I am remodeling a bedroom in an old farmhouse. I am unsure what the best vapor barrier would be on the interior wall behind the drywall.
The interior is studded 2×4’s. The exterior is some kind of fiberboard approximately 1.5″ thick (attached to the studs) with tar paper on the outside of that, followed by brick, and then Permastone (fake stone veneer applied in the 1950’s).
My intention is to do a kind of “flash and batts” approach, spraying closed cell foam onto the interior of the fiberboard and then follow with batts. The question is whether kraft or poly sheeting is the better vapor barrier considering the exterior arrangement described above.
The house is in Southeastern Pennnsylvania. The room is not airconditioned in the summer.
Any suggestions would be appreciated.
M Kniesly
Replies
closed cell foam = vapor retarder
your closed cell foam would be working as a vapor retarder. My concern would be sandwiching insulation between closed cell foam and another vapor barrier/retarder.... you would be creating an area that would not be able to dry in either direction.
If the flash of foam is thick enough, it will keep open interior of the wall above the dew point. I don’t know how thick it would have to be to accomplish that, but I doubt that ½” would be enough. If the foam is not thick enough, and if you don’t stop the vapor from entering the wall from the interior, you will have moisture condensing in the wall and wetting the fiberglass and the framing.
A rather new line of thinking is that if you omit the interior vapor barrier, any such moisture in the wall will eventually escape back into the room during times when conditions moderate and moisture stops condensing in the wall. In my opinion, this is a rather flawed concept. It strikes me as just another excuse to omit the hated interior film vapor barrier.
This thinking is also a natural consequence of sealing the exterior with foam, which becomes a vapor barrier. Then adding an interior vapor barrier creates the infamous “double vapor barrier” that is known for trapping moisture inside the wall. So, to overcome this dilemma, a case has been developed to omit the interior vapor barrier, so the wall can dry out to the room space.
The problem is that when conditions are forming moisture inside the wall, there is no drying taking place, and vice versa. If drying and moistening occurred on a nice, regular cycle of say one day wetting and one day drying, I suppose the situation would remain under control. But it won’t be a regular cycle by a long shot. And the more water that forms in a wall, the lower the R-value. And the lower the R-value, the quicker water condenses. So the problem can accelerate and get beyond the ability of the wall to dry out in a timely manner if the dying cycles happen to be too short to keep up.
One fundamental truth is that there is no problem with a double vapor barrier if the interior vapor barrier is perfect. Of course, perfect is not possible, but good enough is. So probably a well-executed interior film barrier will prevent any vapor from condensing into water inside of your wall.
But what I would do is seal up around the windows and any other detectable air leaks, and omit the foam flash coat. That way the wall can ventilate to the exterior. With brick veneer and a layer of felt, I don’t think air infiltration would be significant enough to warrant a monolithic foam sealing coat just to stop air infiltration. Then I would insulate with batts and apply an interior side film vapor barrier.
Or I would foam the walls full and omit the fiberglass if I wanted a higher R-value.
good post
good points -- highlights why vapor retarders elicit so much discussion.
Speaking of dew points however, I find it tough to see that the interior side of the foam would be low enough to create condensate. Assuming a 65-68 degree interior, in winter time relative humidity is rarely ever about 25% (except for bathrooms) -- creating a dewpoint of about 32 degrees.... chances the inside of the foam could reach 32? I think unlikely.
never the less, I think you have great suggestions to solve the problem... omit the foam or use foam exclusively.
If you want to use foam
as your outermost insulation layer, consult the IRC tables regarding the minimum thickness. They have already done the calcs for you and can tell you how thick is thick enough to prevent forming a dewpoint on the interior of the foam. Note that presence or lack of additional insulation inside the foam layer has an impact.
As to the idea that interior RH is rarely above 25% in the winter, that's nonsense, or at least it can't be applied to every climate, and certainly not to every house in any given climate. Occupant behavior has a huge impact on RH, as does building construction and conditions. Do you own a hygrometer, what type is it, and how often do you take it into other people's homes and measure RH?
Yeah, certainly we run an RH of about 35% (about 39F dewpoint) through most of the milder part of winter. It does drop when the temp drops below zero, though.
You're right it can't be applied to every climate...but nonsense is a bit rough. I do own several hygrometers. We had issues in some rental properties of indoor winter relative humidities over 75% -- obviously creating condensation and mold problems. Since resolving those issues, we monitor humidity very closely, and in winter time it is extremely low. Obviously if you are running a lot of combustion appliances, taking a lot of steamy showers, and cooking a lot of pasta things can be different.
Here's my point --- ok so maybe around here in the mild part of winter time RH runs 35% -- since its mild, its very unlikely that the interior of a wall is going to be at the dewpoint of say 39. As winter gets colder, and the interior of the wall gets colder as well -- the RH drops significantly.
I'm not saying vapor barriers are a farce - but I am going to go out on a limb and say there are improperly installed/designed vapor barriers doing more harm out there than if they weren't present at all.
If dewpoint condensation is such an issue - why aren't our old windows constantly dripping water? when they are, that usually tells us we have a humidity problem, not a vapor barrier problem. A more pressing problem is the issue of vapor instrusion in a humid climate in the summer into a cool house.
What you said
in winter time relative humidity is rarely ever about 25%
It might be true where you live, I guess. It's definitely not true here. I inspect houses for energy and moisture issues on a weekly basis, and see issues with excessive dryness and excessive humidity all the time. This is climate zone 4 marine, and it's fairly mild here, but we still have humidity-related issues that damage buildings. Occupant behavior and building construction details make major differences in how houses perform, whether or not they have problems, and what kind of problems those are. I see a lot of categorical statements being made here lately, and most of them need major qualification.
nonsense
sorry for the triple post
nonsense
see below
fair enough - I got off topic a bit... to be clear I agreed with the 3rd poster's suggestion of either doing all foam or just fiberglass. I am not saying to eliminate vapor barriers, but just that there is a lot of mis-information about them.
Regarding the dew point in winter time & the 39 degree wall that isn't really accurate. For SE PA, average temp in January is 32 and average RH is 65%. That creates a dewpoint of 21 degrees. Very difficult to pack cold air with water. Research shows that in cold non-maritime climates, adding insulation without a vapor barrier to existing homes does not cause moisture problems. My concern is people hearing they need vapor barriers and over-protecting, resulting in a situation that actually increases water retention. How many rotting basement walls are out there because someone sandwiched between a cold wet block wall and a vapor barrier?
again, this really doesn't play into the initial recommendation for the OP... just some healthy discussion
You are apparently not aware that human activities release moisture. Plus many people actively humidify their homes. Plus, just because the AVERAGE temp is 32 that says nothing about what the temp can hit overnight, or for several days in a row.
Vapor barriers
jschatz,
Unless the flash coat could guarantee that the outward migrating vapor would never encounter the dew point at the flash coat or earlier, I would not omit the vapor barrier. I think it is a bit risky to try to predict the humidity and dewpoint within the insulation cavity. Humidity in a wintertime heated house feels good, and with a humidifier inside of a forced air furnace, it is very easy to set the humidity too high to control condensation on windows and inside of walls (if there is no vapor barrier). Without a humidifier, home humidity varies according to number of occupants, bathing habits, and cooking. So, overall, I think predicting the insulation cavity dew point is very difficult.
Everybody wants to omit vapor barriers because they are a difficult and fussy detail to get right. They get torn during the sheetrocking process, and they are very hard to seal around penetrations from pipes and electrical boxes. Seams can get complicated and hard to seal. Too much seam taping and folded layers of vapor barrier on top of framing can interfere with the sheetocking.
With full spray closed-cell foam, you can omit the vapor barrier if you can guarantee that there are no cracks or gaps in the foam alongside framing members. But for all other insulating scenarios, I believe that vapor barriers are necessary on the warm side of walls for houses in wintertime heating climates. And I would use a high quality film with meticulous attention to detail.
I would be highly skeptical of research that shows that adding insulation without a vapor barrier to existing homes does not cause moisture problems, as you say. That may be true in some cases, but there are too many variables for that statement to stand as fundamentally true. In a climate like Minnesota, in a house with permeable insulation and no vapor barrier, if you humidify enough with a humidifier, you can fill the wall and ceiling cavities with ice and water.
And, I do not agree that properly installed vapor barriers can cause problems. In your example of a wall structure rotting away between a vapor barrier and wet concrete blocks the fault is not with the vapor barrier. If the wetness were coming from interior vapor condensing, the vapor barrier would have to be so full of holes that it would not interfere with any possible drying back to the interior that might tend to occur. If the wetness is from ground water, omitting the vapor barrier is not going to help
apparently
It was just an example --- it certaintly gets colder than 32 in pa, and obviously moisture levels are depend on human activity. Point was winter time dewpoints are lower than most interior walls.
Again, please don't avoid my main point...maybe I did a bad job making it.
The OP was considering creating a wall assembly with 2 vapor barriers present --- my point is, in their case (in climate zone 5), they would be better off with no vapor barrier vs. 2. Regardless, the advice I have complemented is very good (hence the complement).
Your numbers
Regarding the dew point in winter time & the 39 degree wall that isn't really accurate. For SE PA, average temp in January is 32 and average RH is 65%. That creates a dewpoint of 21 degrees
OK, average EXTERIOR temp and RH might be 32F and 65%, but the issue is INTERIOR humidity, which can be much, much higher. For instance, right now here, exterior humidity is 27 grains per pound but interior humidity in my house is 46 GPP. It's a little lower in my shop but still quite a bit higher than exterior. That's the humidity that needs to be considered relative to the dewpoint in the wall assembly.
I think you understand this but since I think a lot of people read these discussions I would like it to be clear. I also take your point about vapor barriers. I hardly worry about them in this climate, and in most houses, because conditions are mild and most houses leak a lot of air. Bulk water infiltration is a much larger problem.
First and foremost, "dual
First and foremost, "dual vapor barriers" is a red herring. Everything is a "vapor barrier" to one extent or another, and the real issue is the humidity profile across the wall section, compared to the wall's temperature profile. The potential "badness" (in winter) is not having a plastic vapor barrier on the inside wall,but having the foam (a relatively good vapor barrier) near the outside wall, and having additional insulation inside of that. A vapor barrier on the inside wall can only improve this potentially bad situation.