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Non-vented crawlspace w/ICF


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By Kari in Colbert, WA on 4/8/2005


I want not to vent our ICF crawl space and treat it a conditioned space, sealed of course.  Has anyone done this?  Any opinions?  I have heard this is better for air quality, moisture and energy efficiency but would like to hear feedback from those that have an insulated non-vented crawlspace that is conditioned... Thanks! Or plan to do this...
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By Lauren in Bedford, VA on 5/20/2006


Kari,

I know its been a while since you posted this question.  I cannot believe the overwhelming response.  :-)  One of the ICF distributor/contractors that I have talked with has suggested this very thing.  Did you end up doing it?  Will you use it for storage space?  I can't remember from your other posts if you were going to do radiant heat.  How would it work with radiant heat since there are no heating vents?

Lauren, first timer


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By Gina in Yakima, WA on 5/22/2006


This is for anyone thinking of non-vents in crawlspace. Just my impressions.

During a two-year run, most of the contractors in our area used this type of non-vent crawlspaces.  65-75% of the homes without the vents now have MAJOR mold problems.  The moisture gets trapped during the building process, maybe and never evaporates or escapes and...voilĂ ...a breeding ground for mold.  Just on our block, 7 of the 13 homes have had to have professionals come to remove the problem. 

An interesting note, the homes now under construction ALL have vents.  (Side note: We live in non-humid Idaho.)

Gina


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By Kari in Colbert, WA on 5/23/2006


Well, we do have a non-vented crawlspace and it is fine - we had the radon people seal it up and have an active radon system. We have had water in it for a bit due to the fact that we re-dug our French drain and the septic people didn't put it back in right; plus we have no gutters as of yet and horrible decomposed granite (think clay but worse). Once we fix all of the above plus add some more drainage ditches in our landscaping we should be fine - even with the water - no mold! It is evaporating quickly.

But Spokane is very arid. Our floors were warm through the winter, and we put herringbone cherry wood on the whole main floor - we did not insulate under the floor joists since the crawlspace is insulated and non-vented; later if we want to install staple up radiant heat (our plan so far) we can do so very easily. Would I do so again? Sure, seems to work for us and our ICF house - we are mostly pleased with the ICF.  But do your research - we only built this last year, so who knows what will happen - we came from Oregon and I might not do it in wetter moldier climates but the science on it says it should be fine.
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By Kari in Colbert, WA on 5/23/2006


Oh, we stored a pallet of leftover cherry planks and they didn't get wet since we had such little amount of water and they were on a pallet but most of our storage will be in our attic which is huge and very dry. I don't like spiders so storing stuff in crawl spaces isn't my favorite. :)
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By Jack in Trumann, AR on 12/6/2007


I know this thread is old but I wanted to interject in case others run across it later.  I'm still building my home in Arkansas which is very humid and very hot and humid in the summer but I'm not overly worried about the crawlspace, which I've built un-vented.  Since I'm doing a lot of work myself, including the plumbing, I've spent a lot of time under the house this year and so far, humidity hasn't been a problem down there.  I'm not finished with it but I'll be insulating the crawlspace walls and putting down a vapor barrier to keep it dry.  To each his own I say but you need to research this if you are considering doing it.  I'm sure that different parts of the country would require different methods but if it isn't done right, you are going to have problems more than likely so be careful. 

On the other side of the coin, putting in vents, at least in the humid South, doesn't mean you won't have mold problems.  We've had mold problems in our current house and it is on a vented crawlspace, built just like it's "supposed" to be built according to standards.  The problem is, there is a lot of water in the air down here and introducing it into your crawlspace, which is many times at a temperature at or below the dew point, just causes more moisture to enter your house, not less.

I believe the science behind sealed crawlspaces is good but I think many people that have problems with this method only half do it and they don't follow the steps correctly to prevent moisture from building up.  It's sort of like those folks who claim to be on the Atkins Diet but have never read the book and really don't know what the Atkins Diet entails; if you don't do it right, it won't work.


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By David in Ocoee, FL on 10/18/2010


Jack hit the nail on the head with this one. And it is what I have said so many times before on OBB. The failure of a particular system is almost always due to improper use of a material or improper installation and maintenance, not the product itself. That is why I would prefer to call a non-vented crawlspace a "conditioned crawlspace", because it is vented, just not from the outside. Please allow me to explain all of this. If you have decided to make your crawlspace a "conditioned space", the first thing you will need to do is put down a vapor barrier, not a vapor retarder.

