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Wrong sided vapor barrier??


Bryan
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On a recent modular building inspection there was a "plastic type" barrier on the bottom side of the floor system with about 3 1/2 inches of fiberglass insulation between the joist. This barrier appears to be on the wrong side, or am i missing something? Is it possible it was just there to hold the insulation in-place during transportation.

Bryan

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I don't do manufactured home inspections. But if I understand your question, it doesn't matter. A vapor barrier should be applied against the heated side of the wall. In your case, I suspect that the plastic material holding the under floor insulation in is perforated. That keeps it from being a true vapor barrier.

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Originally posted by Bryan

On a recent modular building inspection there was a "plastic type" barrier on the bottom side of the floor system with about 3 1/2 inches of fiberglass insulation between the joist. This barrier appears to be on the wrong side, or am i missing something? Is it possible it was just there to hold the insulation in-place during transportation.

Bryan

Was it a "modular" home as you say it is or is it a "manufactured" home with a metal chassis and long stringers over a steel backbone for a floor system. If it was a manufactured home, they place that barrier on the underside of the insulation to

1. hold it up under the floors.

2. minimize the amount of moisture that gets up under the floors from the usually uncovered earth beneath.

It's normal and they are rarely intact; some ijit usually cuts through them someplace and never repairs them or some animule will get up in there.

If it is a manufactured home, please make a conscious effort NOT to call them modular homes because the difference is night and day.

ONE TEAM - ONE FIGHT!!!

Mike

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This is under an heated alcove between the garage and house that I did last night.

One side of it is skirted, the other side is wide open to the prevailing wind. Should this poly be slashed, or removed or what do you think? This is bigtime snow country.

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Originally posted by gtblum

This is under an heated alcove between the garage and house that I did last night.

One side of it is skirted, the other side is wide open to the prevailing wind. Should this poly be slashed, or removed or what do you think? This is bigtime snow country.

If it were my house, I'd yank the plastic and the strips of lath then I'd sheath the underside of the joists with plywood. This would keep out the critters, protect the insulation and reduce airborne moisture movement. It would increase the risk of condensation on the top side of the plywood, but I think that the risk would be small.

Alternatively, I'd wall in the open side and turn it into a crawlspace.

- Jim Katen, Oregon

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[ quote]Was it a "modular" home as you say it is or is it a "manufactured" home with a metal chassis and long stringers over a steel backbone for a floor system. If it was a manufactured home, they place that barrier on the underside of the insulation to

1. hold it up under the floors.

2. minimize the amount of moisture that gets up under the floors from the usually uncovered earth beneath.

It's normal and they are rarely intact; some ijit usually cuts through them someplace and never repairs them or some animule will get up in there.

If it is a manufactured home, please make a conscious effort NOT to call them modular homes because the difference is night and day.

ONE TEAM - ONE FIGHT!!!

Mike

Mike,

It was a modular building converted into an assisted living facility, which was constructed out of "pods" of approximately 12' x 24'. A very interesting set up and design. When they converted it to an assisted living they had to install all new plumbing throughout the crawl areas. During this period of the project the crawls were left a mess with debri and holes throughout the under deck covering.

Bryan

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If it were my house, I'd yank the plastic and the strips of lath then I'd sheath the underside of the joists with plywood. This would keep out the critters, protect the insulation and reduce airborne moisture movement. It would increase the risk of condensation on the top side of the plywood, but I think that the risk would be small.

Alternatively, I'd wall in the open side and turn it into a crawlspace.

- Jim Katen, Oregon

Plywood and OSB are vapor bariers, so besides the critters why is this any different than the poly?

Tom

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Originally posted by Tom Raymond

If it were my house, I'd yank the plastic and the strips of lath then I'd sheath the underside of the joists with plywood. This would keep out the critters, protect the insulation and reduce airborne moisture movement. It would increase the risk of condensation on the top side of the plywood, but I think that the risk would be small.

Alternatively, I'd wall in the open side and turn it into a crawlspace.

- Jim Katen, Oregon

Plywood and OSB are vapor bariers, so besides the critters why is this any different than the poly?

Tom

The plywood is certainly a vapor retarder. In this instance, however, it would be much better than the plastic for one big reason. It will do a much better job of reducing air movement. This, in turn, will prevent a lot of moisture movement which, in turn, will reduce the likelihood of condensation.

Moisture can move downward from the house into the joist spaces through diffusion or via air transport. Moisture movement through diffusion is usually a non-issue in buildings. The big culprit is air transport. If you can reduce air movement, you'll reduce condensation.

