Vapor barrier installed on ceiling of crawl space question.

[Michael Potter]

Just saw a new home inspection (WA state) with a half unfinished (dirt) and half semi finished (Concrete floor) crawl space with the vapor barrier installed on the ceiling.

There is no division of these 2 parts of the crawl space.

This is the first time I've seen the vapor barrier on the ceiling and I can think of the drawback of doing it this way (trapping moisture, areas inaccessible to check for rot/WDO's in wooden members.

How common is this?

Shouldn't the dirt floor portion have a barrier installed?

[Claude Lawrenson]

It's more common in that condition to put the vapour barrier over the dirt floor. It's also common that moisture will be trapped if the vapour barrier is sealed to the bottom of the floor joists over a crawl space.

However if the VB on the joist is not sealed, it can breathe. But than again - what would the point be of having the VB on the joists? In my opinion perhaps only to hold batt insulation which is not the ideal location for the insulation.

BTW: In my area (Ontario, Canada), the VB over the earthen floor is required by code; that's been the norm for many, many years.

Mike Holmes: The down-low on crawl spaces

Crawl Space Moisture Barriers

[Michael Potter]

It's more common in that condition to put the vapour barrier over the dirt floor. It's also common that moisture will be trapped if the vapour barrier is sealed to the bottom of the floor joists over a crawl space.

However if the VB on the joist is not sealed, it can breathe. But than again - what would the point be of having the VB on the joists? In my opinion perhaps only to hold batt insulation which is not the ideal location for the insulation.

BTW: In my area (Ontario, Canada), the VB over the earthen floor is required by code; that's been the norm for many, many years.

Mike Holmes: The down-low on crawl spaces

Crawl Space Moisture Barriers

Yep, vapor barrier over the floor is all I had seen until this one.

I swung by and took another look. With all the easily openable seems and penetration by staples, ducting etc... it doesn't seam like a good way to keep moisture from going up into the flooring.

Thanks

[Vern Heiler]

It's more common in that condition to put the vapour barrier over the dirt floor. It's also common that moisture will be trapped if the vapour barrier is sealed to the bottom of the floor joists over a crawl space.

However if the VB on the joist is not sealed, it can breathe. But than again - what would the point be of having the VB on the joists? In my opinion perhaps only to hold batt insulation which is not the ideal location for the insulation.

BTW: In my area (Ontario, Canada), the VB over the earthen floor is required by code; that's been the norm for many, many years.

Mike Holmes: The down-low on crawl spaces

Crawl Space Moisture Barriers

So how do you write up manufactured homes, which are all built with the vb attached to the floor framing?

[Billy Stephens]

So how do you write up manufactured homes, which are all built with the vb attached to the floor framing?

Manufactured Homes are built to a differant set of codes. HUD- Manufactured Housing and Standards

[Vern Heiler]

Manufactured Homes are built to a differant set of codes. HUD- Manufactured Housing and Standards

So because it is a different code the moisture doesn't act the same

[Jeff Zehnder]

Hey, I gotta go with Vern on this one...
It doe not mater what the code is, we still have to address the underlying issue.
Codes, manufacturing specifications and intended use are all important as well as will it perfrom.

[Billy Stephens]

So because it is a different code the moisture doesn't act the same

Vern,
Are You Questioning The Infinite Wisdom of the U.S.Government ?

*if you see a Big Bulge under there whatever you do Don't Poke it.

[Claude Lawrenson]

My apologies...it depends upon geographical location. Here's a good reference source for - charlotte Building Profile: Mixed-Humid Climate: Charlotte ? Building Science Information

Great info if you take the time to search around.

[Vern Heiler]

My apologies...it depends upon geographical location. Here's a good reference source for - charlotte Building Profile: Mixed-Humid Climate: Charlotte ? Building Science Information

Great info if you take the time to search around.

No apologies necessary Claude; I was just playing a little of the devils advocate and making everyone think.

[John Kogel]

Vapor barrier under the floor joists is normal for a modular home or manufactured home. One difference is that those homes are built inside a dry factory, so it works for them. Until something above starts to leak.
Poly stapled to the underside of the joists on site, that is always wrong.

There should still be poly laid over the bare dirt. And the crawlspace should either be ventilated or heated. I hear the crawlspaces in Washington state are real dirt holes as a rule. Like Claude in Ontario, we have had vapor (vapour) barriers and almost always a concrete skim coat over the dirt for many years.

[Terry Beck]

Hey, I gotta go with Vern on this one...
It doe not mater what the code is, we still have to address the underlying issue.
Codes, manufacturing specifications and intended use are all important as well as will it perfrom.

Difference between manufact home and stick built home --
Code or no code, the difference between manufactured and stick built homes is that the manufacturer of the manufact homes does not know where and how the home will eventually be set, and what will be under the home. Dirt, concrete slab, ..... The belly wrap is included at the factory as a vapor barrier, but more importantly to hold the insulation, and protect elec, plumbing, hvac ducts, etc. Until there is a leak, and the plumber leaves a big gaping hole in the belly wrap and plumbing exposed to freezing temperatures.

I agree though that it doesn't make sense to me to put a vapor barrier on the underside of a sub floor in stick built home. As a side note, code allows insulation at the underside of a sub floor, and I see it all the time, but I think that it creates more problems than what it is worth. Besides, it always falls down eventually anyway.

[John Williams]

In the homes that I have worked on the ones with wet crawlspaces have the insulation fall down because it is saturated. I have seen poly stapled to the bottom of the joists over the FG insulation and the insulation is dry. The condensation is on the poly, not in the FG. Right or wrong it seems to work with damp locations.

[Don Putnam]

Just saw a new home inspection (WA state) with a half unfinished (dirt) and half semi finished (Concrete floor) crawl space with the vapor barrier installed on the ceiling.

There is no division of these 2 parts of the crawl space.

This is the first time I've seen the vapor barrier on the ceiling and I can think of the drawback of doing it this way (trapping moisture, areas inaccessible to check for rot/WDO's in wooden members.

How common is this?

Shouldn't the dirt floor portion have a barrier installed?

Michael,

In my work investigating water intrusion issues we deal with this issue regularly. Every crawlspace, or basement for that matter, should have a vapor barrier at the the soil floor. If the floor is concrete there should be a vapor barrier under the concrete. We often encounter existing crawlspaces with no vapor barrier installed over the soil floor, or a poorly or incompletely installed vapor barrier. The first order of business is to correct that. A loose sheet of poly that is not sealed at all walls and penetrations is a wasted effort - the moisture will simply enter around the edges of the sheet.

On the subject of the underside of the floor of the home or building, unless the crawlspace is climate controlled the installation of a vapor retarder is a good idea. Note that I said 'retarder', not barrier. All walls of a building need a vapor retarder to control the diffusion of moisture and to help control air movement in or out. The design of the wall assembly, including the placement and perm rating of the vapor retarder, depends on the local climate. When a home or building is elevated off the ground the floor assembly should be treated as a wall. A properly installed vapor retarder should be placed at the exterior side of the floor insulation and sealed to serve as an air barrier.

Don Putnam
www.roofconsulting.com