crawlspace ventilation clarification questions

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Old 12-30-13, 10:45 AM
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crawlspace ventilation clarification questions

I've been doing some research regarding the ventilation or conditioning of a crawlspace. It seems there has been a paradigm shift regarding which is better. Some people, it seems, think a ventilated crawlspace is the way to go, which necessitates a vapor barrier of 6 mil plastic (minimum) over the bare earth. Some even argue that's a good start, but a layer of gravel of even concrete should be placed on top of that. In conjunction with that, the floor joists should be insulated, but not sealed with a second vapor barrier. A ventilated crawl space can be passively ventilated or actively ventilated with a small fan. I've even found foundation vents which automatically open or close depending on the temperature.

The newer (I think) school of thought is that the crawlspace should be conditioned, almost as part of the living area of the house. This again requires sealing the crawlspace area from the bare earth with a vapor barrier, but this vapor barrier continues up the walls and any pier support blocks. Insulation between the floor joists is not needed or wanted

It seems to me that the conditioned crawlspace approach is to eliminate the moisture condensation that occurs when the hot humid air (during the summer) moves into the crawlspace and becomes cooler. It also eliminates the heat loss during the winter because the air that is in the crawlspace does not easily exchange with the outside world.

My concerns about the conditioned crawlspace are:
1. A climate control system (i.e. heat pump) is sized based on the volume of the house. When the crawlspace is incorporated into the conditioned volume, the heat pump will then be undersized and less efficient.
2. Every crack in the barrier layer (insulation will never be 100% effective) becomes a much bigger energy loss than it otherwise would be because air is now freely circulating between the crawlspace and living area.

I currently live in a 1600 sq ft Cape Cod in central Virginia built on a crawlspace in 1994 with a single heat pump and no additional aux heat (no fireplace, wood stove, etc). We've been in this house since 2006. I'm wondering how to make my home more efficient. I've never blocked the foundation vents in the winter, but I'm wondering if I should. That is what started this whole research project. I haven't done anything yet.

It makes sense to me that you wouldn't want to ventilate in the winter because you're bringing in colder air which will inevitably carry away heat from the floors. It also makes sense to me that you don't want to ventilate in the summer because the hot, humid air will condense in the cooler crawlspace and wick moisture into the floor joists.

I'm really torn about what to do. I'm having a hard time separating the good info from the bad. The government and building codes are slow to change, so I don't know if I can really trust that. People and businesses who want to sell me something (automatic foundation vents, ventilation fans, conditioned crawlspace materials) have a vested interest in promoting their views, so I can't trust them.

HELP!!!
 
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Old 12-30-13, 12:20 PM
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just wait a few yrs & the thinking'll come back as, impo, no 1 really knows which's best.

personally, a 15mil woven vapor barrier [ stego - NOT 6mil apron/vest store plastic ) is best as it withstands foot/crawl traffic w/o tearing/becoming brittle & breaking,,, run it up the walls to the rim joist & seal overlap seams. I like humidistat-controlled power vents, too. insulation up 'tween the floor joists w/vapor side to the crawl is something else we've done.

attic insulation is down in the joists - NOT up on the trusses. there should be soffit/gable/ridge vents allowing air flow. just turn the house upside down & do the same in the crawl.

we do most of our work in atl if that's helpful.
 
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Old 12-30-13, 12:47 PM
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Hi Frank,
I vote for the conditioned crawl space.

Insulation between the floor joists will result in colder floors. If you heat a well insulated crawl, the floors will be warm and total heat loss less. In addition, all of the stuff that is often in a crawl, pipes, ducts, and such is now protected from the cold.

Rather than comparing your crawl space to an attic, different functions, compare your crawl to a basement. How many ventilated basements do you see? Search this forum for discussions about living space over a cold garage. Despite major efforts to insulate and air seal, those bedrooms or whatever are cold, reason, the heat above wants to stay above. Living spaces over a heated basement or crawl have warmer floors because the heat below is pushed up to the ceiling/floor above.

Bud
 
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Old 12-30-13, 01:54 PM
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It is my opinion that the only reason NOT to have a sealed, conditioned crawlspace is cost. It is far cheaper to insulate the floor with crappy fiberglass and ventilate the area than it is to properly grade the earth, add a proper vapor barrier and seal the crawlspace. I have seen figures of five to ten grand to properly retrofit an existing ventilated crawlspace to an enclosed space. It would have been less had it originally been built as a sealed, conditioned space.
 
