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Plenum crawlspace


Brandon Whitmore
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I had an interesting job this morning, that I could use some input on.

The crawlspace was a plenum crawlspace that had 6mil black plastic with dusty pea gravel installed on top. The registers were still in place, but the screens on the duct boots? that extended into the crawlspace were mostly plugged with debris/ dust. The original heating system was replaced with a heat pump with an air handler in the attic. New ductwork was installed, with ceiling registers were installed for this new system.

As soon as I walked into the home, I could smell a musty odor. First thing I did once I realized I was not inspecting a slab, was open the crawlspace hatch-- whew........

There was a very slight whitish substance on the subfloor and framing--- probably fungal matter, but I can not be sure as it was pretty light. The exposed metal at plumbing hangers and nails that penetrated the subfloor were pretty rusty. There is obviously a moisture / humidity issue in the crawlspace, possibly due to the fact that the crawlspace no longer has air flowing through it.

To make matters worse, the only toilet drain was leaking heavily under the home, so there was a 5 square foot area that was moist with waste water--- yuck. I know that this leak had something to do with the smell, but do not believe that this is the only cause of the humidity issue in the crawlspace.

Since the crawlspace is no longer acting as an HVAC plenum, I think that the crawlspace should either be vented to the exterior, or a de- humidifier type system (http://www.basementsystems.com/crawlspa ... difier.php) should be installed. What do you all think?

I recommended that they replace the pea gravel and contaminated vapor barrier. I told them they should talk to an indoor air quality expert to see what their options are for the crawlspace, etc.

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I had an interesting job this morning, that I could use some input on.

The crawlspace was a plenum crawlspace that had 6mil black plastic with dusty pea gravel installed on top. The registers were still in place, but the screens on the duct boots? that extended into the crawlspace were mostly plugged with debris/ dust. The original heating system was replaced with a heat pump with an air handler in the attic. New ductwork was installed, with ceiling registers were installed for this new system.

As soon as I walked into the home, I could smell a musty odor. First thing I did once I realized I was not inspecting a slab, was open the crawlspace hatch-- whew........

There was a very slight whitish substance on the subfloor and framing--- probably fungal matter, but I can not be sure as it was pretty light. The exposed metal at plumbing hangers and nails that penetrated the subfloor were pretty rusty. There is obviously a moisture / humidity issue in the crawlspace, possibly due to the fact that the crawlspace no longer has air flowing through it.

To make matters worse, the only toilet drain was leaking heavily under the home, so there was a 5 square foot area that was moist with waste water--- yuck. I know that this leak had something to do with the smell, but do not believe that this is the only cause of the humidity issue in the crawlspace.

Since the crawlspace is no longer acting as an HVAC plenum, I think that the crawlspace should either be vented to the exterior, or a de- humidifier type system (http://www.basementsystems.com/crawlspa ... difier.php) should be installed. What do you all think?

I recommended that they replace the pea gravel and contaminated vapor barrier. I told them they should talk to an indoor air quality expert to see what their options are for the crawlspace, etc.

I'd suggest one of two courses of action:

A. Seal the old floor registers, insulated the underside of the floor, and install conventional vent openings in the foundation walls.

B. Seal the old floor registers and install something like a Clean Space system with a super-duper air-tight pool-liner vapor barrier. John's Waterproofing installs these and they do a very nice job.

I'm seeing more & more of these abandoned plenum systems.

- Jim Katen, Oregon

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Thanks Jim,

The purchaser is a friend of mine, and the house is a foreclosure. They are gonna purchase no matter what, but this is on a starter home. I told them that it would be ideal to add the vents, subfloor insulation, etc., but said that they could probably get away with a dehumidifier system.

I'm not seeing any signs of water intrusion from the exterior, although it's tough to tell for sure with pea gravel on top of the plastic- the plastic runs up to the sill plate.

I think that if it were my home, I'd probably add a liner/ thick vapor barrier over the pea gravel and add a dehumidifier. It would probably be much cheaper than punching in vents and insulating. It would stay cleaner down there as well.

B. Seal the old floor registers and install something like a Clean Space system with a super-duper air-tight pool-liner vapor barrier. John's Waterproofing installs these and they do a very nice job.

Would you skip the de- humidifier in that case? I'd be concerned with future plumbing leaks or other sources of moisture on top of the pool liner.

I'll have to check the building code to be sure this complies. Wouldn't installing the pool liner and sealing up the registers violate the building code by creating a sealed crawlspace?

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. . . Would you skip the de- humidifier in that case? I'd be concerned with future plumbing leaks or other sources of moisture on top of the pool liner.

I'll have to check the building code to be sure this complies. Wouldn't installing the pool liner and sealing up the registers violate the building code by creating a sealed crawlspace?

Well, you could always install the minimum required vent opening. With a good vapor barrier that'd be 1/1,500 of the crawlspace area.

- Jim Katen, Oregon

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I didn't have a clue what you where talking about, so I looked them up. The only reference I could find was regarding the Pacifc Southwest, are they common that far north?

