Thread: Sealed crawspaces

Todays inspection has a sealed crawlspace. Ventilation is provided by an exhaust fan vented to exterior. A floor vent is cut into floor at other end of home. Home is 6 years old, no signs that the system is not working properly and all looks well. However I thought that a HVAC supply duct needed to be installed in sealed crawlspaces.

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Originally Posted by Trent Tarter

Todays inspection has a sealed crawlspace. Ventilation is provided by an exhaust fan vented to exterior. A floor vent is cut into floor at other end of home. Home is 6 years old, no signs that the system is not working properly and all looks well. However I thought that a HVAC supply duct needed to be installed in sealed crawlspaces.

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The Floor vent serves that purpose. #19 of attachment.
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Originally Posted by Billy Stephens

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The Floor vent serves that purpose. #19 of attachment.
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BS: I do not think so. Read the doc you posted more closely:

2) No passive ventilation openings shall be allowed between the exterior and the interior of the crawl space.

A floor vent which is not an active supply register is a passive opening, right?

Originally Posted by A.D. Miller

BS: I do not think so. Read the doc you posted more closely:

2) No passive ventilation openings shall be allowed between the exterior and the interior of the crawl space.

A floor vent which is not an active supply register is a passive opening, right?
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Oops,
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Originally Posted by Billy Stephens

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Oops,
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As I read that it sounds like they are talking about standard passive crawlsapce vents installed at exterior only. An open vent in floor at interior would be allowed. At least thats the way I read it.

I'm no crawl space expert, but, from an energy standpoint, doesn't it seem a little nutso to suck conditioned air into the crawl space and exhaust it to the outside?

Originally Posted by John Arnold

I'm no crawl space expert, but, from an energy standpoint, doesn't it seem a little nutso to suck conditioned air into the crawl space and exhaust it to the outside?

JA: Yep. It is better to do it Lstiburek's way with a supply register, no return, and not vents. The ICC has just no quite gotten the picture yet, I think.

Originally Posted by A.D. Miller

JA: Yep. It is better to do it Lstiburek's way with a supply register, no return, and not vents. The ICC has just no quite gotten the picture yet, I think.

Ok...................Ok..................... I won't say a word.....................

I enjoy reading the Q/A posts and don't really want to interject but this time I'm having trouble grasping the benefit here, so hopefully you fellows will be able to enlighten me. I also hope I can explain my concerns intelligently. Sorry for the long post.
Question: how can creating a vacuum in a crawlspace be a good thing with the assumed make up air coming from the conditioned space. In this case,the exhaust would create a low pressure condition in both the crawlspace and assumes a positive pressure in the living area. Assuming continious operation of the powered crawlspace exhaust vent.
Now, with an existing low pressure condition in the crawlspace, the HVAC when operating, creates an even greater vacuum in the living area, assuming the rest of the house is extremely tight.
With these conditions established, where is the make up air coming from.
If the house, were tight, possible sources would be the crawlspace, provided the living space low pressure condition were greater, conventional draft fuel fired appliances vents, dryer vents, range hood vents, electrical wall penetrations, window/door openings ect..
Also, with the crawpace exhaust and the HVAC system both in operation it would also seem the crawlspace relative pressure should be dropped to the point it (crawlspace) would be screeming for make up air and it will get it any way it can. Inside, outside, ground, house ect. As long as the air was conditioned then the relative humidity in the crawlspace should not be so bad but if it comes from ground and/or exterior sources how is it controlled.? I acknowledge in this case the crawlspace is supposedly sealed with the exception to the two stated vents.
I've also been told some horror story's of crawlspace dehumidifiers operating without regard to relative humidity and causing significant issues to framing material.
This seems like a fix for one problem with out thinking about the entire building envelope and interaction with other systems. Something tells me this type of system would not respond well to house air tightness testing (blower door) either. I also wonder if the HVAC contractor new about this when figuring his manual J.
The overall outcome I would think would be considerable energy loss without the use of some sort of energy recovery system, high system operating costs. Information rarely disclosed by the seller.

Depending upon the crawlspace and the way it is set up, I'm not a fan of every one being vented to the exterior. I've seen some crawls with a concrete floor and some typical run-of-the-mill moisture penetration stains on the side walls. If the space is relatively dry, why open vents and introduce the damp humid air our part of the country is known for come summer? Also, many of the crawls I see are open to the basement and have no door or access panels. Venting air from the outside into these is venting the entire basement year round.

I'd rather see crawlspaces tightened up and sealed up as much as possible from moisture penetration and migration and treated like a basement. I know this isn't feasible with all crawlspaces in all areas but I had a crawlspace in my house, I would do what I could to do it this way.

