Insulating Interior Garage Wall

Our master bathroom is on the opposite side of a wall from our garage. 

I've got R-19 6" fiberglass batts in the stud bays of the Master Bathroom, but that room gets COLD. It's always the coldest room in the house. It's got one 8' exterior wall, and one 11' exterior "garage" wall. 

I plan on doing a lot more insulating on the exterior wall, but I've been reading up on how insulating the exterior of your house is just as important. 

I thought "hey, my garage wall isn't doing much, and it'd be real easy to slap up some 2" polysio along it." 

Downside is that I've got hanging shelves mounted to that wall. 

Could I just fit polysio around my shelving mounting locations? They are only 2" of an otherwise 8' tall by 11' wide wall. 

It will thermally shortcut at those locations.  But the more important factor here is that you can't leave the polyiso exposed, because even though it's not inherently a fire hazard it will burn and that E36 M3 is toxic when it does.  If you're going this route, put furring on the wall and drywall over the insulation.  At that point you might just as well remove the shelf standards and reinstall them on blocking after you drywall it.

Polyiso is also susceptible to water absorption, so make sure the wall does not get damp.

What about something like Rockwool's Comfortboard? I could get that down to 1-1/4"

It's non-combustible. 

Rock wool is what they use for insulating joints and penetrations in fire rated walls.  It's pretty benign.

The best car guy solution is to insulate and heat the garage.

Is this wall going to be open at some point during your bathroom reno? Spray foam inside that wall would air seal and insulate at the same time and give you the best result.

The garage already is insulated. The problem is the huge air gaps present. 

I also don't want to pay to heat it. That's part of my problem currently, my hydronic radiant system isn't very efficient and it's costing me a lot more than it should because the house (and slab) isn't well insulated despie being built in 1997.

Just got off the phone with a local spray foam installer. I was surprised to hear he has no issues spraying just two walls in a small remodel. 

In reply to pheller :

What am I missing here? I live in a conventional stick-built house in Michigan. It has 6" batts in the walls and while the upstairs can be a bit cool due to the longer ducts, the house stays reasonably even in temperature. The rooms that share walls with the unheated garage are actually warmer to the touch than the other walls. Only thing I've done since the original construction is add 12" of insulation to the attic.  Is your problem leakage, which the foam could well fix, or a lack of even heat distribution around the house?

The heat is one of the issues. I've got in-floor hydronic radiant heat within a slab-on-grade. I'm AZ, but at high elevation and our overnight winter temps dip into the single digits. That bathroom is at the end of the loop, and it also has insufficient floor coverage. Apparently it also isn't thermally isolated from the unheated slab in the garage.  So when the temps dip, the garage slab gets cold, and that pulls heat out of the adjoining bathroom floors. 

Fixing the heating issues in that bathroom is going to be hard. Better insulation is easy (especially while the walls are open.) 

If I can fix the insulation issues and retain the heat better, then I can focus on some sort of supplemental heat source. 

I've actually considered abandoning the radiant system when its time to upgrade/replace its water heater because I just feel like the install was so poorly done that it'll never be efficient, and I could install two seperate ducted mini-splits for almost the same price as a modcon hybrid/dual-loop water heater or boiler. 

In reply to pheller :

I remember seeing some construction details for slab-on-grade housing in Sweden that involved rigid insulation placed vertically and then horizontally (a couple of feet below grade) all around the slab. It kept the ground beneath the slab at 55 degrees or somesuch through the winter and made it easier for the in-floor heating to function. I suppose retro-fitting something like that , including cutting the garage floor, would end up costing more than the min-splits solution.

I'm with Dead Skunk. Something else is wrong. 
 

Sounds like you are saying you don't have an insulation problem, but a lack of heat in the first place (end of loop), and a conductivity problem (no thermal isolation from garage slab)

Also, your garage may be insulated, but I'll bet the garage door isn't. That's a BIG hole.  And it's not heated. 
 

I'd add a small auxiliary heater in the bathroom. Electric is easy. 

The beauty of the hydronic radiant slab is it uses the slab's thermal mass and natural conductivity to effectively store heat. 
 

