View Full Version : Crawl Space Ventialatoin and Vapor Barrier

mathew stouffer
09-17-2009, 07:30 PM
This place was built in 06. 8300 sq/ft. The crawl space was not vented and the buyer asked why the vapor barrier was on the ceiling. I told him the installation was incorrect. Do you ever see this. The whole crawl space was a mess.

Matt Fellman
09-17-2009, 09:28 PM
Wow, that is a mess. I'm not sure what/if the differences are from Oregon to Utah but in Oregon the vapor barrier is to be on the ground, lapped 12" at the seams and be a minimum of 6 mil black plastic. None of those appear to have been met in the photos.

I do see vapor barriers (retarders, etc.) on the underside of manufactured homes but always on the ground as well. But those are black 6-mil and you have clear of unknown thickness.

I may over-react to crawl spaces but in a lot of instances it's best to just haul everything out and start over. Trying to hash and cobble some other guys hack job is just a nightmare. In the end there's really nothing of any value in there... it's just a few hours of labor.

David McGuire
09-17-2009, 09:55 PM
Wow, never seen plastic applied to the ground like that. Did they have any ventilation in there at all? Great place for mold development, all that moisture, no air circulation. Wouldn't go here in Kansas either.

Michael Schirmer
09-18-2009, 02:23 AM
408.1 Ventilation.
The under-floor space between the bottom of the floor joists and the earth under any building (except space occupied by a basement or cellar) shall be provided with ventilation openings through foundation walls or exterior walls. The minimum net area of ventilation openings shall not be less than 1 square foot for each 150 square feet (0.67 m2for each 100 m2) of under-floor space area. One such ventilating opening shall be within 3 feet (914 mm) of each corner of said building.
408.2 Openings for under-floor ventilation.
The minimum net area of ventilation openings shall not be less than 1 square foot (0.0929 m2) for each 150 square feet (100 m2) of underfloor space area. One such ventilating opening shall be within 3 feet (914 mm) of each corner of the building. Ventilation openings shall be covered for their height and width with any of the following materials provided that the least dimension of the covering shall not exceed 1/4 inch (6.4 mm):
1. Perforated sheet metal plates not less than 0.070 inch (1.8 mm) thick.
2. Expanded sheet metal plates not less than 0.047 inch (1.2 mm) thick.
3. Cast iron grills or grating.
4. Extruded load-bearing brick vents.
5. Hardware cloth of 0.035 inch (0.89 mm) wire or heavier.
6. Corrosion-resistant wire mesh, with the least dimension being 1/8 inch (3.2 mm).
1. Where warranted by climatic conditions, ventilation openings to the outdoors are not required if ventilation openings to the interior are provided.
2. The total area of ventilation openings may be reduced to 1/1500 of the under-floor area where the ground surface is treated with an approved vapor retarder material and the required openings are placed so as to provide cross-ventilation of the space. The installation of operable louvers shall not be prohibited.
3. Under-floor spaces used as supply plenums for distribution of heated and cooled air shall comply with the requirements of Section M1601.4
4. Ventilation openings are not required where continuously operated mechanical ventilation is provided at a rate of 1.0 cfm (10 m2) for each 50 square feet (1.02 L/s) of underfloor space floor area and ground surface is covered with an approved vapor retarder material.
5. Ventilation openings are not required when the ground surface is covered with an approved vapor retarder material, the space is supplied with conditioned air and the perimeter walls are insulated in accordance with Section N1102.1.7.

A.D. Miller
09-18-2009, 03:04 AM
Hack job by someone long off his meds.

David DelVecchio
09-18-2009, 07:02 AM
Why... that looks perfectly reasonable, if the intent is to grow mushrooms in the floor joists! Was there a kraft paper vapor retarder on the batt insulation, too?

Redundant vapor retarders = trouble! Three vapor barriers = disaster!

I inspected a vaulted ceiling with no less than 5 layers that acted as vapor barriers / retarders. Paint, poly, roofing felt, paper on the batts, etc. There were ventilation baffles above the insulation, and a ridge vent, but no soffit vents (there was a valley on either side). Then the contractor pierced the ceiling membrane with uninsulated, non-air-tight recessed lights, all within the 2x8 roof rafters. Obviously enough for the rafter span, not nearly enough depth for the minimum required insulation.