Now let me explain what the difference is between a vapor barrier and a vapor retarder. The duct tape and cheap 6-mil clear plastic sheets that are sold in the big-box stores is a VAPOR RETARDER, not a vapor barrier, because water vapor still permeates through this product, albeit at a slow pace. A VAPOR BARRIER such as this one: 20MilDrySpace does not allow any water vapor to pass through it, provided the product has been installed... you guessed it... according to manufacturer's specifications.

Once the vapor barrier is property installed and the stem walls are properly insulated, the next thing you need to do is protect the vapor barrier from damage or movement by covering it with pea-sized gravel.

The next thing you need to do is direct a 4" forced-air vent from your air handler to one end of your "CONDITIONED CRAWL SPACE", and a 6" return vent on the opposite end of your "CONDITIONED CRAWL SPACE". The return vent does not need to be ducted all the way to your return air grill if you don't want to. It just needs to penetrate the floor somewhere, anywhere that is far away from the 4" forced air vent. Are you getting the picture now folks? It is not a "non-vented" crawl space. It is in fact vented. It's just that the vents are a closed-loop system that stay within the conditioned space of the entire house.

If all you do to your "non-vented" crawl space is put down $80 worth of plastic sheeting and duct tape it together and do nothing else, it would not surprise me to hear that you eventually develop mold issues in the crawl space and moisture issues with the first floor's subfloor.

Often, when I read of issues such as these, I cannot help but think that we humans try to impart human qualities to construction materials. We do the same thing with our pets. Folks, a construction material is going to do what it was designed to do and nothing more. Wishing will not make the materiel do something or prevent something that is beyond its design limits. So the idea is to familiarize yourself with the material and what your objective is before deciding to use it. I call that my "common sense" approach. I am not an engineer. I am just a guy who knows a lot about what goes where and why.

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By James in Waynesboro, VA on 10/29/2010


I finished my ICF house in Virginia four months ago and have a "conditioned crawlspace". I have a vent and return in the crawl space to get the air from my HVAC system (which also has an ERV) into the crawl. The problem so far is that I have not used my HVAC system except for three days of air conditioning, so I do not force any air into the space or get circulation.

Right now I am sitting in my house with an inside temperature of 67? and an outside temperature of 45?. I have not had the heat on yet all year. Unless you turn on the HVAC system (I have multi-zones and two separate air handlers), you will not get the air circulation and drying action in the crawl space. I placed a dehumidifier in the space set to automatically turn on at 65? to help. I installed the plastic sheeting over the dirt floor myself and taped the plastic to the side walls and foundation piers, so I know that a good job was done.

It seems like the energy-efficiency benefits of ICF may not equally apply when trying to control humidity in a crawl. I'm not sure what else to do. Any recommendations??
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By Belinda in Augusta, GA on 10/30/2010


James,

I have a question that's a little off the subject. We are wanting to build ICF and we are anticipating to rarely use the HVAC. With the ERV, is it possible to bring in fresh air without conditioning the house?


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By James in Waynesboro, VA on 10/30/2010


Let me offer two different points. First, I used spray foam under the roof deck and in my "attic" spaces. My ICF only goes up to the top of the main floor. I used 2x10's for all spaces built out above this. During this past summer my second floor was noticeably warmer (four or five degrees) than the main floor, so I periodically used a whole-house fan to get rid of the warmer air there. I have the Airscape 2.5e, which is excellent and does not adversely impact my tight envelope.

Because of the WHF, I only needed to use AC two or three nights. I say all this to say that you need to carefully consider your envelope, air quality, and how the HVAC system will work as part of your house system. My ERV does not have a feature allowing it to operate separately from the air handler, which, like the ERV, is in the crawl space. If it did, I wonder how much electricity it would draw. This is what causes me concern about my crawlspace moisture in the summer. 

If you already have house plans, I recommend you send them to Energywise for an evaluation of what your HVAC needs will be. I did it, and it gave me a lot of peace of mind when I was getting quotes from HVAC subs on what I needed. There are not that many subs with a lot of experience doing Manual J calculations on ICF structures.


