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Vapor Barrier Psuedo Science?  RSS feed

 
gardener
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Perhaps someone can shed light on this, because I have read so much conflicting research.

Building scientists get all wierd about vapor barriers. In zero energy homes you: yes, want a vapor barrier over everything. In fact, you want it so tight and thorough no air can get through. In traditional housing thought you absolutely do not want a vapor barrier because houses are meant to breathe and such a barrier will trap moisture in your walls causing mildew and mold.  

Additional research on green housing evaluating the air barrier affect of multiple coats of paint of various types shows paint (already applied to just about all housing stock) is an air barrier. This seems to blow the whole barrier = mold thing out of the water. This old thinking seems to expect, perhaps no insulation (or limited) and perhaps less mold resistant building materials and perhaps more gaps allowing air to travel through, I'm thinking.

Anyone else have opinions?

 
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Here in Alaska we use poly sheeting sealed at the seams and overlapped at the seams on the interior of all exterior walls and the ceiling before wall coverings such as sheetrock go on.  Moisture migrates from warm to cold and in our and other areas cold climate that warm moist air will migrate into the wall cavity.  At a certain point inside the wall cavity the water vapor will reach a point where it will condense and freeze inside the wall.  This renders the insulation useless and sets the stage for mold growth.  Standard construction type houses are built as tight as possible and use active ventilation to exchange the inside air instead of relying on a drafty house to do it naturally.  I've seen the affects of compromised insulation and it is not pretty.
 
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Amit, in Germany "Vapour barriers" are used everywhere. (They are not just barriers, but more often only slow water vapor down, which otherwise cannot escape). I have no doubt that they work… how long they work is an entirely different question.
And I am sure that they are not needed when the choice of wall material and plaster was done with vapor permeability in mind.
 
Amit Enventres
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Good point Walt! In a child's climate the hot (humid) air miche from inside to outside, so even in a traditional home setting a moisture barrier might help your house.... unless your house is really drafty so all the warm, moist air is sucked out and quickly replaced with cold air that when heated becomes skin-crackingly dry air....oh wait....that describes our housing stock. Haha.

So if that's the case, insulating old drafty housing in a cold climate basically should also involve an air barrier.

 
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Give me fresh air! Indoor air pollution can’t be good for your health. I enjoy air flow even in zero degree weather. I’d rather invest in thermal mass. One thing I enjoy about strawbale construction is the air flow! I might have too much due to my wood windows, but maybe next time. Still makes a cozy little nest!
 
pollinator
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Give me fresh air, but let's do it intentionally, not haphazardly.

I want to go the thermal mass route, too, with an insulative layer between it and the cold. My ideal, for now, is a rammed-earth wofati-like structure. I think that if the rammed earth is a moisture moderating influence, and won't rot and fall down on me, that my main concern is draught. I don't care how warm the house, if it's draughty, it's going to need to be kept five to ten degrees warmer than it would were it properly sealed.

Which is not to say that I want to smell my own two-week-old farts coming in the door, but that I think that deliberate intentional ventilation, with different seasonal intakes (sheltered cool intake for the summer) and a design which takes full advantage of thermodynamics for heating and cooling, is the superior approach to building, versus coating a decomposition-prone medium with cob to create a structure whose air exchanges are at the whim of weather and a danger to my structure's integrity.

-CK
 
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Amit Enventres wrote:
Building scientists get all wierd about vapor barriers. In zero energy homes you: yes, want a vapor barrier over everything. In fact, you want it so tight and thorough no air can get through. In traditional housing thought you absolutely do not want a vapor barrier because houses are meant to breathe and such a barrier will trap moisture in your walls causing mildew and mold.  

Additional research on green housing evaluating the air barrier affect of multiple coats of paint of various types shows paint (already applied to just about all housing stock) is an air barrier. This seems to blow the whole barrier = mold thing out of the water. This old thinking seems to expect, perhaps no insulation (or limited) and perhaps less mold resistant building materials and perhaps more gaps allowing air to travel through, I'm thinking.



In traditional housing you HAVE to have a vapour barrier, by code.  It's on the inside, just inside the wall cover.  This prevents moisture from passing through the wall cover and into the wall.  If that moisture is able to get into the wall, it will condense at the point where the temperature gradient reaches 100% RH.  If that point comes where there's batt or celulose insulation, you'll get condensation in the walls.  If you've got closed=cell foam, the moisture can't condensate out.  Once you get above about 60% RH, mould will start to grow.  You typically need about 85% for Stachybotrys to start to colonise.  