For those similarly puzzled by the concept, check out this PDF. There are really good diagrams and descriptions of the way they built on the left coast in the 70's. http://www.fs.fed.us/psw/publications/d ... _rp073.pdf

Brandon, the plenum diagram shows the vapor barrier extending well up onto the first floor walls, not just to the sill. It is conceptually brilliant, but I think it would be difficult and expensive to pull off with today's materials, and virtually imposible to repair. It's no wonder they are being abandoned.

Tom

Tom

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I've come across maybe 5 or 6 crawlspace plenums - not really an amount to make me an expert.

Each time, there was something significantly wrong.

My take now is it's kind of like EIFS; everything must be installed perfectly right to make it work. That usually doesn't happen or simple neglect and deterioration defeat the original intent of the plenum crawlspace.

To be safest, I'l always recommend a traditional, conventional northwest crawl space and heating system. Keep the crawl separate with proper ventilatoin, vapor barrier, etc. Modify the heating (sometimes cooling) system with conventional returns and supplies.

It'd be interesting to know if anyone has come across a crawl space plenum that is actually performing well.

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It'd be interesting to know if anyone has come across a crawl space plenum that is actually performing well.

Yeah,

I've had two. One was a custom-built home in Blueridge and the other was a custom-built home down on Lake Samammish. One homeowner had a creeper down there and it was lit with flourescent lighting.

The home in Blueridge had a return air duct that ran from the second floor ceiling straight down to the crawl and little wall registers on the first floor with open boots under the floor joists.

The thing that I don't like about them is the amount of dust all over everything in the crawl. On one home, the owner's young children found that the register in the dining room was a good place to dump all the stuff that they didn't like when mom was out of the room. The stuff was growing whiskers and everyone in the house was breathing whatever kind of yuck was being given off by the fungi growing all over that old food.

ONE TEAM - ONE FIGHT!!!

Mike

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Brandon, the plenum diagram shows the vapor barrier extending well up onto the first floor walls, not just to the sill. It is conceptually brilliant, but I think it would be difficult and expensive to pull off with today's materials, and virtually imposible to repair. It's no wonder they are being abandoned.

Hi Tom,

I wouldn't say that the plenum design is common as I've only seen a couple/ few in the last 8 years or so. It sounds like Jim runs into them more?

I now can't remember whether it ran up the wall or whether it was tucked under the sill plate. I would want it tucked under the sill plate. If the plastic runs up the wall such as in the diagram you posted, I would think that moisture would condense and travel up onto the walls, in turn damaging the walls. Heck, I'm no building science guy though. Anybody?

There is a Street of Dreams community built in Tualatin that used this design (or maybe just the one I inspected years ago). I believe this was a late 70's / early 80's neighborhood-- sound familiar to any local inspectors?

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It'd be interesting to know if anyone has come across a crawl space plenum that is actually performing well.

Hi Randy,

The irst one I ever saw looked pristine-- it was in the Street of Dreams community mentioned in my last post in Tualatin. The crawlspace was one of the nicest ones I've had the pleasure of crawling through. Framing looked brand new and there was no dust to speak of that I can remember, nor were there any smells. This one still had the original heating system hooked up, so it was actually being used as designed.

What are the main installation errors that you are running into with these things?

The thing that bothers me is that I can't see the plastic beneath the pea gravel. It seems like any damage to the vapor barrier would pose a

pretty big problem, especially if there are drainage issues.

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Hi,

What you don't know is if there's a drain under that pea gravel. A sealed crawlspace like that needs a drain or it becomes a swimming pool liner filled with gravel and whatever else drains into that gravel.

I'd recommend that they fix the leak, use a long piece of rebar to punch that barrier material in a few dozen places, install a drain to the outside, put down another barrier on top of that pea gravel and seal it to the barrier material at the walls, make sure the plastic at the walls is sealed to the walls but doesn't extend up over the top of the foundation, and then connect the drain to the barrier.

Once that's done, clear the mesh so that the air in there can be conditioned and things can dry out, and then treat all of that wood with a fungicide (BoraCare or TimBor) to make sure that the fungi is knocked back.

What's up with the repair scabbed onto the end of that beam? Did they fail to isolate the end of the beams from the concrete and it came under attack?

ONE TEAM - ONE FIGHT!!!

Mike

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What's up with the repair scabbed onto the end of that beam? Did they fail to isolate the end of the beams from the concrete and it came under attack?

About 8 beams were like this. It looks like they did not have long enough beams.. You probably can't tell from the picture I posted (I've got others), but the plywood that is sandwiching the beam is the original T&G subfloor material, so I'm fairly positive this was done when the place was built. Very odd, but there is no evidence of movement.

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What are the main installation errors that you are running into with these things?

Usually, the crawl has a rat slab on the floor. The concrete has cracked which has allowed moisture or in some cases, water to infiltrate - end result is musty, mildewy air.

Then there's the inevitable rodent infestation. Now there's rodent feces and pee cycling through the air system.

Next comes un-faced fiberglass batts. More debris in the air.

Worst case, there's long term water infiltration which has led to rot, insects and mildew. All that stuff goes right in to the house via the air system.

Finally, the un-insulated foundations and rat slab. The heat sytem now is fighting extra cold that doesn't need to be introduced in to the system.

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