Vapor and an exposed floor assembly
Tiziano
| Posted in General Questions on
I’m in zone 6 and the floor assembly is on piers, 6″ off grade. Underneath is 6 mil poly and 4″ of trap rock.
From the bottom up:
ZIP sheathing
1 1/2″ rigid (EPS or XPS?)
Rigid edges air sealed
10″ Unfaced fluffy
Advantech X-Factor subflooring
I’m nearly ready to order my floor assembly materials but am confused regarding EPS, XPS and vapor.
It seems to me that I’d want the assembly to dry to the inside and therefore unfaced fluffy.
But, which rigid? I’m assuming XPS given its lower perm rating than EPS.
And/or, do I add a smart vapor retarder layer somewhere in the assembly?
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Replies
Tiziano,
Any type of foam will do.
Have you thought through how you will install the Zip and foam that close to grade, and what happens to those layers at the piers?
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Zip is not a good product in this application. The board itself is cheap, crappy OSB, that requires sealed edges. Even if the edges were sealed, I'd be awfully nervous about this application.
Consider Advantech instead. Same manufacturer. It, too, is OSB, but is produced with different resins and will be much more resilient to moisture.
Malcom has a good question regarding piers. Curious.
I'd not realized that. Would either the roofing version or X-Factor be even better for my top-down application?
Roofing version? No such thing, it's all the same stuff. X-factor would would be far better- not sure it's available is 1/2" thicknesses though- this stuff is superior to Advantech, either will have better longevity in your application than Zip.
I thought this was a different version but maybe its just the brown color :) :
https://www.huberwood.com/zip-system/roof-sheathing
X-Factor comes in 22/32".
Thanks Malcolm.
We're building it in 2 halves, upside down. Each half is 8' x 12', rimmed with 1 3/4" x 11 1/4" LVLs. The outer 12' side of each half is 2-ply.
After installing joists and blocking we'll add the ZIP (or a better sheathing) and tape. Then we'll flip the halves and put them on the pier L-brackets.
Then we'll add 16' LVLs spanning both halves, and connect the central abutted common ones to each other. Finally we'll tape the outer edges.
The result is 12' x 16', 2-ply LVL rims all around the perimeter, and a 2-ply center.
Then it's just a matter of adding rigid to the bays, air sealing the intersections, adding batt, and covering with X-Factor.
The brackets are 6x6 galvanized and we'll bolt through them into the LVLs.
Image is missing the ZIP tape and insulation.
Tiziano,
That sounds like a well thought out plan, except for the foam.
The main advantage of using a layer of foam on the underside is to reduce thermal bridging - and you lose that if the foam is in the joist bays instead. If it goes there, don't think it's even worth including.
I would skin the whole underside with foam before sheathing it - except for 2"x4" blocks where the piers are so you have solid bearing. You then have a 99% continuous layer which is completely protected by the sheathing, and can still secure it to the saddles the same way.
No need to air-seal the joist bays, just the intersection between the two halves of the floor, which you can do with a gasket or sealant between the LVLs, or by taping the joint at the sub-foor.
What a great idea Malcolm! It gives me CI under the floor like I'll have on the walls.
Between your suggestions and climbing carpenter's I think I've finally got the floor assembly sorted out. Thanks.
Tiziano,
I've end up building a few of these similar floors on outbuildings over the last decade, and with poly underneath they perform surprisingly well, even when very close to grade.
That looks wonderful.
Another question. Faced or unfaced batts?
Tiziano,
No need for faced batts as you have vapour-retarders on both sides of the floor.
The basic idea is here: https://www.finehomebuilding.com/project-guides/insulation/air-sealed-and-on-piers
He used faced batts and I think styrofoam rigid.
Thanks again Malcolm.