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Subfloor thickness and radiant staple-up retrofit

Ktaadn Member Posts: 1
Long-time lurker, first time poster. We are in the process of planning a switch from an ancient oil-fired FHA to a modern NG mod-con boiler with indirect for DHW. We live in a very old (late 1800s) 1200+/- sq. ft. 1.5 story house in Maine. Given our small size, we're trying to decide between copper-fin baseboard, flat panel radiators, or stapling up radiant with aluminum plates. We have current access to the underside of each floor due to an unrelated remodeling project (rewiring). We're planning on a temp of 130 degrees from the new boiler. We are working with a professional licensed heating contractor who has indicated some external opinions on radiant might be warranted before we proceed. Our install is planned to begin this spring after the end of the current heating season.

Retrofitting radiant would be a great solution for us in regards to furniture placement in our small house. However, I'm concerned about the thickness of our subfloor. Our old house has two layers of 1" rough sawn pine for subfloor, which is then topped by 3/4" hardwood flooring (maple, if it matters). That makes for 2.75" of flooring. Is this too thick for a radiant staple-up retrofit?

I'm curious if anyone here has some experience or opinions on this. Thanks in advance for any comments.


  • pecmsg
    pecmsg Member Posts: 4,696
    130° is too high. 90 - 120°F water temp. any higher and the floor is too hot to walk on.

    Staple Up with aluminum plates along with baseboard. In floor reacts slowly, the baseboard gives a little more control of temperature and takes higher water temperatures.
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,506
    I don't usually like radiant with baseboard, I feel like they are doing 2 different things-radiate vs. convection. But I could be wrong :)
    How about radiant ceilings?
  • Jamie Hall
    Jamie Hall Member Posts: 22,941
    I am no expert on radiant floors. However, that floor assembly of yours has an R value of around 4, and I'd be very concerned that you simply couldn't drive enough heat through it at any sort of reasonable temperature difference between the space and the circulating water. By my back of the envelope scrawl, with a water temperature of 120 -- about as high as you want to go -- and a space temperature of 70, you'd be hard pressed to get more than 10 BTUh per square foot out of the assembly. Which may not be enough to meet the heat load of the space.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Gordy
    Gordy Member Posts: 9,546
    The only way to know what you need per square foot of radiant output is to do a heatloss for each room. Then divide it by the available square footage for radiant.

    The floors r value is no higher than 3 which is still plenty. What concerns me in a multi layered subfloor assembly is the lack of conduction from a plated underfloor system to the top layer finish floor.

    I don’t think you would be happy with the performance for the cost. It’s yet to be said until a heat loss is done as to whether it could do it with out a second stage back up be it rads, baseboard etc. which is more cost.

    A radiant ceiling will outperform the floor with as much comfort, and lower water temps to compliment the high efficiency boiler you plan to use.

    Another option is panel rads with trvs. Fast response. Convection ,and radiant output. Size the panels to allow for lower water temps to again get the boiler in it’s rated efficiency range. I think this option is the best bet for meeting the load requirements, efficiency, and cost in both labor, and materials. Panel rads are far more elegant than baseboard.