GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter 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

Moisture on closed cell flash & batt – Do I need vapor barrier?

pjpfeiff | Posted in General Questions on

I think the key to the question is, am I being fooled by unusual conditions?

I’m in the middle of renovating the upstairs of a 50’s Cape Cod in Zone 4.  Insulation is flash & batt over the roof and walls as shown in the attachment (with the exception that I think instead of the batts with listed R-value like I asked for, I got 6″ batts on the roof and 3″ in the walls.  Also no dry wall is up yet.).  The area is unconditioned except for whatever heat migrates from downstairs (this keeps it at~60 deg F if the downstairs door is open).  I will have a mini-split installed soon. 

The moisture I found was only on the spray foam on roof deck–not in the walls.  A cheap meter I have says it is 60 deg. with an RH of 69% upstairs.  It’s actually warmer than that outside right now, but was recently much colder.  The same meter brought downstairs has not fully acclimated yet, but says 68 deg, RH=59%.  Outdoors it is 67 deg, 72% RH.

What I like to think is that this is just an unusual case where we had a good snow followed by warm temps.  So all the snow melted causing high humidity and outdoor temps are high enough that the furnace is not doing much–but the temp hasn’t been high enough long enough for the surface of the spray foam to get above dew point.  So the moisture observed is a rare occurrence (esp. once the upstairs is heated) that poses no problems.  Is that wishful thinking, or do I need a vapor barrier….or dehumidifier?

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. Expert Member
    BILL WICHERS | | #1

    If the space is unconditioned, you’re probably seeing natural condensation just like you see dew on the ground in the morning. This would be due to day/night temperature changes just like outdoors. Once the space is conditioned you’ll be maintaining more consistent temperatures which won’t allow condensation to form in this way.

    I think you’re probably going to be ok going forward without changing things, but I’d wait to see some other commenters thoughts on this before you do anything.


  2. Expert Member
    Peter Engle | | #2

    The dewpoint of interior air at 60F and 69% RH is about 55 F. With your insulation stackup, once the outdoor temperature goes below about 52 degrees, the inside face of the foam will drop below the dewpoint, and it becomes a condensing surface. If your interior conditions were the same when it was cold this week, you definitely would be getting condensation on the surfaec of the foam.

    Also, when it warms up as fast as it did this weekend, you've got some time effects. Even if the air in the house was dryer during the week, if the walls were cold and the surface of the foam was cold, you can easily get condensation on the surface of the foam when the air suddenly gets warmer and wetter. Since the surface of the foam is insulated from both directions, it will warm more slowly than the air either outside or in the room. But, with only fiber insulation towards the interior, vapor can move through the insulation much faster than heat can. The result, again, is condensation on the surface of the foam. Give it until the end of the weekend, when the whole wall has warmed up, and the condensation will probably dry out.

    Bottom line is that this is a temporary condition, related to the rapid change in weather, the largely unconditioned space, the moisture of construction, and the lack of drywall on the ceiling to act as an air barrier. The wall is properly designed, and once everything is done, it should behave properly.

  3. Expert Member
    Dana Dorsett | | #3

    >"Also no dry wall is up yet."
    >" is 60 deg. with an RH of 69% upstairs..."

    Air permeable insulation like batts needs an interior side air barrier to limit air-transported moisture transfer. Even with asphalted kraft facers (not possible to truly detail as an adequate air barrier), at 69% RH the kraft facers become quite vapor open at 8-10 perms or more. (When bone-dry they're about 0.5 perms- they are "smart" vapor retarders.) A layer of standard latex primer on reasonably air tight gypsum board runs about 5 perms, and should be good enough in climate zone 4 if at least 30% of the total R in the roof is the foam.

    If it's less than 30% it would need something more vapor tight, such has half-perm "vapor barrier latex" primer, or a broadsheet smart vapor retarder such as Intello Plus or Certainteed MemBrain.

    Running a room dehumidifer should be able to take the RH down to something reasonble (like < 40%) in a 60F attic, and as long as it's been reasonably dry (40% or less) for several days putting up and painting the ceiling gypsum in winter is fine.

  4. pjpfeiff | | #4

    Thanks, all. It sounds like I should be ok. Actually just today a mini-split was installed, so I'll be able to observe how things look while conditioned.

    I should also mention that for the ceiling I intend to put up painted T&G solid wood planks. Would that have much effect on this discussion? For one, I'm guessing it's not quite as air-tight as well-installed drywall, but is the difference significant?

    Also it sounds like it's a good idea to put some sort of air barrier along the roof insulation in the knee walls and the small triangular attic over the flat ceiling. I was thinking of putting up 1/4" sheet good in the knee walls anyway, just to keep from bumping my head against the rockwool when I'm back there. But I did not intend anything like that for the attic.

  5. MattJF | | #5

    “I intend to put up painted T&G solid wood planks. Would that have much effect on this discussion? For one, I'm guessing it's not quite as air-tight as well-installed drywall, but is the difference significant?”

    The difference is a bit like the difference between a bowl and a colander. You can put up drywall and the t&g over it. Technically you insulation stack up is okay though.

    I have a very similar roof, poliso furring included, except r21 ccSPF/r30 rockwool. I put up membrain on the inside for similar reasons of wanting to contain the rockwool. The only problem is that I now feel compelled to tape the Membrain as an additional air barrier, which is a total pain around knee wall joists and other structures.

    1. pjpfeiff | | #6

      Yeah, ok, I suppose the tongue can't make a very airtight seal with the groove. So perhaps a shiplap joint would be more airtight (it's supposed to be the joint used for boats, right?), but maybe only if you face-nail through the joint? Anyway, maybe Membrain can make some cheap insurance for me. How fussy is it to install on a sloped ceiling?

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |