Where to spray foam on bathroom floor above garage

[biker513]

I have been trying to find the definitive answer to this questions, but have found many different opinions.

I am currently in the middle of a master bathroom renovation. The master bathroom is located on the 2nd floor of a home, and about 1/2 of it is located over unheated garage space.

I currently live in Green Bay, WI which I believe is in Zone 6.

When I tore up the subfloor to reconfigure the plumbing, there was blown in cellulose insulation in the floor joists. Over the subfloor, there was a vapor barrier, and under the cellulose there was a vapor barrier creating a double vapor barrier situation. I know this was incorrect, so I am working on how to insulate the area.

There is currently 1 shower trap above the garage area and the toilet. I am planning on doing 5 inches of spray foam for this area but am struggling with where to put the spray foam. Per normal convention, the vapor barrier (5 inches of closed cell foam) should be on the warm in winter side which would be the bottom of the subfloor. This would require me to remove part of the ceiling in the garage and shoot spray up onto the bottom of the subfloor. However, with this method, do I run the risk of my pipes freezing as I have now removed them for the thermal envelope.

The other option would be to spray down into the joists and then put the floor down afterwards. This would keep the pipes within the thermal envelope, but the vapor barrier on in the incorrect side. My insulator says that he has seen in done both ways. My main goal is to have this done right.

 

[Dana]

In the floor over a garage there is no requirement for any vapor barrier. A full 3/4" subfloor that's air tight already meets the Canadian code definitions for a vapor barrier at bout 0.5-0.7 perms but is otherwise referred to as a "class-II vapor retarder", which is the range between 0.1 and 1 US perms. Other definitions of vapor barrier are less than 0.1 perms. (6 mil polyethylene is about 0.05 perms.)

A reasonably air tight subfloor is more than enough of a vapor retarder to protect this assembly once the garage-side vapor barrier is removed.

5" of closed cell foam is one of the most expensive and least green ways to insulate, and runs about 0.2 perms (= "not a vapor barrier,'ceptin' in Canada ") The high-R/inch is largely defeated by the shorter 5" thermal bridge through the framing. It might hit R30 at center cavity, but those stripes of R5-R6 joist-bridges move twice as much heat as through 10" of joist with insulation on both sides.

Closed cell foam's environmental impact is twofold- the most significant of which is the HFC blowing agents uses, which are extremely powerful greenhouse gases (on the order of 1000x CO2 or more), soon to be banned under the Kigali amendment to the Montreal Protocol, but also a high polymer weight per R. HFO blown closed cell foam is available, but about 35% more expensive per R than HFC blown foam. There are times when that's the only way to achieve the moisture control or performance goals in a retrofit, but this doesn't look like a situation that needs it.

HFC blown closed cell foam can't be installed in lifts of more than 2" without becoming a fire hazard during the curing period, and forming shrinkage voids or separation during the cure. HFO blown closed cell foam can usually be safely blown at 5" or more without those issues.

By contrast OPEN cell foam uses less than half the polymer per R, and is blown with water, a fairly low-impact material that we all have experience with. But like HFC blown closed cell it's depth per pass is limited for both safety and quality reasons. Most half pound foam is good to about 6" per pass, with a cooling/curing period between lifts.

Get rid of the vapor barrier at the garage ceiling level- it's only going to collect moisture over the winter. Then air seal the band joists to the joist bays, and any penetrations of the garage ceiling. (Polyurethane caulk or can foam, depending on the size of the seams/gaps/holes.)

It's fine to install OPEN cell foam or cellulose up to more than half-depth on the drain plumbing or a bit higher- as long as there is at least some stripe of the drain exposed to the warm side of the assembly it won't frost up internally. Don't install insulation between the top of the pipes and the subfloor. In the other bays fill it up with cellulose or open cell foam and forget about it.

If using cellulose for the insulation (recommended- it's easily DIY-able and inexpensive) take the time to seal all of the electrical and plumbing penetrations of the I-joists with can-foam or FrothPak.

It's important to keep the plumbing inside of conditioned space wherever possible, or at least in a location within the insulation layers where it won't dwell below freezing for hours/days on end.