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Conditoning a crawl space: process and materials

I'm trying to find a contractor to do a job that I cannot do due to medical issues. But, what I can do is have specs in the proposal consistent with what I expect. I have read Martin Holladay's comprehensive article on how to condition a crawl space.

The contractor may be open to using battens to secure Tu-Tuff, but he is requesting an explanation for use of the battens. He prefers to run the Tu-Tuff (vapor barrier) only up 6 inches and seal edges against wall with a non drying mastic. Then follow with application of Thermax, also adhered to wall with mastic. Personally, I think that this may not be the optimum plan. Concrete walls seep moisture More on this below.

RE: battens:
I, too, am not sure of the rationale for the battens. Is it for keeping the Tu-Tuff taut while using minimum amount of mastic? Is it for having a firm flat surface to later apply Thermax sheathing?
Are the battens to be placed horizontally to form a perimeter at the top of the wall?

RE: application of Tu-Tuff:
Is it overkill to run tu-tuff all the way up the walls since Thermax will also be applied? Or, is this for getting a more complete seal against moisture from the exterior walls?

As aside I heard what seems like a good tip when applying tu-tuff. The distributor (EFI) of tu-tuff recommended the following: caulk and staple tu-tuff to wooden sill; run down the walls, use a 2x4 to (temporarily) hold tu-tuff in place at the bottom of the wall (no battens), and then keep running the tu tuff barrier onto the crawl space flooring. Personally, I don't like covering any of the wood above the foundation, but I do like the tip on using a 2x4 to keep it snug and avoid accidentally tearing it. For attaching tu tuff to the wall: concrete screws and a solid bead of sealant (one that does not dry and has low voc)???? Any specific sealant to recommend?

RE: application of thermax:
Would it make sense to adhere thermax first and then cover with tu-tuff? Which goes on the wall first? Personally, I would opt for thermax to face the interior, not the wall. If so, how should the thermax be applied? Concrete screws?

RE: ezbreathe fan
Contractor suggested this as a way to supplement existing radon mitigation...which is a little outdated...done 28 yrs ago and current average reading over 92 hurs is 11 (4 is the standard). Evaluator did a sniff evaluation to locate source of leaking radon...coming mostly from crawl space. We like the idea of getting fresh air into the house, but I don't like it that this company does hard selling and totally lacks empirical supportive data. Contractor is offering this equipment and installation at cost....he itemized it to back up his claim. Any thoughts on this application. Forget it? Or, other similar products better...like honeywell...better?

But, the more that I ponder this product, the less I like it. Keep it simple and just cut a big enough hole- register in the existing duct work for forced air to come out. This creates some positive pressure which helps seal everything in the crawl space. Whereas, ezbreathe creates negative pressure. However, since I have a radon problem, it might be better to spider the pvc instead of just having a single stub.

Yes, I am guilty of being obsessive and not trusting a contractor. The prices that I have gotten are extremely high. I have secured prices for all the materials---a liberal estimation is 950.00. Yet, two contractors come in at 4k. Plus, both initially fail to use material up to code.

Asked by william phillips
Posted Oct 31, 2012 1:06 PM ET
Edited Oct 31, 2012 1:12 PM ET


4 Answers

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You have A LOT of questions. I'll try to answer your questions in order, then provide a broader response.

1) Battens keep the vapor barrier from falling off the wall. They run horizontally.

2) The vapor barrier can go up the wall, or terminate lower on the wall. It likely depends on the condition of your foundation (type and effectiveness of exterior foundation drains, quality of construction, condition of water proofing / exterior coating, etc).

Polyiso insulation should not see bulk (liquid) water. If your foundation is dry on the interior, you could probably just use the Thermax as the vapor barrier, avoid running the polyethylene vapor barrier up the entire wall.

3) I would not depend on just a sealant, or a sealant and masonry fasteners to hold the vapor barrier onto the wall. These will fail if the vapor barrier has much traffic. That's why Martin recommended using battens.

4) You can use a mastic (UL-181 HVAC mastic) to apply the vapor barrier to the wall, or any number of adhesive sealants. You might try a good quality polyurethane caulk. Whatever you choose, test it first to ensure compatibility. Polyethylene doesn't bond well with many adhesives.

5) The vapor barrier goes against the wall, then you install the Thermax.

6) You can install the Thermax with masonry fasteners and large head plastic washers, or you can use the batten method.

