would having the ridged foam between the drywall and the fiberglass bats prevent moisture from migrating into and condensing in the walls? Perhaps adding a foam strip on the studs to create an air gap between the foam board and the back of the drywall.
    Or should I just deepen my wall to 2x6 and pack it with fiberglass. Here, in the Philadelphia region, it gets very cold in the winter and hellishly humid in the summer.
    I just want to make my bathroom comfortable.
    Any suggestions are greatly appreciated.
    Thanks, all,

  4. Lucas Durand - 7A | | #4

    William, Do my build as outlined in my first post. Done. I have no other input to this thread unless you start writing checks.


    Answered by aj builder, Upstate NY Zone 6a
    Posted Sun, 01/30/2011 - 21:20

    AJ, how dare you solicit money from people visiting this web site?

  5. Lucas Durand - 7A | | #5

    William hang in there - it tends to be quiet on the Q&A over the weekends.
    Though I am not in position to give you all the advice you need, there are plenty of people here who can.
    A few thoughts though:
    Cutting rigid foam board to fit between studs will be labour intensive and will not offer any significant improvement to your wall's thermal performance. Nor will that approach address thermal bridging through the studs.
    Also, squeezing an R19 batt into the remaining space will reduce the performance of the batt.

    If you have the space to the interior, you might consider strapping the interior side with 2x2 for a "cross-hatch" wall (sometimes known as a "mooney wall"). This will give you some extra depth for insulation, and reduce the thermal bridging through the (2x4?) studs. A blown in insulation like cellulose will fill the irregular cavities made by the strapping. Attention to air sealing in the renovated portion of the wall (and through the rest of your house) is critical to good performance and the longevity of the structure.
    It is difficult to say if my thoughts are relevant to your situation without knowing more details. Air sealing could be real P.I.T.A. without resorting to a sprayfoam product.
    Anyway, I hope this helps some.

  6. TJ Elder | | #6


    You are right to worry about trapping moisture with the approach you described originally, rigid foam toward the outer edge of studs. It should be fine to place rigid foam over the interior of studs and screw the drywall through it. In that case the low permeability should be okay for the interior face of a bathroom wall. If you run air-conditioning in the summer then the foam thickness needs to be sufficient to avoid condensation of humid exterior air.

    The cross-hatch method as Lucas described is also a good solution and should be less expensive. It also gives a more positive support for mounting drywall. You would need to install blown fiber insulation to fill the space completely, and use a vapor retarder primer at the interior drywall surface.

  7. Aj Builder, Upstate NY Zone 6a | | #7

    Fiberglass insulation, is insulation though not the best. And Lucas is wrong. Compress a batt and it has a higher R per inch. Willy said he is going to thicken his wall. Good. One layer of foam is fine inserted or maybe under the drywall. The reason I said to add just foam to the studs was to add a thermal break to the framing while allowing a higher perm. for drying to the inside if need be.I don't know his build and only know that he could at least have a build that could dry to one side. One side is better than no side.

    The big thing is to set up the bath fan well and with a switch to run it longer than a shower. And to run the fan every time the shower is used. Then all should work out no matter how the wall is built.

    The end. finito. Sayanara

  8. Bill Verbano | | #8

    A big thank you to all who have replied. A little background. I've been a carpenter/contractor/builder for most of the last 36 yrs. Though, I've been semi retires ever since the recession hit. Most of my experience in construction had been in Calif., but we moved to Pa. 8 yrs ago.
    A couple of things- one, building methods are, to my mind very different in the east than out west. Obviously, So. Calif. doesn't present quite the same problems I currently face regarding my insulating dilemma. Additionally, building methods and materials have changed and continue to change radically over the years as engineers, architects and builders strive to make a better home.
    Trying to stay abreast of all things new in construction is a tough task. And I don't have to run a business in the meantime.
    I am new to this forum but, I know things can get, shall I say, heated (pun intended). We all know there are eight ways to Sunday when it comes to getting things done, or built and we all know, as builder/contractors how bruising to our egos it can be to ask other builder/contractors for advise. I like the idea of a forum like this to find answers and it is my impression that this place should be more like sitting around a campfire and getting your 2 cents in, than a strictly dry information exchange.
    A.J., I appreciate your suggestion, but not knowing you any better than Lucas or Thomas, I can only take your ideas under advisement. I'm sorry, no checks will be forthcoming. Understand, though, that I am in no way looking for anything that is not freely given. We all know there is a wealth of information, both good and bad out here on the internet.
    Again, thanks to you all and if anyone is ever in the Philly area, let me know.
    I'll build a fire.

  9. User avatar GBA Editor
    Martin Holladay | | #9

    Here's my vote: either install horizontal 2x4s as Lucas suggested, to create a thicker wall in a way that provides a thermal break, or install continuous rigid insulation (I like polyisocyanurate) across the interior of the studs, and screw the drywall through the polyiso, as Thomas suggested.

  10. mike maines, so me. zone 6a | | #10

    Bill, you are right to be concerned about moisture in your walls. I wrote the FHB article, and I have replaced bathroom walls that have rotted from the inside out. What part of PA are you in, or what climate zone? (See the map at the beginning of the Q+A section).

    If you are in the colder parts, 1" of foam will not be enough to prevent condensation. If you can afford the space, Martin's suggestion of polyiso across the inside of the studs is a great system, and because it allows the wall to dry to the outside it is more forgiving of a little moisture in the wall. The Mooney wall that Lucas mentioned is another great system and minimizes thermal bridging.

    If you decide to use foam against the wall sheathing in a flash-and-batt type installation, the more foam you use the better. Make sure you don't install a poly vapor barrier but do seal edges and penetrations in your drywall or backer board. Additionally, you should be sure to run your bath fan until no moisture remains in the bathroom. If you can't meet all of these requirements you should go with one of the more-forgiving insulation methods.

  11. Bill Verbano | | #11

    Thanks, once again to all. Particularly to Anonymous for the good wishes for success and to Mike M. for your input.
    This bath project has been in the planning stages for a very long time. It has had to wait for the kitchen and rest of the first floor to be finished first.
    There are some other elements that will figure into the equation that I haven't mentioned.
    As I've said previously, I live in the Philadelphia area, where the winters are cold and dry and summers are humid and hot. Very different than from So. California, where I used to live and where I learned my trade.
    One of the things I had never known before, in Calif. was vinyl siding. It's everywhere here and I hate it. However, the vinyl siding on my very poorly built track house is at the end of its life and will need to be replaced. This I plan to do in the near future, along with upgrading the windows.
    Something I've noticed in my area that seems to be common practice by siding contractors is the use of foam panels, some foil face, some not, under the new siding. This seems like a good idea as a way of additional insulation and also a way to reduce air infiltration from the outside.
    In my case, regarding my bath remodel, it seems to me that if I used foam panels on the inside of my walls, under the drywall and then added an additional layer outside, under the siding, I'm either preventing moisture from getting into the walls or I'm trapping moisture in them.
    One other thing, since the humidifier on my forced air gas furnace isn't currently working, the air in the house gets so very dry we use room humidifiers to keep our skin from crawling. This said, I think that even when taking the longest, hottest shower any moisture doesn't even have a chance to get into the walls and cause problems. The air is just too dry.
    Presently, I'm leaning towards building out my walls an additional 1 1/2" by sistering the studs with new ones. This will allow me to plumb my out of plumb walls and leaving a slight gap from the original studs create a thermal break. Stuff this new deeper wall with R-19 batts and use foamboard across the studs under the drywall. We shall see.
    Again a hearty thank you to all for your answers, time and expertise.
    Bill V.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |