I read through a number of articles in the FHB magazine and notice some of the diagrams and photos do not show any vapor barrier on the walls or ceilings (I have seen it on basement floors). A lot of focus is on air-sealing, which is good, but little about vapor barriers.
I am not criticising the builders or authors but I am just curious as to the reason. Perhaps it is due to different building trechniques across the border; although I am in southern Ontario, our weather cannot be worse than most northern States.
Alternatively, I wonder if it is because once the house is air tight there is little appreciable moisture penetration through the walls and ceiling.
Perhaps the stricter building codes do not require it anymore.
The reason for my question is that I have built houses and done renovations in the past but it has been a few years. I was always told that a vapor barrier is as important as air-sealing a house (actuallt, the two work together).
Thanks for all thoughts and input!
Replies
Actually, air sealing is almost certainly more important, but often a vapor barrier is a major factor in air sealing.
How necessary an actual vapor barrier (in the form of poly sheeting, eg) is to the success of overall weatherization design depends on a lot of factors. What is, in the final analysis, necessary, is a temperature/dewpoint "profile" across the wall/ceiling cross-section which never allows the temperature to drop below the dewpoint at any spot in the wall/ceiling. In many cases an actual vapor barrier is the simplest way to assure this.
It often happens, however, that people get emboiled in emotional "discussions" about the dreaded "double vapor barrier", raising all sorts of fallacious scenarios, and as a result it seems easier to omit any distinct vapor barrier than to figure out what is really true.
Usually (if it's done at all), the "profile" is worked out by an archie or engineer (or some other techie) based on standard construction practices and tables of material characteristics. And then the design is copied again and again to where any link to the original calculations (if there were any) is lost. What likely happens in most cases is that folks learn "this seems to work, most fo the time" and they keep doing it again and again. (And, of course, they don't really know if it's working unless the wall is opened up or it suffers a serious failure due to poor design.)
And actually a leaky (from the outside) wall is often less apt to suffer from moisture problems (condensation within the wall) since the moisture can more readily escape. This is the deal about "double vapor barriers" -- it's not really having two that's the problem, it's having an outer (colder) layer that's better at stopping vapor than the inner layers, so that the dewpoint profile is high until near the outer surface (while the temperature profile will tend to slope downward at a roughly steady rate). You could have 27 vapor barriers on the warm side with no problem.
Vapor Barriers YES!
I can only relate what I have done and how it is working for me. When I built a place in Northern Michigan where it regularly gets to -10. I went with 2x6 walls and unfaced fiberglass R-21 batts in the walls and 16" of cellulose in the ceiling. I am fully sided with 1/2" OSB which certainly slows any moisture permeation to the outside, this is covered with Tyvec and then vinyl siding. I used a 6mil poly for the vapor barrier starting with the ceiling first and then wraping the walls. I used special boxes around all outlets on the outside walls to ensure minimal air movement to the interior. In other words the place is air tight. I actually believe this is easier and faster than the typical faced batt and it's easier to see if the batt is in the wall cavity correctly. I just recently took my IR temp gun and checked the temps along all the outside walls looking for cold spots will it was -7 outside. My wall temp showed 2 degrees less than the inside temp everywhere I measured. The place has been finished for 7 years and I inspect it carefully every year and no signs of mold or moisture anywhere. I should also mention that being on Lake Huron, in the summer we get some days where the humidity is unbelievable. In totality this system was easy and cost effective and the results are utterly amazing. It costs me about $70 dollars to heat 2300 sq. ft. in the coldest part of winter. In addition, because I'm not fighting air leaks the overall comfort is fantastic. When we take friends up in the winter they can't believe even every room is. I currently live in a builder built home and my wife hates it. She wants me to build another place down state to match the comfort and economy of our vacation home.
The thing is, you need the air barrier more on the outside than then inside, especially with fiberglass insulation.
Why must an air barried be on the outside?
The insulation value of fiberglass is very easily destroyed if there is wind blowing through it. Other forms of insulation do better, but it's still best to keep the wind out of the walls.
Wouldn't the exterior sheathing (OSB or plywood) with the house wrap (plus exterior finish) not keep air movement down?
Thanks!
SnafuBE wrote:
Wouldn't the exterior sheathing (OSB or plywood) with the house wrap (plus exterior finish) not keep air movement down?
Thanks!