The fact that the Crawl Space has a fan venting to the outside makes it a vented crawl space.
Drawing conditioned air from the house and blowing out a crawl space vent would be costly for the HVAC Bills.
Is the HVAC equipment located in the crawl space? Is the floor vent connected to the HVAC System?
Venting or Not, Conditioning or Not depends on local climate. Theres good information on closed Crawl Spaces at http://crawlspaces.org

In our area properly closed crawl spaces work well.

Sealed and slightly 'conditioned' - as described without any opening or vent to the exterior and a 4" or 6" register (depending on size of the space) to keep the crawlspace under positive pressure. Seen quite a few down here in moist Charleston SC and they perform wonderfully.

Quite a nice place to hang out after spending time in the attic :-)

If my home were not fully elevated it would be as described above.

Sounds like this is a system called Humidex. It uses the theory ofconditioned air drawn from the living spaces to force humidity from the crawlspace or basement by use of a fan forced system. In theory it creates a negative air pressure from the living space to exit moisture before it can enter the structure.

Humidex

We have several wood foundation homes with sealed crawl spaces in my area that do quite well. The HVAC system is vented into the crawl space and the floor registers are open to the crawl.Using no ducts the HVAC provides positive pressure which then heats/cools the home. Moisture barrier is sealed at seams and runs up to the top of the foundation block. I have seen several that use Icynene in the band joist area to complete the seal. I always come out saying "why can't they all be this nice!!"

Thats where the climate comes in.

Where I live, vented, unconditioned, crawlspaces often have moisture problems and rot. I inspected a brand new house this year with a vented crawlspace. The insulation under the floor and the floor joists were soaked.

The hard part is explaining to the builder and client that although the house was built to code, it was probably best to close the crawl space.

Originally Posted by Trent Tarter

As I read that it sounds like they are talking about standard passive crawlsapce vents installed at exterior only. An open vent in floor at interior would be allowed. At least thats the way I read it.

I agree. The point being that for the system to work as intended, there is no unconditioned air source in the crawl space.

I just last week did a final visit to a crawl space in a new home. The builder read the code to mean conditioning the crawl space with ducts from the HVAC system straight into the crawl area. Well, that was a pure disaster. Beards of mold. 40% moisture content. We won't even get into trying to figure out how they'd balance the system.
They cleaned the framing. Sealed off the ducts. Installed a de-humidifier.
That cleared up the problem. But to meet the code, it is still necessary to inject conditioned air. That is best done with a 60cfm fan in-line in a duct from a floor register in an inconspicuous location. Yes, it has to run all the time. Yes, in a perfect world that might be pressurizing the crawl space, but you just can't get them that tight.
Now all that presumes proper sealing, proper water proofing, proper insulation, proper duct design. I just don't know why they didn't just build on a slab and be done with it.

JLMathis

Question: what code are you referring to? Also with that fan in continous operation does the door hit you in the face when opened because of the low interior air pressure.
Still where is the benefit?

Local (Greenville) inspection dept. required conditioned air into the crawl space if sealed. I don't have a new code book yet. You can't seal a house tight enough to blow the door off of anything. No matter how fastidious you might be. It just ain't happening with a 60 cfm fan.
Now note: I never implied that this was a good idea. I don't have enough practical experience. But once you seal the crawl space, Jeff Tooley and all the founding fathers of sealed crawls will dump conditioned air into the crawl with good success. Again, implicit in that is a totally dry crawl with suitable drainage system as needed and 100% sealed vapor retarder. If you create an atmosphere roughly equvalent to the interior space, you should maintain a dry, clean area.
We have a ton of problems in this area with wet crawls and in exisiting homes, sealing can be benificial. Now bang for the buck--that's a whole different question.
JLMathis

Originally Posted by Jeffrey L. Mathis

Local (Greenville) ...

I realize that you have divulged your location in this particular posting, but this thread is an excellent example of why everyone should fill out their User CP so that every time they post the reader can see where they are inspecting.
Locations such as "USA", while patriotic and all, aren't that helpful.

Is that Greenville SC? Also is that a local or state code? Curious, cause somebody is always messing with the codes here in NC and they always seem to goof it up. Usually, after there have been a couple of major lawsuits and/or insurance claims they get it right.

Originally Posted by Lawrence Transue

Is the HVAC equipment located in the crawl space?

Lawernce had a good question. where is the HVAC equipment? and if a fuel fired furnance, where does the combustion air come from?