7) I'm confused on your ventilation strategy. There are two issues here.

a) Ideally, you should install a passive radon vent. A collection pipe (a perforated pipe bedded in gravel) sits below the crawlspace floor vapor barrier; this connects to a vertical pipe that exits your roof. This system draws the gas out of the soil, reducing the amount of radon entering your crawlspace. Depending on your location, you may need an active system. You should probably consult with a radon mitigation professional. Many states license this trade. Otherwise, deal with a contractor that is knowledgeable of the state / local codes and best practices. But anyway you do this, it is both difficult and expensive to do this work in an existing house.

b) The code requires continuous exhaust ventilation for a sealed crawlspace (i.e. 1 CFM per 50 square feet of crawlspace floor area). This requirement is separate from the radon vent. You need an exhaust fan. And it is supposed to make the crawlspace slightly negative to the main house---helping to minimize the amount of radon making its way into your living space.

You can use a high efficiency exhaust fan, like the Panasonic Whisper series, and couple this with a few floor registers (transfer grilles) to draw air from the house into the crawlspace. Or, you could install a small heat recovery ventilation system, so you don't lose a lot of conditioned air. The recommendation would depend on your climate, thermal enclosure, cost, and a number of variables.

If you want to educate yourself further, this organization has come great reference material. Look at the crawlspace section on this page:


This one is old, and reflects older building codes, but it has some great illustrations:


You should also go back a read Martin's article again. There's a lot there that you missed.

Depending on your climate, there is also a concern for termites. You may need to leave an inspection gap at the top of the foundation---or provide an alternative means of inspection.

Finally, I don't want to sound rude or arrogant, but if you don't trust your contractor, maybe it's time to find one you do trust? One that is familiar with this type of work? It's great to educate yourself, and make sure you are hiring someone that is competent. But, the contractor should be specifying the work, not the other way around.

You didn't mention your location / climate zone information. It's difficult to provide guidance without this information. One size doesn't fit all!

Good luck!

Note: Edited to remove typo.

Answered by Daniel Ernst
Posted Oct 31, 2012 8:07 PM ET
Edited Oct 31, 2012 8:09 PM ET.


Daniel...Wow...I truly appreciate your patience an thoughtful responses. Yes, every time I read Martin's article I get more out of it.
If it's of any additional help, should you have any inclination to further respond...My location is Louisville, KY.

Re: contractors I just talked with a home inspector that I used almost thirty years ago. He's still in the business and expanded into radon assessment. He fully believes in crawl space conditioning. And, he has had a similar experience to what I have come up against when getting proposals from contractors. The workman that he sees is shoddy with very poor understanding of materials and application. And, homeowners are being excessively charged. This individual sees a lot of homes and he has a very strong reputation with local realtors. Yes, I agree about the direction of knowledge. It's not a good idea to be specifying and managing the contractor. This home inspector is going to do some research and make a referral.

Re: radon. I already have an active radon mitigation system that was installed when I purchased the home.There is a fan drawing air from the pipe that a pipe that penetrates the crawl space and from another pipe that penetrates the basement floor. Unfortunately, the vapor barrier was only four mil and not properly sealed to the foundation. The pvc pipe that goes into the crawl space is one inch, lies under the gravel/rock, but only extends a few feet into a crawl space of about 700 sq.ft. I have talked to two certified radon specialists. One says to leave as is given that there will be a new vapor barrier well sealed; another wants to re-do the entire system only leaving in place the pipe that penetrates the basement floor. As part of the re-do he wants to re-size the pvc pipes and have a more extensive pipe configuration that into the crawl space.

Re: ventilation. the crawl space is not completely sealed and separate from the basement. There is an opening currently covered with a plywood barrier.(4x6ft) Would it be adequate and effective to have properly sized registers in the duct work within the crawl space and sending air into the crawl space, along with knowing that there the current wooden plywood barrier to the crawl space will be removed allowing for air to move into the basement which has an open stairwell to the rest of the home? I ask this knowing that you patiently point out that I need to read and re-read cited material.

No matter, you have filled in some holes for me. THANKS.

Answered by william phillips
Posted Oct 31, 2012 9:00 PM ET


hi william, in regards to the minor point of low VOC sealant: we used about 2 cases of the large format tubes on our current project. of those tubes, ten of them were Le Pages green series low VOC acoustic sealant. of the ten tubes, three were defective, i.e., they were dried up or otherwise non-operable. we were using the sealants, et al, to seal the t&g joints of plywood panels. some weeks after application, the squeeze-out of the le pages product is rock hard. meanwhile, even in freezing temperatures, the 'black death' is almost as tacky as when first applied. hardly a scientific experiment but the conclusions were convincing to us, nonetheless...

Answered by erik olofsson
Posted Nov 1, 2012 12:27 PM ET


Erik, appreciate the heads up on tremco sealant.
To confirm..
Most local contractors just spray on a foam. Years later, I hear from the home inspector frequent problems with mold. Contractors argue that foam becomes one with the wall...a perfect sealant from moisture. Which type of foam may be the issue. (I thought that I had posted a reply..but it looks like it didn't make it...so this may be a repeat)

Answered by william phillips
Posted Nov 1, 2012 4:11 PM ET

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