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By Kenneth in Lees Summit, MO on 10/31/2010


If you plan to rarely need HVAC, is there a particular reason you want to precondition the ventilation air? There is more than one way to achieve ventilation; and ERV (or HRV) is not the only answer.

However to answer your question directly; yes it can be done. Think about houses that use hydronic radiant heat as an example. I have seen HRVs used for ventilation in these situations, and clearly they are not attached to HVAC systems. Obviously, you still need ductwork, but it is very small compared to HVAC ductwork.

And to the suggestion about Energywise; my recommendation does not fit with the other poster here. Suffice it to say I think you get better service by hiring a commercial HVAC service to run the numbers for you. I am certainly glad I did not follow the Energywise recommendations for my house.


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By Belinda in Augusta, GA on 11/7/2010


What would you suggest be used to ventilate? We are wanting to go with geothermal and our sub used the Manual J calcs and submitted a report. Our biggest concern is the winter; we will have a wood stove and rarely use our other heat. However, if our envelope is tight, we will need fresh air coming in somehow.


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By Kenneth in Lees Summit, MO on 11/7/2010


Ventilation is easy; use a fan. How do you ventilate a bathroom to remove humidity? Why not let that fan do double duty - just put it on a timer. This solves your ventilation problem with equipment you are already required to install. And read the above problems related to ventilation when the HVAC system is not running; your ventilation fan is not dependent on your HVAC, and you can run it anytime you turn on the switch. Sometimes we overthink problems, when the solution is both simple and obvious.

Now then, you need makeup air or you risk going negative pressure (I am a proponent of positive pressure, or at least neutral). So put a hole in the wall for makeup air, introduce it to your HVAC duct and you get to balance your whole system. If you are over-the-top, put a flap valve in there somewhere. If you want to buy the unit as a whole, look at a skuttle make-up air control for your return duct of your HVAC system, about $20 total.

Unless you detail everything just right, and bring a blower door in to verify that everything is detailed just right, I doubt you are near as "tight" as you think.


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By David in Ocoee, FL on 11/7/2010


James,

You either have a functioning vapor barrier in place, in which case you do not have a moisture issue in your crawl space; or you don't have a functioning vapor barrier and moisture is accumulating as a result. You asked for a recommendation, and here it is:

If you are getting consistent humidity readings under 60% relative humidity in your crawl space without the use of a dehumidifier, do nothing. You are fine. If you are getting consistent readings above 60%, I would check the following items:

1. Is the soil inside your crawl space below grade? If so, you will need to take remedial action to re-install the vapor retarder ABOVE GRADE.

2. Has the plastic sheeting been securely attached to the top of the ICF stem wall as well as any penetrations with approved fasteners or tape, leaving no gaps between the plastic and the EPS foam or penetrations?

3. Did any tears or punctures develop in the plastic when it was being installed? And if so, were the punctures or tears sealed with approved tape?

4. If there are any seams in your vapor retarder, were the seams overlapped a MINIMUM of 16", sealing both the BOTTOM seam as well as the TOP seam of the overlap with an approved sealing tape? (Duct tape is not an approved sealing tape for vapor retarder.)

Assuming that the above mentioned has been checked and corrected, a dehumidifier should be more than enough to keep humidity levels well below 60% in the winter, provided that the catch basin is emptied on a regular basis.


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By Mark in APO, AE on 8/12/2012


I've recently come to the conclusion that a conditioned crawl space will be the best choice for my planned 5,000* sq.ft. ranch house in Georgia. I plan to use ICF, and since the idea of a crawl space is new for me I'm doing a lot of research and thinking.  


David - When you say the vapor barrier needs to be above grade? Are there steps that can be taken to avoid this? One end of my house will be below grade, and I'm thinking that if the wall is treated for moisture in the correct way that I should be all right. I haven't quite figured out how to treat the wall, but I'm thinking about it. Maybe something like running the vapor barrier up the inside of the ICF wall and sealing it near the top of the wall. I think code requires that I leave a three-inch inspection gap for termite tunnels. I could seal and paint the exposed three-inch gap and possibly terminate my vapor barrier at that point.

I've got plenty of time to think about it, so I welcome others' opinions.

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