The problem is that the perimeter insulation and thermal breaks are often overlooked.  The slab is just as capable of conducting and radiating it's heat to the exterior. If the perimeter was not properly insulated, the radiant system will be compromised. 

BTW... I wouldn't consider that an interior wall. The garage isn't heated (and probably has no insulation in the ceiling)- that's the exterior. 

I have some concerns about the slab's lack of insulation making further investment into that system financially inefficient. 

One of the concrete guys I talked with, who was really helpful, also mentioned that he had built a few homes in this neighborhood. He said back in the late 90's it was very common to just insulate just around the perimeter of the slab. No exterior insulation, no insulation under the interior slab. He said after a few years they got guidance to start insulating the interior, and more recently, everywhere.

He was pretty confident that my slab had only been insulated around in the perimeter. 

He said "I've had radiant systems in every type of slab insulation setup, and with every improvement with insulation styles, I saw big improvements in efficiency and lowered cost to operate." 

Great.

I've thought about how I might rectify this. One would be to not waste money heating the slab, and instead switch to a ducted mini split. Another was to insulate the floor somehow. My wife and I are removing carpet from the house, so it'd need to be some sort of thin insulating layer under the LVP we've been installing. 

Another idea was to convert every room to WarmBoard. Basically dig up the hydronic loops (or abandon the loops in the slab and route new loops through Warmboard Subfloor), route them into the WarmBoard, and then I can retain the hydronic setup but get insulation over the slab with quicker response times. 

I've got a good radiant engineer/designer who I've been talking with, and he's going to flush out the details of which options might work best in my case. 

In reply to pheller :

That's good. I'll be interested to hear his recommendations. (I doubt insulating the slab will be one of them)

In reply to pheller :

Did the contractor who told you about the perimeter insulation give any indication how deep it goes?

I didn't ask about stem wall perimeter insulation, but seeing the thermal images, I don't think there is much. 

The insulation I did see is 2" thick under 4" slab but only extends 2-3' from the exterior wall.

Well, I'll be following along to see what solutions you can come up with. A few years ago I spent several weekends going to open-house tours by a couple of local builders who specialize in building LEEDS certified homes and always found the various construction and insulating techniques interesting.

pheller said:

I didn't ask about stem wall perimeter insulation, but seeing the thermal images, I don't think there is much. 

 

The insulation I did see is 2" thick under 4" slab but only extends 2-3' from the exterior wall.

That's quite typical and adequate for perimeter under slab insulation, but it needs to be in conjunction with stem wall insulation. 
 

There is no need to insulate under the whole slab. Dirt is just more thermal mass. 

I'm not sure radiant designers would agree with you here, Paul. 

https://www.finehomebuilding.com/2014/07/09/insulating-a-slab-on-grade

In reply to pheller :

There's a lot more to it. 
 

You have to define foundation type and climate zone before you can make blanket statements like that. 
 

According to the article you posted (which I agree with 100%), climate zone is the #1 factor, and foundation design is second. 
 

I'll admit, you are slightly more north than I was thinking. I was thinking monolithic slab in climate zone 3. (My suggestion and the recommendations from your article completely agree for that). You are in climate zone 5 (which does change things a little- my bad), and you haven't told us about your foundation design. You are giving partial information, and getting partial answers on the internet. If it's monolithic, then the best you can do is exterior vertical wall.  The recommendation would be 2' turndown. 

 

But most importantly, you aren't building new. What difference does it make what new radiant designers do?  You are remodeling, and you can't cut up the floor and do it over. You need to deal with the existing conditions, and come up with the best solution you can. I love Fine Homebuilding and never disagree with them, but these recommendations aren't really relevant to your problems. 
 

In reply to pheller :

BTW... excellent choice on your reading material. Taunton Press (and Fine Homebuilding) is top notch. 

If you lay new loops on top of the existing slab you're going to spend a bunch of money and will still lose heat into the non insulated slab. 
 

Can you post a photo of the exterior wall from outside? I suspect your slab edge needs some exterior insulation + stucco a lot more than your stud walls need more insulation. 

In reply to OHSCrifle :

I agree