Insulating the Barn Floor

[Brian_Weekley]

I've been thinking about how I might insulate the floor of my barn.  The flooring over the joists will be T&G hemlock boards.  I don't want to add rigid insulation between the joists and flooring because it will further reduce the headroom.  I also don't want to use spray foam underneath between the joists because carpenter ants like to nest in it.  I was considering batts of mineral wool (Roxul) between the joists because this stuff is very inert and doesn't support mold growth (it's made of spun metal (recycled slag) and basalt (volcanic rock)).  I would probably need to cover it with a wire mesh to keep mice from nesting in it.  Any thoughts or suggestions for a better alternative?  Also, would it be prudent to use either a vapor retarder or some kind of house wrap on one side or the other?

[beenthere]

What R value are you trying to achieve?

And what will be below this floor, and what temps are you thinking you will want above this floor?

I'd control mice with poison and traps.  ;)

[D L Bahler]

You must be VERY careful insulating a floor like this. This is a high moisture location, and if done wrong insulation will cause the wood to rot by holding moisture to it, or causing it to condense on or near the wood. Any foam or plastic should be considered out of the question.
mineral wool or even raw sheep's wool will work here
Also an infill of loose dirt will work very well, or sawdust.
Fiberglass is no good.

[VictorH]

Sawdust?  I would think that sawdust would draw in moisture - just how would you do this to keep that from happening?

Victor

[D L Bahler]

This i'm not sure of. I just know that some cultures have done it quite a bit.
Personally, I wouldn't, unless it was intermixed with some powdered clay or something like that, and borax.

[Thehardway]

Jueston,

Can you give us a better idea as to what your purpose for insulating is?  Is this a new build? Are you experiencing problems? What purpose will the barn serve and will it be heated/cooled?

Typically, if you can provide a thermal break and keep things dry, you would not need to insulate. Is there a lot of moisture there?

[Brian_Weekley]

Jueston,

Hardway, Jueston is the other cat avatar!  This is Brian. 

This is a 20x20 barn I just raised a few months ago in Maine.  It is intended to be a cabin or workshop down the road.  I plan to put a wood stove in it to keep things comfortable when I'm staying or working there during the winter.  Before I put the floor down, I just wanted to add some minimal insulation to help keep things toasty and minimize air infiltration through the floor without causing any moisture issues.  Because the structure sits on a traditional dry-fit stone foundation on gravel, there's airflow, but nothing to keep ambient humidity or pests from getting into the crawl space.

[beenthere]

but nothing to keep ambient humidity or pests from getting into the crawl space.

They just go along with a crawl space.
But rigid foam panels between the joists would be my first choice.
Once you button it up, you won't know what is going on down there anyway. ;)

[Philbert]

In  modern construction you would insulate the foundation walls and put a vapor barrier over the ground below.

With the construction you describe I don't know if putting an impermeable vapor barrier between the joists and floor would lead to condensation and rot under the floor.  Maybe some type of breathable membrane, like Tyvek?

Interested to hear what others have to say.

Nice looking foundation by the way!

Philbert

[D L Bahler]

I would never recommend the modern approach on a timber frame, or anything else.

Te modern approach is geared to highest r-value and lowest vapor travel, the sacrifice that must be made is that it is very bad for the wood.

I have seen first hand the effects of spraying the foundation and putting down a vapor barrier. You get water ON the vapor barrier. Don't know how many times I have seen that. Yes, you get a warmer floor but at the cost of the longevity of the structure. The theory is that it is supposed to stop moisture from migrating out of the ground. In many cases it traps moisture that migrates from above. Moisture will travel and settle into the cooler region, always. The only way to stop moisture from above is to have a barrier in the floor, which can itself become a dew point.

Of the options listed by you, I think the best is the wool because it is a good insulator, but also it breathes and does not have moisture problems. I would attach it between the joists, put something over the foundation walls to slow down pests (you will never stop them) and give the dirt underneath a good coating of AG lime. The lime will keep down fungus and molds and such, and also make it less friendly to mice and spiders. It will indirectly preserve the wood.

If you seal off the crawlspace, you make a moisture trap. The moisture will migrate from the space above into the crawlspace underneath. DON"T seal off the crawlspace!

Your suggestion of wire is good. I would put a fairly tight wire mesh around the inside perimeter of the foundation tacked to the framing above and buried a few inches in the dirt. That will do a lot to keep out mice and rats.

[Thehardway]

Sorry about the mistaken identity Brian,

Some others have made some good points.  I would not try to "seal" the crawlspace.  I would put down a sheet of 5mil poly just on the ground.  Cover it with about 2 inches of pea gravel and put a flowable fill layer over it.  This will keep things dry and clean and eliminate the likely hood of vermin wanting to get in there.  Your dry stack rock will allow air to filter through and keep condensation from rotting your wood. After you put down your sills and joists, I would put a layer of felt paper down before installing your T&G.  This gives you a vapor barrier and stops drafts but still allows things to breathe fairly well.  Sealing up crawl spaces tight is never a good idea and there is no way to avoid vermin if you have a material desireable for nesting or as a food source.  If you are concerned about cold winter floors, go old school and stack straw bales around the foundation perimeter in the winter.  Let the air flow in the summer.  I would leave an access panel in the interior floor so you can inspect from time to time or just in case you decide to run wiring or plumbing underneath in the future you can get under it.  having a reasonable smooth concrete surface under there works great for access as you can roll a mechanics creeper around real easy while laying on your back.

[jueston]

Hardway, Jueston is the other cat avatar!  This is Brian. 

all us cats looks the same....

[Holmes]

  How about foil bubble/bubble foil insulation on top of the joist before the flooring goes down?  I do not know an exact r value but they claim an r6 with an air space, r4 without and its 1/4" thick.  I've used it under radiant heat. Personally I can not tell you it is great or poor.

[Brian_Weekley]

I had no intention to seal the foundation.  Although humidity or wind-driven rain will get into the crawlspace, the air flow through the loose rock foundation will help keep things dry.  I did plan to put some sheeting on the ground to reduce the ground moisture.  If I do add insulation, I think the mineral wool batts between the joists would provide some insulation without being a medium for mold growth.

Hardway, I like the idea of putting some felt between the joists and flooring.  This would act as an air break, but still be permeable to allow moisture vapor to escape.  With that thought, I would think any semi-permeable membrane would also be OK to put between the joists and flooring—maybe Tyvek or a synthetic felt (e.g., roof guard).  The only reason I'd prefer the synthetic alternatives is I think they would hold up better stretched and stapled across the joists.  Holmes, I don't think I would go with the bubble/foil stuff because it isn't permeable.

For the "pest deterrent", I was originally planning to staple wire mesh on the underside of the joists.  However, I think Bahler's suggestion is better—I'll tack the wire mesh to the inside sills and bury the other end into the ground around the foundation perimeter.  Adding some lime wouldn't hurt either.  I also like Beenthere's comment:  once you button it up, who cares (if the mice are playing poker down there)...

Finally, I will most definitely have a door in the floor to access the crawlspace when needed—useful if I ever electrify that thing...