We are going to have some closed cell insulation sprayed into our second floor exterior wall and dormer roof areas. This will be covered with a 2″ rigid insulation. Should we put in a 6 mil vapor barrier over this (or under I guess, in the roof area)? My understanding is that the closed cell acts as a vapor barrier.
Discussion Forum
Discussion Forum
Up Next
Video Shorts
Featured Story
A custom finish carpenter shares how he designed and outfit his two-car garage shop with all the tools, jigs, and accessories he needs to comfortably do great work.
Highlights
"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.
Replies
Aside from all the other issues, where you put a vapor barrier depends on your climate, which you failed to specify.
Foam sandwich?
Are you talking about rigid foam on the outside and spray foam on the inside?
The spray foam is going to go between studs and rafters, then the rigid foam will be put up over that. We won't be doing anything to the outside of the house this time around. Just trying to maximize the R value.
We live in Brunswick, Maine. When we insulated the house we put up a 6 mil vapor barrier over the insulation - walls and ceilings - but never taped up any of the overlaps. We had many different opinions on this issue. One opinion was that the paper face on the insulation would act as a vapor barrier and we didn't need anything else. Another was that we needed to put in the paper faced insulation and use the 6 mil also, to prevent any warm air from going through to the exterior sheathing. One problem we have with this house is a lot of condensation on the windows (on some of the coldest winter days that is often ice). We have a concrete floor in the basement, but wasn't sure if the vapor barrier might be causing this condensation.
haw
I may have missed it, but what type of heat do you have in the house?
Aquariums,plants, other moisture producers?
Heat
The house is heated by an oil furnace and backup woodstove. We tend to leave the thermostat around 63°.
Some thoughts on your project
I assume you are going to finish that foam with drywall. If I am not mistaken, that would be required for fire protection. If you pay for the spray foam going in first, would it be that much more to just complete the foam filling of the insulation space with the spray foam, as opposed to filling part way with the spray foam, and then finishing the fill with rigid foam board?
With the approach you describe, using spay foam first and then covering that with foam board, I would still use a film vapor barrier at the warm side of the wall. The spay foam would be a vapor barrier, but you have to be concerned about where the dew point would be in the wall. If you have too much R-value in that foam board and/or not enough in the spayed foam, the spayed foam might be cool enough to condense any outgoing vapor. If that happened, you could get moisture in between the two foams. And it would not dry out very easily.
Or you might consider just filling the whole cavity with the foam board cut to fit nice and snug, and then cover with a film vapor barrier under the sheetrock. With the vapor barrier, there won’t be any air infiltration that could make it all the way into the heated space. The infiltration that gets in around the foam won’t actually penetrate the foam and degrade its R-value like it would if the insulation were fiberglass or cellulose. And if any outgoing vapor did get past the vapor barrier, it would be readily ventilated out through the small air leaks in the sheathing and siding.
Although, maybe I am not understanding your plan. You say you would add spay foam between the rafters, and then add foam board over that. Does that mean you would put the foam board on top of, or spanning over the 1-1/2” faces of the rafters? If that is the case, I am not sure how you would intend to finish the interior surface.
The fact that you added a vapor barrier in previous insulating work, could have increased window condensation. Prior to that vapor barrier, vapor had a lot more places to ventilate out. Adding the vapor barrier would have helped contain the vapor, leading to higher humidity, and more condensation on the windows. But the vapor barrier is needed in conduction with insulation, so the true remedy for the window condensation is to try to lower the humidity in the house. If you have a humidifier, turn it down and try to find a dryer setting that you can live with that does not frost up the windows significantly.
The walls and ceiling will be finished with sheetrock. My understanding is that the spray foam would go between the studs and rafters (filling the cavities) and then the layer of rigid would span over the 1-1/2" (actually they are 2" because they are full 2x4's) faces and then the layer of sheetrock.
I've tried to convince the contractor to use 4" rigid insulation cut to fit snug in each cavity, but he doesn't seem to be hearing it. I think it would be less costly than the spray foam. If there were any gaps with the rigid, he could fill them with some expanding foam. I may try this approach again.
The space we are going to be working on is currently insulated with fiberglass, covered with a vapor barrier and sheetrocked, but we haven't done any painting. So, the initial thought for this project was to just put up some strapping over the sheetrock, attach some 2" rigid insulation and then sheetrock again. Because our proper vents aren't installed correctly, I decided that it would be best to take a different approach. I guess the thinking behind the strapping and rigid under (which is it when you're talking about a ceiling that is sheetrocked?) what is already in place would be the increased R value.
We get condensation on the windows even when the temperature outside goes down to the low 30's. We don't have a humidifier, but we do have a dehumidifier. Unfortunately we just couldn't afford to run it this summer. We use our dryer about 5 times a year. Most of the summer the clothes hang outside and in the winter they hang on racks.
Thanks for your thoughts and help.
and in the winter they hang on racks.
Might be your source for excess moisture.
thermo at 63? you aren't married to my wife.
Yep, if you're drying your clothes inside on racks, that would be a major source of humidity in the house.
Haw1, note that some condensation on double-pane windows is reasonably normal when temps drop near zero (especially if curtains are closed), and frost on the windows will occur at below-zero temps, but it shouldn't be "a lot".
If you're getting "a lot", your problem is simply that the humidity in the house is too high -- the vapor barrier has nothing to do with it (and may, in fact, be saving your house from serious damage). You need to identify humidity sources in the house and deal with them -- run vent fans when showering, run a kitchen vent hood when cooking, etc. If the house is excessively "tight" you may need to install an air exchange system.