Air from inside the house made its way into the various layers of vapor barrier as the roof warmed up. It "rained" inside these two rooms every day around 10:30 or 11 AM, even during bone dry weather. The house was on a lake to boot.

The architect (not me) and builder both blamed faulty roofing / flashing.

They would have been better off for all the money they spent doing it wrong to invest in SIPs, eliminating the need for ventilation where it was not possible anyway.

Better no barrier than wrong barrier.

mathew stouffer
09-18-2009, 07:16 AM
There was mold on the TJI's and sub floor. The builder was there and said it was because the house was framed during the winter. I laughed and asked him how the hell this thing passed inspection, where is the ventilation, what happened to your "vapor retarder", and what the hell is the plastic on the ceiling. He was not too happy. The Client asked what was the best solution. My thoughts were starting over, like Matt said, and installation mechanical ventilation.

Cobra Cook
09-20-2009, 07:26 AM
Never saw any thing that bad. Did the builder try that was done correctly?
It is ok not to have vents if you are adding conditioned air to the space, but how was it supposed to work it the floor joist are covered? It appears that there is already mold growing on the sidewalls already, by getting moisture through the sloppy way the dirt was covered. Tell the customer,owner and the builder that all of the plastic needs to be removed and new plastic of the proper thickness needs to be installed on the dirt portion only. I suspect that when the plastic is removed from over the insulation, it will be wet and most likely need to be replaced also. Good luck

mathew stouffer
09-20-2009, 07:59 AM
They are gutting it. It's a four million dollar house house, so what ten grand to fix the crawl spaces.

Cobra Cook
09-20-2009, 02:41 PM
Good idea!:rolleyes:

Gunnar Alquist
10-02-2009, 04:23 PM
I have exhumed this topic to post a related question.

Referring to just a basic moisture retarder that is loose-laid under a home: Is there any requirement as to whether or not black plastic must be used? Or, more to the point, is there any reason that clear plastic cannot be used?

Cobra Cook
10-02-2009, 06:26 PM
I think black is recomended more so to keep the light out which prevents grass and other plants from growing, otherwise you could have a solorarium under the house. I hope I spelled that correct

Gunnar Alquist
10-02-2009, 07:05 PM
I think black is recomended more so to keep the light out which prevents grass and other plants from growing, otherwise you could have a solorarium under the house. I hope I spelled that correct


I just spoke with another inspector who said pretty much the same as you. I had never even paid attention to the type of plastic, except if it was too thin.

mathew stouffer
10-03-2009, 05:28 AM
I thought is had to be a certain thickness.

David McGuire
10-03-2009, 05:47 AM
I thought it had to be 6 mil or better.

mathew stouffer
10-03-2009, 06:47 AM
that's what i read, but it does not specify clear or black.

Cobra Cook
10-04-2009, 07:44 AM
I have seen contractors use 4 or 6 mill. 6 does not cost that much more
so i would use 6. The main objective is to cover the entire surface from corner to corner no matter the thickness.I would rather see 2 or 3 mil that covered every sq inch of area than 6 mill leaving 3 foot around the paramemter exposed like i receantly saw on an inspection.

Jim Luttrall
10-04-2009, 08:36 AM
Foundation Drainage
Wood Foundations
R405.2.2 Moisture barrier A 6-mil-thick polyethylene moisture barrier shall be applied over the porous layer with the basement floor constructed over the polyethylene.

R504.2.2 Moisture barrier:
Polyethylene sheeting of minimum 6-mil thickness shall be...

R408.2 exceptions: 2&4&5...approved vapor retarder material...

The only specific thickness I can find for under floor vapor barriers is 6-mil with all other references that I find is "approved" with the energy chapter N1102.1.7 changing to mention ...the exposed earth in all crawl space foundations shall be covered with a continuous vapor retarder having a maximum permeance rating of 1.0 perm...

No mention of color or other performance factors, just 6-mil vapor barrier or minimum 1.0 perm vapor retarder.