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pextron

Eastern USA

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Joined: 06/27/2010

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Posted: 06/29/10 01:58am Link  |  Print  |  Notify Moderator

It seems that I may be on the right track about using house wrap as a vapor barrier.

While watching a series of travel trailer restoration instructional videos titled Restoring The Deville, I found some where they use house wrap and vapor barrier beneath the aluminum skin to prevent condensation from rotting the wood beneath.

Restoring The Deville Part 13a (Installing the Metal):
http://www.youtube.com/watch?v=pKz-1PikFbA

Restoring The Deville Part 23d (Insulation & Vapor Barrier):
http://www.youtube.com/watch?v=HUNl_hAQCW8

* This post was edited 06/29/10 02:36am by pextron *





smthbros

WI

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Posted: 06/29/10 11:33am Link  |  Print  |  Notify Moderator

While I agree with the need for vapor barriers in RV construction, I disagree with the location of the vapor barrier being between the skin and framing. Fiberglass insulation will inhibit the transfer of heat but not moisture. The moisture inside will pass thru the insulation and then condense on the now cold (because it is not insulated from the outside temps) vapor barrier, causing the same problems as if there were no vapor barrier. The vapor barrier should be on the conditioned side of the insulation.

pextron

Eastern USA

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Posted: 06/29/10 06:48pm Link  |  Print  |  Notify Moderator

smthbros wrote:

While I agree with the need for vapor barriers in RV construction, I disagree with the location of the vapor barrier being between the skin and framing. Fiberglass insulation will inhibit the transfer of heat but not moisture. The moisture inside will pass thru the insulation and then condense on the now cold (because it is not insulated from the outside temps) vapor barrier, causing the same problems as if there were no vapor barrier. The vapor barrier should be on the conditioned side of the insulation.


This got me to thinking so I did further research and it would seem that the proper location of the vapor barrier, or vapor retarder greatly depends on the climate. According to the US Department of Energy, the vapor barrier should be placed on the interior side of walls in cold climates but it should be placed on the exterior side of walls in hot climates.



It seems to make sense because cold climates tend to be very dry while hot climates tend to be very humid. As hot air meets cold air, moisture or condensation will form, but the source of the moisture seems to be the most critical element in determining the proper placement of the vapor barrier. In other words, because the source of the moisture is on the inside in cold climates, it makes sense to put the vapor barrier on the inside and since the source of the moisture tends to be on the outside in hot climates it makes sense to put the vapor barrier on the outside. However, because the location of RV's and travel trailers tend to vary a lot, it could be very difficult to know exactly where to put the vapor barrier.

Another interesting problem comes to mind too. Because RV's and travel trailers are prone to leaks, it would also seem to make sense to have a vapor retarder (if not vapor barrier) on the outside, as a secondary protection layer to help wick moisture away from the interior framing.

What if one used a less permeable vapor barrier (6 mil plastic) on the inside and a more permeable vapor retarder (house wrap) on the outside to get the best of both worlds?

smthbros

WI

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Posted: 06/29/10 08:54pm Link  |  Print  |  Notify Moderator

The exterior skin of an RV is a vapor barrier. If one were to add a vapor barrier to the inside as well, then one has created a trap for any vapor that may leak past either barrier. Trying to get a perfect seal may be problematic. It may be easier to manage the interior relative humidity with ventilation in cool outdoor temps, or air conditioning in warm outdoor temps, or a dehumidifier in moderate temps.

pextron

Eastern USA

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Posted: 06/30/10 12:15pm Link  |  Print  |  Notify Moderator

smthbros wrote:

The exterior skin of an RV is a vapor barrier. If one were to add a vapor barrier to the inside as well, then one has created a trap for any vapor that may leak past either barrier. Trying to get a perfect seal may be problematic. It may be easier to manage the interior relative humidity with ventilation in cool outdoor temps, or air conditioning in warm outdoor temps, or a dehumidifier in moderate temps.


The only problem is that the aluminum skin tends to have condensation form on it which could not only penetrate the studs but also the fiberglass insulation.

Wes Tausend

Bismarck, ND

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Posted: 06/30/10 01:59pm Link  |  Print  |  Notify Moderator

...

Those are some good links, the DeVille and the US Department of Energy on vapor barriers. A more correct term for "barrier" would be "retarder", since very few substances approach perfect "barriers" to dry humidity.

I agree with you guys. I believe that the best place for a vapor barrier would be either directly under the interior paneling or paint-coated on the interior wall face of its surface.

Polyethelene plastic sheeting might work under interior paneling if it isn't glued to the studs, just as it works in full size homes under the gysum board in northern climates.

An alternative barrier retarder, is a specicically designed paint, such as Glidden, if you can still find such a product. The advantage is that it can likely be applied directly over the existng paneling, if the present surface is not too slippery a substance. There should not be gaps that leak vapor or air/vapor, so some caulk might need be applied under paint. I used this type barrier in my house, when I remodeled, to avoid removing existing drywall. This was combined with new exterior walls that I added built-in polyethelene (aka: Visqueen).