Not necessarily. If detailed properly along with the rest of the air sealing techniques-then yes.
There are millions of homes with all that you mention done to them and you can put out a match by holding it next to an exterior wall outlet.
It's all in the detailing of the details.
Yes, house wrap, properly applied, is an air barrier. That why you use it.
Thanks folks!
Proper installation is key ;)
I wish I could say the builders followed that advice when they built this house. I cannot believe an inspector did not see some of this stuff :o
Yes, installation of the housewrap needs to be obsessive -- taping or caulking every seam or joint.
Right ... air barrier on the outside. Vapor retarder (not barrier) ... is on the inside. They provide two different functions.
The vapor barrier, if any, on a below-grade masonry foundation wall should be against the masonry, on the "outside" of any insulation. The "moisture drive" is from the soil into the building, except in desert conditions.
Something that hasn't been discussed in these great threads is the difference between outside moisture and inside moisture.
Rain and humidity need to be kept out. Out of the OSB, out of the fiberglass, etc. That is done with house wrap, foam sheathing, flashing, etc.
Showers, boiling water, breathing, combustion from gas appliances etc. needs to get out. That is done with venting. Bath fans, Range hoods, HRVs, ERVs. If you seal the house so well the no air can get in (with moisture in it to be sure), no moisture will get out either. That is what condensates at the dew point. I believe(contrary to some) that if you have outllets, switches, windows, and doors that penetrate your inside vapor barrier(visqueen), you will get moisture in your wall cavity. Air will go there through convection cycles and take moisture there and create mold there. The tighter the outside vapor barrier, the more readily this occurs, unless the dew point is kept away from the moisture. Or take the inside moisture away from the dew point with venting. Recovery ventilators do this best because if your house is tight (2ACHr at 50pcl), bath fans, range hoods, and drier vents can't replace the air (moisture )they are trying to push out.
Of course, the critical factor is the dewpoint, not the relative humidity. So long as the dewpoint below the temperature then no condensation will occur.
As you suggest, a significant problem is outlets and other pentrations of the smooth (interior of the) exterior wall -- these can let significant amounts of humidity out and outside air in. But moisture will diffuse from such imperfections into the rest of the wall and, in the process, get "diluted" to a large degree, so if the overall moisture "leakage" rate is low then the danger of condensation in the walls can be kept similarly low.
An important thing to remember is that heat, moisture, and air all move through wall materials at different rates -- what stops air (eg, housewrap) does not necessarily stop moisture, and what stops moisture (eg, vapor barrier) does not necessarily stop heat. It's a balancing act to minimize heat transfer and air infiltration while "managing" moisture.
(And, of course, in northern to temperate climates there should NOT be a vapor barrier near the outside surface of an exposed-to-the weather wall.)
The chicken or the egg? Obviously at this level we are splitting hairs, butt... is there moisture in the air or air in the moisture. What goes through the house wrap? The H2 or the O? And why only one direction?
We made boxes once from house wraps, several kinds, and their tapes. We filled them with water and some leaked like seives (woven). Some did better, but all were empty in two days. That is why we use 1"XPS sealed with foam (TAPE HAS FAILED US TOO MANY TIMES) outside of our OSB sructural sheathing. We still use a vapor barrier inside for the inspector, but rely on fresh air circulation for moisture control. As with everything, if yours ain't broke don't fix it, but after our winter this year, our customers are making joyous noises about us.
As you found, housewraps let moisture through but are relatively impervious to air and liquid water (though some better than others). It's like a good overcoat -- it sheds rain, keeps the wind from blowing through, but "breathes" enough that you don't get all sweaty like you would in a "slicker".
(No material is "directional" in its tendency to let stuff diffuse through. It's simply that the H2O vapor pressure is generally higher inside the structure than out, at least in cooler weather, and gas diffuses from high vapor pressure to low.)
Transmission of water in liquid form is far different than moisture in air. Need to think of it as water dissolved in air. When teh temperature of that air reaches a certain point (dew point) the water 'precipitates' out of the air (ex. warm water can dissolve more salt but if the water is cooled the salt is forced out of the water).
All electrical boxes on an insulated (above-ground) wall should also have a vapor barrier if you use a vapor barrier on the wall. There are boxes made for this and can be sealed after the wires penetrate the box. The box is then sealed to the vapor barrier (acoustic sealant?). I have seen some use pieces of the vapor barrier material itself to wrap electrical boxes. This is to help minimise moisture in teh air from transferring through the wall.
