I may have a variation on the usual VB vs no VB discussion. I plan on having electric radiant heat in my bathroom floor, so I will be insulating the floor (in the joist bays). The twist is that the kitchen is right below, lots of moist air around meal time. Let’s take FG off the table right away, I see 2 alternatives:
(1) Cotton insulation (www.bondedlogic.com, basically cells-like) sitting on a floor between the joists (kinda like the one I ripped out during demo, sigh….). The problem is the bathroom floor will be impermeable (Ditra map with Kerdi around the edges) so I can’t put a VB underneath the insulation without creating a moisture sandwich. I worry about this insulation becoming soaking wet and that getting into the joists.
(2) Foam insulation, 2″ or 3″ of styrofoam. The VB problem sort of disappears (or at least is greatly diminished) but there’s the obvious fire risk. In this case we have DW suspended ceiling below where the original plaster and lath kitchen ceiling was (recently demolished). With that open space under the joists, I will need to provide a thermal barrier, probably 3/4″ plywood. Or is that another VB, perhaps it should be 5/8″ Type X, 5′ long and 10″ wide in some places, sitting on a floor between the joists.
I’m inclined towards the cotton insulation rather than the foam just because the plan is already to surround this bathroom on 3 sides with foam (2 outside walls and ceiling). However cotton poses the VB problem. I don’t see a problem with foam under electric radiant heat, there’ll be 1-1/4″ ply between them, please educate me if I’m wrong.
Replies
If both sides of the floor are interior, which it sounds like they are (bathroom above, kitchen below), then you don't need a vapor barrier at all. You only need a vapor barrier on the warm side of an exterior wall or floor.
You need very little insulation to do what you want to do, that is, direct the heat into the floor above rather than into the ceiling below. If you are worried about fire, I don't know why you wouldn't use fiberglass -- it would be the most reassuring choice. However, if you could set any insulation on fire you'd have to do it by burning through the wood first, in which case you're already gone. So I don't think it matters unless a combustible insulation would psych you out.
If it were me, I'd use 3½" unfaced fiberglass pressed up against the underside of the floor.
The cotton insulation has flame-retardant chemicals as with cells. That vs FG: it's a question of which I believe is worse, the fibres or PDBEs. But at least I believe the cotton insulation is effective.Point taken, condensation should not be such an issue on an interior (heated) surface.
Taylor
Any VB here will potentially cause problems. I would use a 3-1/2 foil faced Fg batt installed by friction fit between the joists and held up by wire stays. Leave a gap of about 1 " blow the subfloor to allow for air circulation should any moisture find it's way to the subfloor. don't staple the edges of the foil or you'll have to tight a VB sandwiching the subfloor. The foil will reflect the heat back up rather effectively.
You don't say why no FG. If formaldehyde is the issue you can get FG that has none. It is a White color as opposed to the normal yellow or pink. I believe you can get it foil faced.
I'd just use unfaced fiberglass.
Good grief Taylor.
For your marriage's sake.......in the words of Larry the cable guy.
Get 'er dunnnnnnnnnnnnnn.
as usual, best of luck on the project.
Remodeling Contractor just outside the Glass City.
Quittin' Time
Cal, you are either DW or my mother using a pseudonym.... :-)
I hear you.
But alas, just an interested third party.
Isn't it fun...........Remodeling Contractor just outside the Glass City.
Quittin' Time
Actually DW has been very very patient.... Every now and again she goes out, buries her head in the snow and lets out a few screams.....
The woman is a saint.Remodeling Contractor just outside the Glass City.
Quittin' Time