Hmm. If I were you, I wouldn't insulate (just) between joists, insulate below them, or even better just on the floor.

e.g. See photo 5 in here:

BSI-009: New Light In Crawlspaces — Building Science Information

or the alternatives e.g. fig 6, fig 7 discussed in the same article. You'll need to swapout your supporting timber columns tho' (e.g. with AAC blocks?).

Keep your timber warm and keep it happy.

Seen it all go wrong twice with insulation between joists here (UK)... Your local climate will be different of course, but have a good read of that article...

HTH,

Tim.

Thanks for your post, Tim.

And thanks very much for the info. Regarding the local climate here, it actually is a marine weather system, and almost identical to that of UK... We just don't get BBC, the real BBC.

I am well aware of temperature gradients and dew point, and the potential harm to wood.

This year's project is actually the beginning, not the end.

I am staying away from fiberglass, I have seen the dreadful photos of water soaked fiberglass... horrible.

I'm using EPS because it is non-hygroscopic and a good insulator. I am air-sealing all around, and at all seams.

Right now, my floor is going to be like this:




Which I take to be more or less equivalent to this:




But my final floor will be like this:




Which looks to me to be along the lines of this:




What I have not yet decided is what to use in the cavity below the EPS and above the bottom Polyiso layer.

I'd be interested in any other comments you might have, since we are practically 'Climate Brothers'.

Best,

-AC

Great thread!

Subscribed.

Erich

A/C why the air space? Would dense packed cellulose work with your humidity?

The air space is to allow for the foil reflector to do it's radiant heat reflecting job.

Dense Pack is used for wall insulation, but the thought of it for crawl space makes me very nervous.

-AC

Would mineral wool do?
Roxul

It's one of the filler materials I am considering. It is pretty nice to work with.

-AC

[great detailed post BTW]

And unfortunately not as avail and pricey. I'd like to use it too and Menard here started to carry it, but it's almost 4 times fiberglass price. Even more than the expensive denim batting.

Any reason you didn't just buy one of the foam spray tank kits and do a 1 or 2" spray underneath?? It's about a $1 a board foot. Might require some planning to move the tanks timely to prevent clogging.

And I'm confused by the radiant barrier, I thought it was suppose to go on the warm side(floor) of the air space to prevent heat radiation through the air gap??

I don't think there's any problem with using fibre glass, so long as it's kept pretty warm. i.e. the lowest PIR board needs to provide (rule of thumb) approx 50% of the insulation value, so that the timber is warmer+dryer.

Given your crawl space, I think I would put EPS underneath the polythene, and also stick it to the walls, and leave the timbers open. Air seal between the polythene and the floor. No ventilation. Cut out the bottom of the timber where it would protrude through the EPS, and replace with AAC block.

The "airspace" in the image which AC quoted from the building science article has two functions.

1. To keep the timber warmer i.e. insulation is towards the lower side. Filling the gap would make the timber cooler, and thus damper.

2. Any points with increased moisture (e.g. spill, moisture ingress from walls etc.) will readily spread out along the airspace and exit the structure over a much wider area (so quicker) than if it were full filled.

Radiant barriers aren't that useful on upward facing horizontal surfaces, as over time they get dirty and stop working. Also their contribution is pretty minimal anyway.

In A/C's proposed sketch, I think I would omit the EPS, as it's relatively high position will keep the timbers cooler, plus it's expensive + labour intensive to install. Just use batt (what ever is cheapest - so long as it has the PIR beneath it, it'll be fine.

Incidentally, for fixing the PIR upwards, I would use plastic discs and stainless screws. Don't put any timber down there, it'll soak. I think the timber support columns will have similar problems and will probably need replacing with something which won't mind the relatively high humidity environment...