And yes, with a well sealed (air sealed and moisture sealed) house an HRV is important to replenish air within the house.
I do not think there are uni-directional materials but as with anything if there are differences across a barrier, nature always tries to make both sides equal be it moisture, pressure or temperature...we are always fighting nature's push to make things equal :D
Won't really make much difference, so long as the wall doesn't leak.
Hahaha...yeah, I certainly hope it will not leak after I get a crack fixed.
Thanks!
The vapor barried can act as an air barrier too.
I have been reading a bit about variable-permeable vapor barriers.
But with something like fiberglass insulation (or even poorly-fit styro panels), a leaky exterior "envelope" can negate much of the effectiveness of insulation.
To an extent I can understand this point. If water (rain) or moisture from high humidity penetrates the wall from the outside then most insulation is pooched until it dries (and hopefully mold does not decide to grow until it dries).
Southern Ontario and Northern States enjoy weather that is very challenging to address. In winter the moisture drive is to the outside. However, in the summer on those hot hazy days the drive is to the interior. The only thing that provides some help is that the temperature difference across the wall is greater in the winter than in the summer. So my thinking (perhaps wrong) is that the moisture drive to the interior during the warm months is less than that in the winter, As a result during the warm humid months there would be less chance of condensation from external moisture occuring in the insulation on the cool side of the wall.
It must help to adjust the indoor temperatures during the summer to follow the outside temps rather than leave the A|C at the same temperature all summer (I usually set it about 5-10C below the daytime high during the summer).
Maybe variable permeability barriers are part of the answer
Love the discussions and searching more into this.
BTW I know that differences in location/climate plays a big part. I try to keep this in mind when reading articles.
It might make sense to adhere the foam to the wall the same way that foam is adhered outside for use as a base for synthetic stucco. That would mean applying synth stucco base with a trowel with 1/2" diameter cutouts in such a way that the mortar went on in more or less vertical lines.You press the foam into that, then, if any condensation occurs the water will fall to the floor. I don't know what happens to it then.
Use the vapour barrier inside. When there is a difference in relative humidity between inside and outside - and there is unless your windows are wide open - there is a physical pressure driving water vapour from high RH to low RH. That is mitigated somwwhat by most houses having a slightly lower air pressure inside than out, but the vapour is still being forced outward in winter as a rule.
Anyway, in Ontario I think it might be required as i believe Ontario has adopted the National Model Energy Code as provincial code.
That stucco base chould also work OK as parging with the addition of glass fiber mesh, too. It's pretty tough stuff. Any ICF supplier will have a product or two to choose from. They are all much the same but I have worked with Senergy Alpha Dry Base Coat quite a lot and found it just fine.
Ron
Thanks Ron.
Definitely continue to use vapor barriers on above ground wood framing when doing repairs. On the foundation (below ground) side my concern is that vapor/moisture from the poured concrete would become trapped between the barrier and the wall as the concrete would be the higher moisture side.
At least without a vapor barrier the moisture can pass through the wall to the interior and it would be addressed with air exchanges
Snafu (aka Ron as well)
(Grrr -- At least the 4th time in the past week that I made a post and clicked "Post" and it sent me back to "Reply to Comment" with an empty comment window.)
Anyway, moisture will not "become trapped" between vapor barrier and masonry unless the wall actually leaks. Once a "steady state" is achieved, no more moisture will transfer through the masonry. And masonry does not promote mildew. If it did then masonry would be a lousy choice for foundations.
Hey Dan, you're not getting upset with the software here are you?
Ron
This is a good question and a good discussion. Lots and lots of good information being shared, and it makes me happy to know that people understand the function of these layers! Bravo, guys!
Snafube - to answer your question in it's simplest form, I'd say that a vapor barrier is typically not necessary unless you live in a very cold climate. Why? Well, Martin Holladay will do a much better job of explaining: http://www.greenbuildingadvisor.com/blogs/dept/musings/do-i-need-vapor-retarder
Justin
While the discssion can be fun in itself, the OP lives in Ontario and I believe a poly vapour barrier is required in a wooden house there. If the work is done under a permit and inspected then the plastic must be installed, seams taped, edges sealed to window and door framing, electrical boxes installed in air seal boxes - the whole nine yards.
