hi, I am playing with heat design ideas for bedrooms and bathrooms. This house the main living spaces are radiant hydronic concrete floors. All exterior walls are 8" concrete with vapor barrier exterior. And 5 1/2" foam outside of that. Plenty of thermal mass inside. I have been toying with the idea of building hydronic radiant heat concrete benches below Windows to heat small bedrooms and bathrooms. Benches would be 8' long per room. I liked this idea for the reasoning of keeping all radiant pex tubes in a mass, rather than higher heat radiators. I was hoping to avoid radiant floors, partially for finish flooring options. Also the cost of gypcrete floors here. The benches would not have the surface area of a whole floor, but would have a large mass, Opinions? Expertise?

Mass in an active radiator is a bad thing. Surface area is beneficial, but you could also do that with more conventional radiators (ie, they don't have to be higher temperature).

As jonr suggests mass can lead to response time issues. In your case overshoot would be the thing. Even with ODR such massive radiation can be a handicap in all but the coldest climates where the base load might justify some type of full-time load condition. In my basement bathroom for instance.

For very well insulated homes aluminum panels under a sub-floor will run at low temperatures and not require much care in floor product selection. I have used every radiant panel, sub-floor, sandwich, gypcrete, concrete and Warmboard but chose extruded aluminum sub-floor panels for my own remodel.

Many masonry heaters often incorporate heated benches into their design. They are truly wonderful places for people and pets to hang out. The Btu/h that heated benches generate depends on the surface temperature and the exposed surface area. 80-90F is a typical bench design surface temperature range to target. You would first need to accomplish both a heat loss analysis and a bench heat gain analysis to determine if the bench design will be suitable for heating each room. We have software on our website that would enable you to do this.

I wonder if you could ever get the radiant heat and air temp/drafts uniform enough to make next to the window the most comfortable place to sit. In any case, you could build them with lower mass radiators (like wood and aluminum panels).

It is amazing how proximity affects comfort. I have used radiant floors, walls, ceilings and various panels to offset the inevitable discomfort created by windows. Spot heating is the key to comfort in a well insulated home. There is no better "spot" to heat than below a window. Even in low-mass homes radiators rightly are placed at the high-load windows. Window seats have been warmed for a hundred years. Doing the heat load for each room is the first step in any HVAC design. More especially in DIY radiator construction.

I agree - someone installing radiant heat around windows understands the importance of a uniform radiant temperature being delivered to the occupants. It's not just about air temperature and mean radiant temperature. You might also get enough convective heating to counter the usual sheet of cool air falling on the inside of a window (important when you are really close).