Also, even with the poly, there's little risk of a problem in this location. Air very rarely moves downward from the inside of a house into the crawlspace. It almost always moves upward due to the stack effect.

I doubt that plywood under the joists will be any worse than plywood on the outside of walls.

- Jim Katen, Oregon

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Originally posted by Bryan

Mike,

It was a modular building converted into an assisted living facility, which was constructed out of "pods" of approximately 12' x 24'. A very interesting set up and design. When they converted it to an assisted living they had to install all new plumbing throughout the crawl areas. During this period of the project the crawls were left a mess with debri and holes throughout the under deck covering.

Bryan

That description still troubles me. I don't think it's a modular home because manufacturers of modulars don't cover the underside of the floor joists with any plastic - double-wide manufacturers do all do that though. Also, the modular dealers try to eek out every last inch of space and will typically build 14ft. wide modules.

OT - OF!!!

M.

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Originally posted by randynavarro

Jim - clarification?

Plywood has a perm rating of .36

6 mil plastic has a perm rating of .06

Plastic is a much better retarder than ply.

You suggest that ply will do a better job than plastic at reducing air movement. Do you say this because there may be more seams and tears in the plastic?

Air movement has nothing to do with the perm rating. The perm rating affects vapor diffusion. We've learned that vapor diffusion is vastly less important in buildings than we used to think it was.

Even though the plastic has a better perm rating, it will develop holes, tears, rips, and so forth. That will make it pretty much useless.

The plywood is sturdier, has a decent enough perm rating to prevent wild swings in moisture (from the outdoor air) and will protect against wind, weather & critters.

If the interior of the building is a steam sauna and if the stack effect magically reverses itself there might be some condensation down there, but I suspect that those conditions will be unlikely.

Perm ratings have very little to do with the effects of moisture in buildings. It's all about air movement. The amount of moisture that passes through materials via vapor diffusion is minisule compared to the airborne moisture movement that occurs through the vast great gaps & holes in most buildings.

- Jim Katen, Oregon

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Perm ratings have very little to do with the effects of moisture in buildings. It's all about air movement. The amount of moisture that passes through materials via vapor diffusion is minisule compared to the airborne moisture movement that occurs through the vast great gaps & holes in most buildings.

Agreed. Air movement is much more important than vapor diffusion. So to be exceptionally clear. . .

If one were to skin a wall perfectly with a 6 mil, heck, let's say 10 or 12 mil plastic; no seams, no holes, no cuts, that would be a better performing* wall than skinning with 1/2" plywood. Yes?

*performance is defined as minimal vapor diffusion and basically zero air movement

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If one were to skin a wall perfectly with a 6 mil, heck, let's say 10 or 12 mil plastic; no seams, no holes, no cuts, that would be a better performing* wall than skinning with 1/2" plywood. Yes?

A SIPS wall or ceiling panel has basically the same net effect resulting in very close to zero air movement and vapor diffusion. There wouldn't be a problem with a Thermos style wall either. The problem lies in real life details that allow air and vapor into a cavity where it can't escape.

It all looks good on paper until you throw a SawzAll wielding Bubba into the recipe. Windows are all self flashing..Right?

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Originally posted by randynavarro

Perm ratings have very little to do with the effects of moisture in buildings. It's all about air movement. The amount of moisture that passes through materials via vapor diffusion is minisule compared to the airborne moisture movement that occurs through the vast great gaps & holes in most buildings.

Agreed. Air movement is much more important than vapor diffusion. So to be exceptionally clear. . .

If one were to skin a wall perfectly with a 6 mil, heck, let's say 10 or 12 mil plastic; no seams, no holes, no cuts, that would be a better performing* wall than skinning with 1/2" plywood. Yes?

*performance is defined as minimal vapor diffusion and basically zero air movement

Your hypothetical wall would have less vapor diffusion. I don't know if it would be better in any measurable way though.

- Jim Katen, Oregon

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Hi Randy,

It's all been done before and the Canadians, including Dr. Joe Lstiburek, probably understand it better than anyone else because they've been studying the issue longer.

Here are a few documents in chronological order that should be enlightening.

http://irc.nrc-cnrc.gc.ca/pubs/brn/brn211/brn211.pdf

http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd175_e.html

http://irc.nrc-cnrc.gc.ca/pubs/bpn/30_e.pdf

http://irc.nrc-cnrc.gc.ca/pubs/bpn/31_e.pdf

http://irc.nrc-cnrc.gc.ca/pubs/fulltext ... ral813.pdf

ONE TEAM - ONE FIGHT!!!

Mike

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