I don't understand the last bit about the paint and how the barrier = mould thing is blow out of water.  Can you try to explain it a different way?  Sorry for being dense.
 
Amit Enventres
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Hi Timothy!

So code varies from region to region and era to era. Therefore, there's no official vapor barrier in our old home.  It was designed to have air flow freely and so no moisture should build up. In warm regions I hear the moisture barrier should go on the exterior if you air condition to keep moisture from getting trapped in the walls due to the reverse temperature gradient.

However, certain paints, especially in multiple coats, act as air/vapor barriers rather effectively.  So, our home now essentially has a vapor barrier even though it wasn't designed for one.  This might be why some of our exterior walls show some water damage without there being any official leeks. This should be fixed by weep holes in the brick exterior (a newer design element than our house and a fairly easy retrofit).

On a side note on cob/strawbale. Some where in our neighborhood there is a straw bale house which supposedly is doing fine. I would also worry about rot on strawbale, but then I have a house of wood, so who am I to talk?


 
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Let us say you are in a car in a very cold place. your heater is on. Inside is hotter than outside, where does the condensation occur? It is inside, right? the opposite is true here in our hot country. condensation occurs outside because we use  air conditioning. Take one wall of your house, cold outside hot inside. Vapor will condense inside and soak your wall with water thus the wall loses its insulating property. Worst if there is enough water in the pores of the wall and it freezes your wall can crack. So yes vapor barrier is needed where there is a differential temperature and you need it on the hot side not the cold side.
 
Amit Enventres
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I love your analogy julian! The crazy part of building science here is that there is so much air exchange that the air heated is basically the air outside which has low moisture content, so there's no moisture issues, until you lower your air exchange rate, like in a car.
 
Chris Kott
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julian Gerona wrote:Let us say you are in a car in a very cold place. your heater is on. Inside is hotter than outside, where does the condensation occur? It is inside, right? the opposite is true here in our hot country. condensation occurs outside because we use  air conditioning. Take one wall of your house, cold outside hot inside. Vapor will condense inside and soak your wall with water thus the wall loses its insulating property. Worst if there is enough water in the pores of the wall and it freezes your wall can crack. So yes vapor barrier is needed where there is a differential temperature and you need it on the hot side not the cold side.



I agree, Amit. A very straightforward analogy. I hadn't thought of it that way, but of course, that's exactly it.

-CK

 
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In a really old house with poor insulation and sealing, air being able to move freely through the walls will reduce the chance of condensation and rot inside the walls, at the cost of very poor thermal performance. A vapor barrier on the inside surface, like paint, can only slow the entrance of vapor into the wall cavity and thus lower the chances of condensation and rot. (All predicated on cold winter temperatures being the controlling factor.)

If the interior humidity is very high, or the insulation so poor that interior surfaces are cold, then you may get condensation and mold on the surface. This is not the fault of the vapor barrier, but of poor construction that doesn't keep cold out or humidity down.

If there is no interior vapor barrier in the same situation, then the vapor can travel into the wall cavity and possibly freeze inside and soak the wall and cause hidden rot.

A strawbale house that is in good condition likely has, in addition to good roof and foundation, vapor-permeable surfaces outside and probably somewhat less permeable surfaces inside, thus allowing more vapor to exit than gets in from the interior. Good air exposure on the exterior would help greatly, while a wall closely shaded by bushes or vines might slow evaporation and let rot start.
 
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there is an excellent video on the subject: "dont do stupid things": https://www.youtube.com/watch?v=rkfAcWpOYAA this whole thing is several hours in length. I want to point out that moisture barrier and thermal barrier usually coexist and both are a defining feature of what I call 'artificial buildings'. Earth block and rammed earth buildings, what I call 'natural buildings' have a thermal gradient, even if they have a break. I would love to discuss this stuff.
 
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Howdy everyone !
Riffing off what Glenn Herbert said. Traditional houses were not designed for the kind of water intensive lives we now lead. In modern houses the wet areas, baths, kitchens, are inside. Cooking produces an immense amount of water vapor, so does bathing. Not to mention the general water vapor from our respiration.   Traditional houses everywhere had no vapor barrier and were generally built of vapor permeable materials. The thing is, the ways of life in the past were quite different from ours.

