An above grade portion (wood framing) of my basement walls (in unfinished furnace room) is currently insulated with faced R19 fiberglass batts. I live in nothern climate zone 5.

Unfortunately there is currently nothing but wood behind these batts and there is no air sealing, so when i looked behind the batts the wood walls have frost on them and some are soaking wet.



Before (behind faced fiberglass batts):

Before - behind insulation

I begun a project to air seal behind the batts gluing 1" R5 XPS foam boards to the wood and spray foaming around the gaps, then removing the face from the batt and replacing the unfaced batt on top of of the XPS.



After a few cold days i looked behind some of the batts that I had replaced and noticed that there was some condensation on the XPS foam boards (towards the vertical center of the boards). The boards themselves didnt feel cold at all.

Is there something else i should have done to prevent this / how can i correct it?


  • What's behind the foam? You say this is above grade, but the basement floor is visible... that's below grade, right? Is there a perimeter drain system or ventilation? Could this be a simple water penetration/wicking problem that needs to be addressed either at the exterior or on the interior of the concrete? Commented Jan 24, 2018 at 16:36
  • @ShimonRura there is wood behind the foam, i can take a picture later and add it if that helps. My house has a walk-out basement and so that part that i took photos of is at ground level, ie behind that wall there is exterior siding (no portion of that wall is below ground). I am not sure what might be causing it, how would i diagnose this? There is a draintile around the basement and sump pump but i think this should matter because this wall that i am insulating is not below ground? Thanks for your help
    – skimon
    Commented Jan 24, 2018 at 18:32
  • My last home had a daylight basement, the upper floor had a deck that went the length of the house and across 1 end. When the deck was installed they did not properly seal the wall so water would drip down the wall on the weather side of the house. Once I figured out this was the problem I pulled the deck board closest to the house and fitted some flashing to keep the water out. I did have to rip the deck boards so they fit with the flashing in place but that stopped the water then I was able to repair the rotted studs. If you have a leak in the wall it could be quite a way above the damp area
    – Ed Beal
    Commented Jan 25, 2018 at 19:55

2 Answers 2


The water is coming from the air inside the room, not from outside. It is condensing on the colder surfaces on the exterior walls (even if they don't feel cold). What you are doing is fine, it will block any air seepage and add some insulation value, but won't stop the problem you are having. What you need is a vapor barrier on the interior surface of the insulation to prevent the moisture in the room air from coming into contact with the colder layers of insulation inside the wall and condensing.

  • what sort of vapor barrier might i use ? Should it be on top of the foam in this case? Is the XPS foam itself not a strong enough vapor barrier ?
    – skimon
    Commented Jan 25, 2018 at 19:50
  • they sell plastic sheeting vapor barriers, it needs to be on the room side of the wall, over the insulation. The foam will make a good vapor barrier against the outside, but it will cool down and water from the room air will still condense on it.
    – Josh King
    Commented Jan 25, 2018 at 20:09
  • Thanks - over the foam or over the fiberglass batts?
    – skimon
    Commented Jan 25, 2018 at 20:23
  • over the fiberglass
    – Josh King
    Commented Jan 25, 2018 at 20:25
  • doesnt this create a double vapor barrier which is problematic? Since the xps - fiberglass - plastic is a sort of sandwich with vapor barriers on both sides?
    – skimon
    Commented Jan 25, 2018 at 22:20

You may find that are creating a new problem trying to solve the existing problem. This is because water vapor movement is somewhat complex because there quite a few factors to take into consideration. I strongly recommend you do some searching and reading the free information available at https://buildingscience.com/documents/guides-and-manuals I have several hundred dollars worth of design for moisture references and lot of the information in these expensive books has come from the once Federally funded research at Building Science. Trying to sum it all up in a message here is simply not practical.

But, know be aware that with the foam glued to the wood, water vapor from the outside can be forced into the wall, either wind driven or vapor pressure or both and now the foam is the "warmest" side and water vapor will condensate there and the wood will rot. It becomes very important that there be adequate measures in the exterior building envelope to prevent the moisture getting in from the outside. The more sealed up a wall, the harder it is for moisture that gets into the wall to dry out and this is a huge problem that we have in our efforts to design and build energy efficient buildings. There much information on the website I linked to that goes into great detail about it and the test houses that have been built in all the climate zones and then dismantled to find out what works and what doesn't. Code has not caught up to building science in all regions of the US, by the way.

So, you must deal with the moisture vapor inside and outside when construction a "perfect" wall system. As already pointed out, you've got two huge moisture sources inside and that moisture has to be dealt with vary pragmatically. There is good information in the link on how to deal with internal humidity effectively. You just need to do some searching on that site as there is a wealth of information there.

  • How is this answering the question?
    – Ed Beal
    Commented Jan 26, 2018 at 6:21
  • It really isn't directly answering it and my preference would have been to leave a comment but I don't have the reputation required to do that. I discovered this site a day ago kind of by accident. As 20+ year licensed/bonded/insured general contractor in Oregon I have dealt with a lot of moisture problems. So have most contractors here. Oregon has several climate zones, I am in Portland, we are considered coastal. It is wet here. Water and walls rotting because of mistakes made by builders and diy'ers alike have resulted in new continuing education requirements for contractors ...
    – C Knight
    Commented Jan 26, 2018 at 9:23
  • ... and substantially increased liability insurance costs for any contractor that works on projects that effect the building envelope. The original poster MAY be creating a bigger problem. I don't know what the exterior is or what kind of house wrap (if any), exterior insulations system (unlikely). I am simply pointing the poster to one of the best free sources of information on the web that is directly relevant to the problem they have, and the one they COULD be creating later on. Removing siding and replacing sheathing is expensive.
    – C Knight
    Commented Jan 26, 2018 at 9:29
  • @skimon Sorry about that. I was trying to link to the "manuals and guides" the main site is [buildingscience.com] (buildingscience.com). There are several different areas on the site that has free information. Start in tab "Guidance" and probably "Guides and Manuals" but "Enclosures that Work" and "Designs that Work" will heavily influence building codes eventually. The Papers tab has a lot of research information, most of it is easy reading. The books are all good though some are now not as up to date as the research papers and can be expensive to buy. Explore the site.
    – C Knight
    Commented Jan 26, 2018 at 21:02

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