Vapor barriers have been traditionally applied to the "warm side" of the exterior home wall. In this position, they retard harmless dry humidity from migrating toward the cool side and condensing to harmful liquid water within the wall cavity.

The reason I think the vapor barrier should be on the inside in a camper is that, up north in cool weather, any migrating vapor tends to collect as wet moisture near the cool outside where the Dewpoint is reached. The alternative is also true, a cool air conditioned camper interior, also attracts humidity from warm moist air from the outside(like a cold glass of lemonade), whereas full size deep south homes might have a vapor barrier directly under the exterior siding. But many campers have filon (fiberglass as in boat hulls), or similar polyester resin exteriors which act as fairly efficient vapor barriers, not unlike a paint vapor barrier. Campers that have aluminum siding allow humidity in (and fortunately out) easier in between their lap mounting grooves. While they allow vapor in, they also dry sooner, than filon, within the cavity. So I reason that the best overall specifically added barrier will be an inner barrier.

The exterior home wraps are not intended to be vapor barriers, but rather the opposite. Tyvek, a popular brand, is designed to breathe any trapped vapor out, but resist both wind load penetration and perhaps some actual driven liquid. It is an economical version of Gore-Tex, Gore-Tex clothing being intended to breathe body moisture out, while at the same time resisting liquid water penetration. An example, in hunting boots, seems to water-proof the boots, while still breathing and venting foot perspiration away. So Tyvek might indeed make a good exterior barrier, as it does in full size northern homes, resisting liquid rain leaks and yet breathing most cavity humidity away, at least in the case of breathable aluminum trailer siding.

Wes
...


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DavidP

Raleigh

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Posted: 06/30/10 02:12pm Link  |  Print  |  Notify Moderator

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pextron

Eastern USA

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Posted: 06/30/10 02:22pm Link  |  Print  |  Notify Moderator

Wes Tausend wrote:

The exterior home wraps are not intended to be vapor barriers, but rather the opposite. Tyvek, a popular brand, is designed to breathe any trapped vapor out, but resist both wind load penetration and perhaps some actual driven liquid. It is an economical version of Gore-Tex, Gore-Tex clothing being intended to breathe body moisture out, while at the same time resisting liquid water penetration. An example, in hunting boots, seems to water-proof the boots, while still breathing and venting foot perspiration away. So Tyvek might indeed make a good exterior barrier, as it does in full size northern homes, resisting liquid rain leaks and yet breathing most cavity humidity away, at least in the case of breathable aluminum trailer siding.

Wes
...


This was the reasoning behind my idea to use plastic on the inside and housewrap on the outside. Since the housewrap can breathe, it would allow any trapped moisture to escape but at the same time, it would resist air movement and moisture penetration from the outside.

* This post was edited 06/30/10 02:54pm by pextron *

Bonefish

Midland, TX

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Posted: 06/30/10 03:03pm Link  |  Print  |  Notify Moderator

pextron wrote:


When I do eventually find one, I am seriously considering the possibility of completely stripping it down to the frame so I could start by cleaning and painting the frame and go from there. My thinking is that after the interior is removed, I could remove all the exterior siding and the roof, then remove each wall in one piece.


If you are planing on going to this extreme...Why don't you buy a good flat bed trailer with good tandum axles. If you are handy then just build a camper up from there. Look at a number of camper designs you like and design your own. It would not be hard to improve on a lot of the ones out there to suit your likes.

No matter which route you take you will not be camping this summer in a fixer-upper.

Bonefish





pextron

Eastern USA

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Posted: 06/30/10 04:05pm Link  |  Print  |  Notify Moderator

Bonefish wrote:

If you are planing on going to this extreme...Why don't you buy a good flat bed trailer with good tandum axles. If you are handy then just build a camper up from there. Look at a number of camper designs you like and design your own. It would not be hard to improve on a lot of the ones out there to suit your likes.

No matter which route you take you will not be camping this summer in a fixer-upper.

Bonefish


I have considered building one from scratch, trailer frame and all, and considered building on on an existing trailer platform but when I did the math, it just seemed to make more sense (financially) if I bought one that had extensive water damage but was otherwise in tact with working appliances, A/C, awning, etc... to use as a basis for my build. The prices I have been seeing for damaged units have been in the 1-2k range for models in the mid to late 90's or early 20's which seems reasonable enough when I factor in the cost of everything except the wood, paneling, cabinets, etc....

I posess all the skills (carpentry, plumbing, wiring, welding, etc...) needed and have built several houses, including the recent remodel of the house I currently own so doing a travel trailer should be easy by comparison. Granted, there is a lot that I don't know that is specific to travel trailers but I am learning fast. I am still very early in the planning stages but my timeline for completion will be approximately 1 year, after I manage to find a suitable trailer to use as my project donor.

Currently, my plan is to find a suitable travel trailer to use as a donor, then strip it down to the frame after taking detailed pictures of everything to use as reference material later. I am also considering various floor plans but don't want to get too involved in the actual design until I find a donor because the length and other variables are subject to change. In the mean time, I am taking the time to learn everything I can about travel trailers, especially about how they are constructed.

* This post was edited 06/30/10 04:17pm by pextron *

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