Some estimates are that an average family of four releases about 3 gallons of water a day in pure humidity, just vapor. Some lifestyle choices may actually release more.
In the modern world we shower, cook and boil water at home, and the amount of water we sometimes use is prodigious. A fun experiment is put a large plastic tub or bucket in your bath tub or shower stall, aim the shower nozzle at it so that most of the water hits it, and let the shower run for 10 minutes. Take a funnel, and a couple of one gallons milk jugs or pitcher. Pour all of the water into a jug or pitcher until its full, then go empty it on the houseplants or garden. Count the number of times you fill up a jug. We do that every day in taking a bath.

Traditional houses do allow air and water vapor to move freely, there is a thermal cost but people tended to insulate their bodies, wearing more or less layers of clothing. Also space radiant heating was more common. Even in the 20th century, up until recently whole house heating was a middle class or wealthy luxury. Older poorer houses had gas space heaters, or even coal and wood stoves. Up until the 1960s and 70s coal and wood stoves were an economic necessity, not a lifestyle choice, for millions of Americans. People "bundled up" in the winter. In the summer they opened windows and doors, which encouraged air circulation. Poor thermal performance, even in an age not concerned with the ecological impacts of our energy use, was not a major factor. People probably consumed less fuel with their stoves and fireplaces. Radiant bed pans with coals were common up until the 1930s and 40s.

Today our lifestyles are quite different. We don't bundle up and self insulate indoors during the winter, unless we are poor. Whole house gas or electric heating is the norm. Our bodies are used to operating at different temperature ranges than our parents or grandparents, much less than great grandparents. And we bathe a lot more. I think that's a good thing, but it has an impact that needs to be considered.

In the western world, if we look at old house plans and descriptions from 18th and 19th century American houses then something that may hit us is that it wasn't uncommon for kitchens to be in basements, or sheds in the back. Sometimes cooking was also done at a central hearth/fireplace. It depended. But toilet/waste relieving facilities and bathing (when rarely done) were always outside. People bathed in rivers, water holes, or wooden and later metal tubs out back. They used out houses. In Urban houses, when bathing was (rarely) done you may have had a small metal tub and the amount of water actually used was small. Only the rich had large bathtubs. Showers did not exist. Remember, showers aspirate water and hot showers generate a lot of vapor.

In some parts of rural Europe cooking was done in a detached place, but in many others and in typical urban European houses however the cooking was done at the central hearth. Either way a lot of water vapor just goes up the chimney or hearth hole in the roof. As for bathing, when rarely done, it was done in facilities outside. In some European cities from the late Middle Ages on you also had communal bathhouses. Up until very recently, after World War I really, people tended to do one weekly Saturday bath. Even after World War I people didn't bathe as often as we do today. My grandmother was a depression baby, she "washed up" with a wash cloth at the bathroom sink, just scrubbing her arms, legs, face and other parts most days. She only took a real bath a couple of times a week. My siblings and I lived with our grandmother for a few years in my childhood and the first thing she did was teach us how to "wash up" and stop all of that gosh awful water wasting by bathing daily like our parents insisted.

Outside of the West, in Japan and parts of East Asia communal bathhouses, sometimes in Buddhist temples, were used. People did not bathe at home unless they were part of the social elite. In the Middle East, North Africa and the general Arab world cooking was often done in a central courtyard - and hence an outdoor space. Bathing was done at communal "hammam" bathhouses. The bathhouse was one of the main social spheres, outside of the cofeehouse and mosque, in which people met, gossiped, and socialized. When bathing was done at home, the "ghusl" or bath was done with about one or two liters of water, about half slowly poured over each side of the body, with olive oil soap, scrubbing with a luffah, and then the remaining poured over, with tooth brushing and rinsing the nostrils with a quick sniff of water. So the amounts of water used were small, hence the amount of water vapor dispersed small. (in fact Old people still bathe this way even in cities, in some parts of North Africa and the middle east. Even in modern houses with showers they may have a small plastic tub and small ibrik, or water pitcher. Younger generations shower Western style though...)

You will find similar in many old world urban cultures, we don't even have to look at rural areas. People generated less water vapor, and did vapor intensive activities in different parts of their house than we do today. We live more thermally intensive lives today, we need more heat for our comfort, and we choose to cook and bathe in our intimate living spaces. In exchange for thermal efficiency we must tolerate amounts of water vapor that facilitate mold growth, poor air quality, and condensation. The solution is ventilation. Whether we can do this mechanically or passively is a different mater, but vapor barriers, insulation, and aggressive ventilation are requirements for us today.

I could be seeing things incorrectly and welcome thoughts and feedback!
 
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