If it snows where the house is located, install the cabinet on an interior wall or consider a surface mounted medicine cabinet.

No snow. But the house is already built with custom cabinetry all matched. Done.

I don't know why I didn't think of it before but I've used another class of products oriented towards automotive racing that may help. Thermo-Tec and Dynamat et al offer some thin insulations for automotive racing and high performance applications. They use a thin layer of this stuff usually in areas like the firewall to keep engine heat from intruding in the cabin for example. Might help in my application as well...

Thermo-Tec Aluminized Heat Barrier

I might sandwich a layer of this fabric on either side of a 1" foam board.

And I'm sure there's other radiant barrier products more home construction oriented I could put directly against the outer sheeting before inserting say a foam board.

Thoughts?

OK here we go...

Thermasheath Faced Polyisocyanurate for GA door insulation - R6 at 1 inch. Not bad, halfway to regular batting and better than a 2x4 stud...

Or rigid thermal faced fiberglass boards like thos used to build HVAC plenums... But R-values only look to be between 4 and 5 vs the option above...

This RMAX rigid foam insulation board is another one w/ R6 at 1 inch...

When we built we had to fur out the wall to accomodate insulation behind our medicine cabnets in a snowy climate. Love having the medicine cabinets!

Given your situation, foam board or spray foam would be suitable insulation materials for the space behind the medicine cabinet on the exterior wall. Both have their pros and cons, with factors like ease of installation and moisture resistance playing a role in your decision. Ensure proper sealing for optimal insulation performance, and consider checking local building codes for compliance. Overall, either foam board or spray foam can effectively address the lack of insulation in the existing space.

@justhighstone, thank you AI

OP, you won't like trying to carve out spray foam. Just buy a sheet of foam board and furr out as you describe.

Thanks! 'Furr out' that's the term I was looking for.

The cabinet was apparently installed with a finishing nail gun with 2 nails set at each corner into the sides and into the vertical studs on either side.

Getting those finishing nails without messing up the wood cabinets too much will be fun. They're deeply recessed - I can try drilliing them out or drilling around them to expose the nails so they can be pulled or broken off or ground down. Then I can re-install w/ larger head screws.

The best R value I found for 1" foam was R6 for some sheet garage door insulation foam. R5 to R6 is likely as good as I can get and should be 'good enough' as it would be comparable to to studs. I can probably caulk (or a little spray foam) around it a bit to get a good airtight seal on both sides.

You can see a pair of nail holes in this picture below - 8 nails total - 2 in each corner... would have been a LOT easier if they'd mounted it in there w/ screws. Sigh...

Are you sure the wall is using 3-1/2" studs.

Depends where you live. Maybe convert to surface mount. I don't like these wall barnacles. Prefer to keep items in vanity.

Aerogel boasts R 30 per inch...but costs more than gold.

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'Furr out'

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Here in the land above the USA the term is Firring.

Can't you drive the finishing nails into the studs with a nail set?

The best foam board insulation is Polyisocyanurate.

The first inch of insulation saves more heat than the second inch, etc.

You must not be in a very cold climate if the studs are 2x4's and there is no exterior insulation.

Sure, that small section of the wall is under insulated but the heat loss for a year through it is probably less than one sixty second opening of the front door to let someone in/out. Or less than any one of your windows for that matter.

But if you want a solution just fill it up with a life time supply of cotton balls.

The heat loss would be a bit like a window - and probably less. You are usually getting around R-4 for a window. The sheathing plus mirror might be more than that.

Are you bothered because the contents of the medicine cabinet get cold?

If they were able to get away with this, there are probably way worse places for heat loss in the rest of the house.

Hot environment - lots of triple digit days in Summer. Winter might only barely hit freezing one or 2 nights typically.

Problem is anything you leave in the cabinet in the summer gets HOT. A cream might get pretty soft if not liquify for example. Liquids will be a bit warm.

In the winter moisture can condense once the back panel just like it would on a window in the event of a quick frigid drop with a good amount of humidity in the air. If the windows have some moisture condensing on the inside - this cabinet will too.

The concern here is the high temps hitting the contents of the medicine cabinet and the occasional moisture that might accumulate on the back panel.

I *LIKE* that idea of driving the nails further in until they clear the sides! Thanks for the pro tip!

That is interesting that the 1st inch of insulation provides the most difference and so on. Thx.

My plan is to remove the cabinets, put in an inch of foam, re-mount with an additional piece of trim surround to furr them out. I think that 1-inch foam behind the cabinet should be 'good enough'.

Thanks all!

I don't think any insulation is going to fix the cabinet getting hot. Insulation may help a bit but not enough. You will never get temps down to a safe zone all the time without increasing air flow into the cabinet which seems like too hard of thing to do.

I really hate to criticize decisions after they are made when nothing can be done about it but it happens all the time and I just hope people can learn from others.

There is a reason that medicine cabinets are not common anymore. You might call it fashion but the idea of putting something sensitive on an exterior wall is just wrong. Especially in a hot environment.

Does the sun shine directly on the exterior wall where the medicine cabinet is located? If it does, planting a tree to shade the wall may be your answer.

You may have to consider a surface mounted medicine cabinet instead of a recessed wall cabinet if you want to put any temperature sensitive items inside. Unfortunately, with the triple digit summers, I don't think there's a way to get enough insulation back there to make a substantive difference, as David stated.

I don't know how any portion of an exterior wall fails to get insulated if you have routine code inspections. I suggest you get an IR scan of the entire home and look for suspicious areas while the home is under warranty for such issues. In Virginia, builders are required to come back and fix code violations found up to three years post completion.