Hi, brand new here; I'm researching a remodeling project and found this great forum.  I hope this is the right place to post.

We have a 1960 split level with a 1970 addition on the back over a crawl space.  The crawl space is mostly enclosed, with a door (18"x24") at one corner and a few small screened vents.  No ducts or pipes are in this space.  Crawl space is a literal description; it's not a fun place to work.  Above it is our 20'x26' family room, where we spend most of our time when home and awake.  Next year we will be having a lot of work done in there, possibly including a radiant floor.  We want to insulate the floor now so we can get some benefit this winter, and to help us decide if we really want to spend the $ on radiant floor in the spring. 

We live in zone 5 - Harrisburg PA.  The floor joists are 2"x8"; probably considered inadequate but I've never noticed that the floor sags across its width.  The original homeowner installed fiberglass insulation under the floor but he used wall insulation so it's 3.5" thick and he put the facing on the bottom (away from the living space).  What would be the best way to re-insulate:

> Radiant/vapor barrier an inch under the subfloor and then 5.5" of R-21 high density fiberglass with 1 gap between it and the barrier?
> Radiant/vapor barrier against the subfloor and 6.5" of R-22 an inch below?
> One of the above plus foamboard over the joists?
> Something else entirely?
 

I'd attach taped rigid foam to the bottom of the joists and then add cellulose in the resulting joist cavity.

Is the crawl vented or un vented? It's a shame to throw away insulation, why not (if it's unvented) simply foam the exterior walls?

In your climate it's a lot less foam and generally better to close up the vents, and insulate the foundation walls with 3-4" of EPS or 2" of closed cell polyurethane earth-coupling the crawlspace to the subsoil, then add R19 batts snugged up to the tubing & subfloor. You'll also need to add a 10mil poly ground vapor retarder. If you insulate only between the joists the joist edges will get moldy from the combined load of summertime air moisture and ground moisture, since with greater insulation the temperature of the crawlspace & joist edges will be closer to the deep subsoil temperature, which is more than 10F below the average mid-summer outdoor dew point. If you insulate the the foundation walls AND between the joists AND put down a ground vapor retarder, the air in the crawl space will roughly track that of the living space. You may still need/want to put a dehumdifier in there for the summer, or ventilate the space with conditioned space air (a small blower duty-cycling on a timer).

If the pre-existing batts aren't all moldy you could leave them in place until you'r ready to do the radiant, then re-use them instead of R19s for the radiant, adding another layer of kraft faced R13s (the common loss-leader bargain roll at box stores) below that if/when you tear it all apart to do the radiant. Don't worry about creating moisture traps with kraft-facers- while they are a fairly tight ~0.4 perms when dry, the become more vapor open, rising to 2 - 5 perms in the event that the moisture content of the facer rises. Without the radiant but with insulated crawlspace walls R13 will be enough, but once you make the floor the radiator you'll want a bit more than R13, and the cheapest way to get there is another R13.

No matter which way you go with it, it's essential to air-seal band joist and foundation sill seams with foam- (1-part can-foam or FrothPak). If you don't air seal it, wind currents will use the fiber insulation and any gaps between the fiber & subfloor as a thermal bypass, robbing both the insulation and radiant of performance. Band joists and foundation sills are usually among the largest air-leaks in a house that hasn't been blower-door tested & remediated, usually larger than all the door & window leakage combined(!). That's a bad enough energy performance hit even without a radiant floor, but the hit triples once you start using the floor as the radiator.

There is ZERO value to radiant barrier in this sort of application.

This is what we did. We moved from a basement home to a house with a "warmed" crawl space (4" EPS perimeter insulation, concrete floor). Previous home had heated floors that were sorely missed. The forced air heat pump (4 ton) had a couple outlets in the crawl space but not enough to keep the floors comfortable while heating the rest of the house. So, originally as an experiment, I installed a 7Kw mini split heat pump in the crawl space. This runs 24/7 during the heating season (Oct. to Apr.). It is set to 22°C and increased as the outside temp. drops. Keeps the chill off the floor and we don't have to run the big HP as much.

Thanks, all. I had an insulation contractor in who gave me a quote for R-30 high density batts with a vapor barrier between the joists, and suggested that I plug the vents and put plastic on the ground as vapor barrier. If we decide to go with radiant floor heat I was thinking we'd go with an electric mat on top of the subfloor; the floor needs to be lifted about half an inch anyway (the addition has settled a bit relative to the original structure). So yes, radiant barrier would be a waste. We don't have central air (right now we have a portable AC unit in there but we are thinking of a mini-split) so running conditioned air into a sealed crawlspace may not be practical. Taking that into consideration, is sealing and insulating the crawlspace walls (and putting a vapor barrier on the ground) still the best approach? Also, I've read that if you foam the rim joists you have to put a fire barrier over the foam. Is this the case in a crawlspace with no ignition source?

The floor of a conditioned crawlspace is like a slab on grade floor - you still need to insulate it.

You also need to either vent it to the interior or dehumidify it.