Insulating tile backer over staple-up radiant heat left bathroom floors cold. What are the fixes?

The owner described the problem in an Oct. 31 post on Reddit's r/Homebuilding. Tubing is stapled to the underside of the subfloor throughout the house. In five rooms, a tile installer who has since been fired put 1/2-inch GoBoard under the tile. The plumber has suggested raising the boiler temperature or pulling up the floors.

GoBoard has a polyisocyanurate foam core. Johns Manville's technical data sheet rates the 1/2-inch board at R-2.3 and the 1/4-inch board at R-1.2. With staple-up tubing, heat has to travel up through the subfloor, the backer and the tile, so a foam layer sits right in its path. Uponor's design charts for tubing under the subfloor already assume 3/4-inch plywood under the covering and show the required supply water temperature rising as the R-value of the floor covering goes up. Uponor caps fluid temperature for joist installations at 180°F.

That leaves three routes. The first is hotter water for those five rooms only. Stephen Minnich, writing for PHCP Pros, notes that different floor coverings often need different supply temperatures, and advises keeping the number of water temperatures low because good mixing valves and circulators are expensive. The second is to cut heat loss downward: on the HeatingHelp forum, Edward Young, a retired heating technician, recommends insulation below the tubing rated at least four times the floor's own R-value. The third is to tear out the tile and backer and redo them with a less insulating backer.

Before choosing, Minnich advises running a heat loss calculation for each room, because two rooms with the same heat loss can need different water temperatures depending on what is on the floor. The result shows how much hotter those five rooms would have to run.