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floor made of framing planks turned on edge: radiant heat below and finish above?

simonhuynh Member Posts: 3
Hi Everyone -

Apologies, this is a bit of a cross-post from the Radiant Heat forum, but I also have more general questions so I thought it okay to repost here.

The particular challenge I'm dealing with is that I'd like to install radiant heat beneath an existing flooring that is made of 2x4 douglas fir lumber planks turned on edge. The property is a unique one, and no one I've spoken to has ever heard of flooring made of that much wood. The wood looks quite nice and keeping it as is would save us the cost of more wood flooring, so I'd prefer going under rather than over.

Alan Forbes, with whom I've been discussing this with (and who's been a great deal of help), is concerned that we won't be able to get adequate heat transfer through the 3.5 inches of wood. From what I've read elsewhere, douglas fir has an R-value of 1.2 per inch, so we're looking at an absolute thermal resistance of 4.2, maybe 4.0 after lots of sanding and then refinishing? In case it matters, the wood planks are 50 years old. On the bottom side of the house the wood planks will have seen lots of wear from weather.

The second question is: has anyone had a good experience using lumber framing wood for their flooring? I am pretty certain the floor is strong enough -- portions of one floor we're even used as a garage originally -- but I've heard others express doubt about the wood being too soft. The planks have been put in tightly enough, so far as I can tell, that they look pretty good, though probably some wood filler wouldn't hurt. Any other concerns to be wary of?

So. . . can it be done? And what's the greatest absolute R-value through which you've successfully and happily pushed radiant heating?

I'm happy to attach pictures of whatever you'd like to see.

Thank you all in advance!



  • hot_rod
    hot_rod Member Posts: 22,120
    what kind of heat load

    does the room have? Where is it located and how often is that area at design conditions?

    I had a cabin in Montana that was built with 2X4 on edge like that. A stud mill nearby sold the culls for pennies a piece so it made a unique, affordable, flooring concept.

    Hardwood floor finishers have some amazing fillers and sealers available now, I'm sure a workable finish could be applied. Depends on you expectations are.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Jamie Hall
    Jamie Hall Member Posts: 23,275
    Pretty much any old mill

    you go into, at least in the northeast, had solid wood floors like that.  It is not the most economical use of timber, but the load carrying capacity of that type of construction is really impressive.  As to finish, you can do pretty much anything to them you'd do to any other "softwood" timber floor -- keeping in mind that the term "softwood" refers to a type of tree (pines, firs, hemlocks, redwoods, etc.) and not how hard or soft the wood actually is.  If it's older douglas fir, the chances are that it is actually pretty hard as flooring materials go.  I'd be inclined to sand it and urethane it and celebrate it!  (Actually I'd probably shellac it -- much easier to repair than urethane when the dog scratches it -- but that's my preference; most flooring guys don't want to hear about shellac).
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Gordy
    Gordy Member Posts: 9,546
    edited July 2013
    Douglas fir

    Is strong, and has a nice consistent grain, I love the redish hue, and lighter tones in the grain.

    There's plenty of lumber to work with as far as sanding refinishing. Just make sure you hire a professional floor refinisher that knows how to sand a floor, or it can really get screwed up. There is an art to it. Like hot rod says there is plenty of fillers on the market, and in the right hands can almost go undetected. Example divots can be made into knots if desired for the look.

    It's pricey but I would use street shoe made by basic coatings for the final finish.

    Water based finishes are harder than oil based finishes. Recoat to fixed any damaged areas.

    I would skip the radiant under floor. With out a heat loss it's hard to say if you could get enough btus out of a plated underfloor design. Especially a controlled design with a floor that thick.

    Again there are other elegant radiant options walls, and ceilings.
  • simonhuynh
    simonhuynh Member Posts: 3
    edited July 2013
    so the finish will be great but heat not so much . . .

    Thanks for the responses, Jamie, hot rod and Gordy. It sounds like my family and I have a lot to look forward to once the floor is filled in and refinished.

    Before responding to the heat question I need to make an edit: after more fully exposing the floor beneath I learned the planks are in fact only 2.75 inches thick. After sanding and refinishing, perhaps now we're within field goal range of a plausible radiant system, maybe?

    hot rod, I don't know enough to answer your questions about heat load and design conditions. I can say, though, that the current state of the upper floor of the house is a big heat dissipator. The upper floor -- there are two floors total -- is one large main room with a kitchen nook, about 800 sq ft total. The frame of the house is a steel octagon, 8 large steel beams each at the corner of the octagon. The top floor exterior walls (there is only one interior wall, on the kitchen) are 75% glass from floor to ceiling. These picture frame windows are 50 years old and single pane. The house is located at one of the windiest points in San Francisco. I hope begins to address the heat load question.

    Gordy, can you elaborate on a "heat loss" beneath the floor? The system I was imagining looked something like extra thick pex tubing (1/2" ?) run at, say, 8" intervals on aluminum fin transfer plates with aluminum-faced batts below. Would that constitute a "heat loss"?

    Lots of this terminology is new to me so no offense taken if you happen to explain it as though I was a 5th grader.

    Thank you!
  • Gordy
    Gordy Member Posts: 9,546
    edited July 2013
    Heat load

    Is the amount of heat the room, or structure loses that your trying to heat at a given outside temperature.

    So there are calculations that need to be made using r values of walls, roof , floor. What the coldest temps the structure sees in a typical heating season. How much solar gain you get with structure orientation, wind.

    There are computer programs that will then put it all together, and be able to tell the designer of the system how many btus a square foot the floor would have to produce to maintain the desired indoor temp.

    That's the first step heat load calculation.

    Now that we know your location is San Francisco, and not Fairbanks Alaska there maybe some hope.
  • bill_105
    bill_105 Member Posts: 429
    edited July 2013
    Simon,can you post some pics.?

    I mean plenty of them ,like the neighborhood. Being from there it would be a trip!

    Oh BTW Fairbanks has been over 85 degrees lately. It's been cookin' up here this summer.
  • CMadatMe
    CMadatMe Member Posts: 3,086
    edited July 2013
    Just Run

    It through a radiant design program. Answer in a minute as to will it heat the space. Carpet applications have R-values in the 3 area and require high water temps sometimes 160 to fully heat the space. Might have to go the floor warming route and add panel rads for the supplemental. You could size the panel rads based off the same water temp running under the floor.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Radiant Engineering

    They used to have output charts for their Thinfin C as a function of the R-value of the floor, but I can no longer find them on their website.

    Heatloss on new houses here in the Bay Area typically come in at 17-20 BTU's/ square foot.  I want to see if theses accelerator plates can approach that with the kind of floor that Simon has.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • Gordy
    Gordy Member Posts: 9,546
    High r floor coverings

    Chris sure it has been done, but is it really efficient? Probably lucky to get 15 btus a sf.

    I would also tend to worry about that great of a temp difference from the bottom of the wood to the top of the wood floor. Doug fir is a soft wood, and could create some undesirable movement.
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