Greenville, NC. No nechanical equipment in the crawl space. The modified code, as I understand it allows for sealed crawl spaces within narrow parameters. Interpreted to mean creating a conditioning method. Now I discussed it today with a builder with experience. he doesn't like the idea of injecting conditioned air with a fan. He Likes to put the vent in the floor and pull the air out with a fan at the foundation. his reasoning was that that eliminates pushing crawl air into the house through inadvertant leaks or gaps.
Food for thought if your interested in this kind of stuff.
Sorry about the profile stuff. Thought that was done.
JLMathis

Originally Posted by Jeffrey L. Mathis

Sorry about the profile stuff. Thought that was done.
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JLMathis

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Jeffrey,

If you go to the upper left of this page,

Click user CP,

Click Edit Profile,

Fill in where you live, it will show up on the right side of your post.
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Well Jeffery, thanks for clarifing that, now I have to get busy and learn the new code. Until now I just thought you were in a different state and the code you were talking about did not apply to NC. I just kinda figured a sealed crawl was covered under some sort of manufacurer's exemption. I guess I kinda got lax with all of the recent hub bub about staying away from the codes in HI lately. Thank goodness I haven't had to inspect one of these things yet.

Question: are there any specific points unique to a sealed crawlspace that should be checked?

Jeffery: sorry, I was a little rusty on my NC geography, so I had to find mapquest Greenville, NC. and realized your on the coast. I can understand now why they would be so popular in your area. Here in the piedmont things are just a shade dryer.
I just thought I might pass on something I got from a local engineer I occasionally swap HI stories with.
He was telling me he recently had been involved with a client were the crawlspace was extremely damp, you know the raining crawlspace type damp. So they sealed it, put in a couple of dehumidifiers and life was good, until... As the story went the dehumidifiers, supposedly dried the lumber out pretty quick and for the most part caused it to turn to powder. That's when he got the call to come in and engineer the fix. I'm sure there is more to the story than that but figured it was worth mentioning since you were dealing with dehumidifers.

I really do hate to get into all this crawl space stuff but I will a little.


There are no 2 crawl spaces alike.

All crawl spaces should be evaluated on an individual basis.

Should some crawls be sealed....Sure.

Should all crawls be sealed......No.

Should all crawls have a humidistat operated fan for ventilation in the crawl...I think so.

Should all crawls be vented....I think so.

Do I want to pressurize a crawl and push crawl air up into a home....no.

The only thing left to say is what I always say. There is no such think as a crawl expert to tell you what to do with all crawl.

I repeat.....My opinion is every single crawl needs to be individually evaluated for its own needs. There is no magic *cover all* for crawl spaces no matter where you live or what your climate.

Here, Here Ted. Point well taken.

Originally Posted by fritzkelly

Why bother? What's wrong with a nicely ventilated unconditioned crawl space?

I notice you are in Arizona. Zero humidity.

Here on the east coast, we commonly have 90% + humidity and 90+ temps. When the warm wet air goes into the relatively cooler crawlspace via foundation vents, it tends to condense. The idea is that the foundation vents will provide enough ventilation to keep it dry. The problem is the wind blowing in the crawlspace is almost saturated with moisture so provides little help.

A better idea is to seal off the foundation vents, insulate the exterior crawl walls, 100% vapor barrier taped and run up the piers, and air conditioning. I have seen desiccated frogs in the sealed crawlspaces. Frog jerky if you will. The vapor barrier is typically much thicker than 6 mil and is opaque white. Although not usually 72 degrees like indoors, certainly much less than the 90+ outside temps.

Since the insulation is along the exterior walls, the subfloor is exposed making checking of all the penetrations very easy.

Originally Posted by Ted Menelly

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Should some crawls be sealed....Sure.

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Should all crawls be vented....I think so.

Me confused.

Originally Posted by John Arnold

Me confused.

A little play on words


All crawls need an individual eval.

So, Should some be sealed....sure, why not...if it works for a particular crawl...might not work...I don't think it will. If someone proves it to be better for that particular crawl...seal it.

But what is my honest opinion....All crawls should have some ventilation to the exterior.

All that dribble means that there are no 2 crawls alike.

Ted this is true but...........venting to the exterior not only allows outside air in but also the dew point that air is bringing in, into the c/s.

No expert here, but here in the south the standard could fit into 95% of all crawl spaces.

I had the opportunity to work with one of the best in this field out of Charlotte. Might just have enough info to be dangerous.

Dan

Originally Posted by Dan Popoff

Ted this is true but...........venting to the exterior not only allows outside air in but also the dew point that air is bringing in, into the c/s.

No expert here, but here in the south the standard could fit into 95% of all crawl spaces.

I had the opportunity to work with one of the best in this field out of Charlotte. Might just have enough info to be dangerous.

Dan

The use of the term "sealing off a crawl space" needs serious additions and adjustments.

36 years in Mass, 14 years in Florida and 5 years in Texas.

All of that time was building, remodeling and inspecting for over 35 years.

I have almost never been in a crawl that did not need a plethora of fixes, in the home, under the home and outside the home to the property itself before I would even slightly consider sealing up any crawl. In most case the prep work alone before you even thought of sealing of the crawl would cost a small fortune.

Just my honest opinion for what I have seen in the little experience I have had.

Not arguing here. Just expressing an opinion that has totaled over three decades of experience in the matter.