Ron
Thanks Justin for the words and the link to GBA. There are some good blogs and discussions.
Part of the challenge is the climate. Although southern Ontario can be cold it can also be hot. Same applies to many northern states. If the temperature and outdoor moisture levels were constant then building a good wall would be easier.
I originally brought up the topic only because I noticed articles that do not mention the use of a vapor barrier. I started to wonder if the practice was not necessary (part of my nature to question something different). It is possible that the examples were for homes in different climates zones (would easily explain it)
@Ron,
I do love the discussion and brought the point up for this purpose so I could better understand why vapor barriers are or are not being used.
I do wish all builders took that approach and inspectors enforced the rules. Not all builders are shody and I would not use a broad brush on all. The ones who built this small collection of houses and the inspector certainly did not follow code. So far nothing serious just shody workmanship. If I knew who they were then I would put their names on a black list ;).
Thanks everyone
SnafuBE
I don't recall seeing much in the NBCC about workmanship. I'm sure there must be something in there somewhere but what i recall is all about materials and design issues. Workmanship issues are left up to the conscience of the worker and the discretion of whoever is writing his paycheque. Building inspectors don't even have a lot of influence here except where those issues are dealt with in the code, that is,hardly at all.
Anyway, use the VB below grade too. Use an additional sheet of poly aainst the concrete or masonry outside the insulation to protect against liquid water. Vapour movement inward is not of any concern through a foundation wall, unless it's concrete block. Basements are typically damp in summer not because of the movement of water vapour through the wall but because of outdoor air sliding down the stairs and then cooling, which raises the relative humidity of that air. RH is dependant on temperature.
Poly vapour barriers are used less in the US than in Canada partly because of different climate conditions but mainly, I think, because they have different regulatory regimes in every village, town, county and state. Different codes, too. Here, the NBCC has been adopted as applicable code by almost every jurisdiction. The National Model Energy Code has also been adopted wolesale in many juridictions. Also, the techniques of bulding for a cold climate are much more widely adopted in Canada than in the US, naturally enough.
As an example, I think you can still buy glass fiber insulation in the States with the paper backing meant to be stapled on the studs or joists. That paper is sometimes called a vapour barrier. It haven't seen it in Canada in 30 years. It doesn't work here. The winter is too long, the vapour pressure differential gets too high and that stuff leaves too many air gaps. It also prevents you from getting a good insulation job because you can't see the fit.
Ron
Hi Dan,
I have experienced the same thing in other forums. I try to remember to copy the reply before posting just in case ;)
When I referred to 'trapped' moisture this was in reference to using a vapor barrier on an insultated below grade concrete (non-leaking) wall (basement). Trapped may not be the best word as it suggests that the moisture would never move (unlikely). With a vapor barrier being used the moisture level in such a wall would be higher than in a similar wall without the barrier (presuming the moisture level in the part of the house near the wall is less).
In this specific case it seems it would be better not to use a vapor barrier to allow any moisture to migrate into the house where it can be managed with air exchanges.
I'm talking about a vapor barrier directly against the masonry. If you do that there is no need to allow the moisture to "migrate" into the house -- it will achieve "steady state" rather quickly and no further moisture will enter.
Insulation, water vapor, condensation, absorbsion
Lets try to clear this up.
Water vapor is created by people, breathing, sweating, cooking, washing. So water vapor is almost always higher inside a home (with people) and warmth, than it is outside, where it is normally colder and drier.
This is why we use exhaust fans and blow our warm wet air outside and draw in colder drier air.
The reason water vapor barriers do not often work as designed is down to poor installation.
Water vapor is designed by nature to always move from warm to cold - in cold climates with winter heating it always moves outwards - in hot climates, homes with air conditioning - the water vapor moves inwards.
Water vapor molecules are very small, much smaller than air, things that are air tight are not necessarily water vapor tight.
The water vapor molecules are small enough to move inside the fabric of many things used in building - especially wood, which is hygroscopic and absorbs water vapor which condenses inside the wood, raises its water content and can lead to mold and wood rot.
The problem as you can appreciate, is that water vapor can move through cracks and holes that are too small to see. This problem can be overcome, by fitting sheet insulation on the cold outside of the water vapor barrier, to ensure the barriers room side surface is always above the rooms dew point - this ensures that the molecules of water vapor do not move inside the frame.
Thanks everyone for the good discussion on this complex topic!