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Old 12-30-13, 02:51 PM
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I am in Michigan, and believe that climate, soil conditions, etc. may warrant some subtle differences in how one approaches the situation, but, after quite a bit of research, I am converting the crawl space in our 40+ year old home to a conditioned one. In fact I just came up from there to sit upright a bit, but am working on the first step today, adding some more lights down there. My primary reason for this is, just as an example, we had a high today of about 20 degrees, and it's about 70 degrees down there; actually warmer than we keep the house. Now there may be one difference, because we have hot water baseboard heat, and the pipes for that are radiating enough to warm it up that much. Depending on how your heat is run, that may make a difference. So, for probably 10 years or so, I have wondered why not use that heat for the floor, but it has simply been a matter of making time to do it. Anyway, my plan is to place plastic on the ground, but, while a lot of "experts" suggest 6 mil, I would like to go with 20 mil, but still need to make a decision, based on the cost difference. I am going to bring it up about a foot on the blocks, and glue and tape it at that point. Next will be caulking the joints at the rim joists, and seal them with 2" Thermax sealed in place with Great Stuff. Then we will glue 2" Thermax to the blocks, and finally, remove the original fiberglass insulation from between the joists. I plan to box around the existing vents, so that we can open them in the summer, just as we have always done. As far as the vents that you have now, again, different conditions may warrant different approaches, but, up here, I have never heard of anyone wanting to leave them open in the winter.
 
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Old 12-31-13, 02:04 AM
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aka pedro, suggest 1st insulating pipes. heat will still be in crawl - just not as quickly. lights are heat, too.

good plan impo,,, no 'expert' would rec 6mil,,, stego will do the same & should cost less

I like cross-flow ventilation,,, for sealed spaces, exhaust fans w/timers work well.

would reconsider insulation removal - best !
 
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Old 12-31-13, 07:24 AM
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Yes, didn't mention the pipe insulation, but most of that was done a good many years ago, and will get the rest of it once we remove the fiberglass between the joists. I agree on the fact that a real expert would not recommend 6 mil plastic today, it's a carry over from 20-30 years ago in my opinion, but that doesn't stop people who consider themselves "experts" from writing articles, and I found reference to it a number of times, both in books and on the internet, which is the reason that I typed "experts" in parenthesis. As I said, still need to weigh the cost factor, but will definitely use something heavier than 6 mil.
 
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Old 12-31-13, 07:40 AM
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Where does a homeowner (non professional) buy Stego? I looked at the website, but don't see it on the Home Depot or Lowes websites, nor do I see it on Amazon. I come to believe that if I can't but it on Amazon then I probably don't need it!
 
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Old 12-31-13, 09:23 AM
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It looks like a product intended for under the concrete, but they don't mention an open crawl space floor. A search will turn up several products intended specifically for your application, although every time I have priced them the thick plastic wins the $$ battle. It is kind of a question, will you be there 10 years from now when the plastic needs to be repaired or replaced?

I usually reference photograph 5 on this link as how a sealed crawl space should look.
BSI-009: New Light In Crawlspaces — Building Science Information

Bud
 
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Old 12-31-13, 02:25 PM
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that's because it IS used under conc,,, anything that'll stand up to those jabonies is good in my book,,, NEVER expect much from an apron/vest store OR amazon,,, IF you want pro results, you've got to buy where the pro's do & do the work as a pro would buying cheap stuff is never a good practice as is paying for more product than's required to complete a project properly.

eg, you buy 6mil plastic at the apron/vest store & have ?'s,,, you'll probably get a shoulder shrug,,, buy @ pro sources & they'll either KNOW OR refer you to mfg's rep,,, both have different mkts & diff biz plans.

IF amazon can't sell it in necessary quantities to make $, they won't,,, doesn't mean you don't need it,,, can't buy pro grade tools on amazon from what i've heard but never tried,,, we buy from mfg's authorized sellers, mfg's rep, pro const supply houses, direct, or ebay,,, you're asking for pro mtls/methods & diy help so its a whole new world - best !
 
 

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