Does one need more than the standard amount of attic insulation (R40) when using radiant floors? We're having trouble keeping two rooms at the front of our house warm and we were thinking either the insulation might be thinner at that point in the attic or maybe we just need more insulation when using radiant floors. Since radiant floor "heats you and not the air" is one more susceptible to the "radiating cold" of the ceiling (because the air in the room no longer acts as a insulator from that cold)? Thanks, Colin.

No, it shouldn't.

If the attic above is R40, the ceiling won't be cold, it'll be at room temp, just as it is with other heating systems. If there's a "radiating cold" it's more likely to be from exterior walls, or (as is even more likely) the windows. The air has the same insulating value as it always did, and the radiation temps from the not-floor surfaces are about what they are in any other situtation. (The ceiling might be even slightly warmer than room air temp, since it's the surface taking on radiated heat most directly from the floor, and re-radiating it back to the room.)

With a cheap infra-red thermometer you should be able to point it at the walls & ceilings do tell where the coolth is from. If there's a localized thin spot in the attic insulation you should be able to find it from below. It'll be worth fixing, but it's not likely to be the dominating heat loss. With the same thermometer check the window frames.

If a radiant floor isn't keeping up it's usually because the total heat loss of the room is higher than was anticipated at the design phase. But if your heat loss is dominated by high infiltration rates, too much glazed area, or too little wall insulation, taking the attic up to R100 won't make an appreciable difference. Surface area is surface area- and I'm bettin' the surface area of exterior walls & windows are roughly similar to that of the ceiling (more than the ceiling if it's a corner room, somewhat less otherwise), but has an average R value of R10-12, which means the conducted heat loss through the wall will be roughly 3-4x what you lose through the ceiling. But if the place isn't very air tight, all bets are off.

Yes, look for air leaks in the cold areas. If that isn't it, then look to increase the flow in the heating system for those zones.

Thanks everyone, and sorry -- I really should have add more details.

It's an ICF home (3" insulation / 6" concrete / 3" insulation). New construction. Triple glazed fiberglass windows. The two front rooms in question are corner rooms. Both have one 4x4 window (east facing), and one room has an additional two 2x2 windows (south). 176sq.ft. of exterior wall, 120 sq. ft. of roof.

The radiant floor input temperature is now around 95'F and returns at 80'F. I've increased the flow for the front two rooms to 0.6gpm. I've reduced flow in the other rooms to about 0.2gpm.

I've been wondering: if the attic insulation were thin over the outer walls, would the cold bleed through the concrete from the top (and create cold walls)? I'm hoping a direct inspection of the attic insulation will tell me what's going on. (I can just carefully wade through loose-fill insulation to get to the front rooms, yes?)

The exterior walls are noticeably cooler than other walls so this is my best guess right now.

I'll see if I can track down an IR sensor. Thanks!
-Colin.

those are pretty low temps. maybe that's as it should be, but you also might need to bump it up a bit.

The 6" of insulation on the exterior walls adds up to ~R24, then you have the windows, which are guaranteed less than R24, so your average insulation over the walls is likely ~R22-ish. 

Your attic is ~R40, and you're wondering if thin spots in the attic where the coolth is entering, conducting down the concrete and sapping heat from the room through the ~R12 of interior side wall insulation?  Perish the thought!  Even if it were half-thickness (R20) over the entire attic, the wall's heat loss will still dominate, since the walls have ~50% more area, with an average R value comparable to the half-thickness attic.

The problem is not attic insulation unless it's out & out MISSING (not just thin, or with some gaps), and over a significant portion of the area. This could be determined easily with the a ladder and the palm of your hand.  Otherwise, bump the temp 5F at a time until it keeps up.

Unless your ceilings are rated for the dynamic loading of a walking person (and I doubt they are), don't even think about walking on it.  If the joist tops are exposed (and I'm assuming they're not) you might be able to walk out along the tops of the joists, but it should be obvious if there's missing insulation.  If the roof is ventilated at the soffits without proper chutes it's possible that you'll get some wind-driven thinning at spots, but it would have to have made huge drifts scrubbing other spots bare to have a large effect on the room's heat loss.