Considering heating mat (120V) from Home Depot with a programmable thermostat in a bathroom that's L shaped and probably about 50 sq. ft. on a new construction. I am tiling the floor. Question is: I'd have to insulate the subfloor (I would use fiberglass batts not foam to save money) then install subfloor, cementboard and put radiant under the tile in the mortarbed. Is it worth the extra trouble of wiring it and the cost of buying the unit, which is about 400 bucks with the mats and thermostat. The unit says it could make the tile as much as 20 degrees F warmer. If my subfloor is 65F that doesn't seem that warm. Any thoughts? THanks.

An 85F tile floor is uncomfortably warm. But unless you have large windows &/or skylights in there the floor won't typically need to reach 85F before the thermostat is satisfied, especially if this is just supplemental heat, not the sole heat source for the room.

Whether it's worth it or not depends. Would just insulating under the floor bring the floor temp enough to be comfortable (if not cushy-warm)?

How are you heating the rest of the place?

We keep our bathroom floors about 78-80F which keep the bathrooms about 72F. Personally I would like the tile about 85-90F, but then the room would be about 80F. I should have heated just the area I stand in to shave ;)

Dana, heat will be air source from a GSHP. I have to do about 700 sq. ft. of hardwood or tile flooring in about a week so I'm only looking for extra work if it's worth it. I guess it is comfy but we don't spend a whole lot of time in our bathroom. Wondering if I could achieve a similar effect by installing a vent sized electric unit near the floor.

Our house now has terrible ductwork and the bathroom is often very cold. Our bedroom is about 18 F below other parts of the house on very cold nights. If my new construction goes well I might be trying to fix a problem that doesn't exist.

I'm surprised that a GSHP contractor would utilize existing ductwork in poor shape & unbalanced condition !?! Was the prior heat from an air-source heat pump or a fossil furnace, or...???

Upgrading insulation & air leakage on the building will likely improve the balance issues somewhat, but not completely. If the duct design is OK (per Manual-D), and well sealed against leakage, you can probably tweak the balance a bit without ripping out whole sections. But if some rooms are inherently lossy and others much less so, it's difficult or impossible to balance it perfectly. Truly high performance building envelopes render those issues moot though.

If it turns out that it's still uncomfortably cold in there after the fact you could put in a small hydronic baseboard loop (or radiant, if you like) running off the hot water heater. If your GSHP is using a desuperheater for HW, it'll run as efficiently as the geo- be 3-4x as efficient as running it with electric radiant.

Dana, Sorry I meant my existing house sucks but my new construction will not or I will have my installer's head. I'm thinking now is the time to add radiant heat to the bathroom on the new construction because I won't tear up the tile for decades.

Thanks all for the help. I probably won't do it just for one room. If I had the dough I'd spring for Geo radiant in the whole house. That's big bucks but I can see how that's super comfy!