Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Bituminous building paper in wooden floors

I'm considering installing underfloor radiant heating in an older house (1927) that has a raised floor. There is bituminous building paper between the subfloor and the hardwood finish floor. I've been warned that I would have problems with outgassing from the building paper causing smells and worse. Does anyone have experience with this? Are there (affordable) ways of heating the underside of the floor that wouldn't cause a problem? My design heat load is ~25 BTU/sf. The finish floor is tongue and groove. How much of a problem with uneven gaps should I expect? I live in the Bay Area, so the variation in outside temperature and humidity is less than in a lot of other places.




  • Mark Hunt
    Mark Hunt Member Posts: 4,909

    First: Have a QUALIFIED radiant floor heating contractor do a heat loss calculation.

    I live in upstate New York so we get a little colder here.

    I have been in houses where this has ben a MAJOR problem.

    But you may be able to do the radiant floor because of your location.

    The floor temps where you are may not be high enough to cause a problem, but you won't know until you get a GOOD contractor in there.

    Try the "Find a Contractor" feature. The best in America are listed there!

    Mark H

    To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"
  • hr
    hr Member Posts: 6,106
    It all depends on temperatures

    I have done plenty of older homes with asphalt impregenated felt paper as underlayment, without a problem. Then I had one high temperature staple up (160) degree supply, that was a disaster.

    He did "fix" it by cranking the temperature way up on the boiler for a day, opened all the windiows and let it outgas. The home is fine to this day, 10 years later.

    I would suggest a couple options. Could you remove a piece of the paper and heat it, maybe heat it in an old electric frying pan (with adult supervision :) to find the squirm point?

    This would be an excellent application for aluminum transfer plates and constant circulation to keep the temperatures as low as possible.

    There are some few, rare ocassions, in my opinion, where floor radiant may not be the best choice. Maybe a combination of low temperature floor warming and some panel radiators, celing or wall heating options to supplement on coldest days.

    This asphalt paper question comes here quite a bit. I'm not sure there is an exact answer. Depends on all the above. And your willingness to "trial and error radiant retrofit" :0)

    hot rod

    To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"
  • Philip Haves
    Philip Haves Member Posts: 3
    Staple-up vs. suspended pipe

    Thanks Hot Rod! I'm put off aluminum transfer plates by the cost ($7/sf for Radiant-Trak - is there a cheaper alternative?). Does anyone have any experience with suspending pipe ~3 inches below the subfloor? This seems like a good way to avoid the hot-spots you get with staple-up, but is there a problem with thermal cycling of the joists and is there a proven way to calculate how much pipe you need to meet the load with a given water temperature? (I can use the heat transfer coefficients for cylinders in the ASHRAE Handbook of Fundamentals but I'd like to know if anyone has a simpler method, preferably validated on a real job.)

  • hr
    hr Member Posts: 6,106
    Lets do some calcs

    The experts seem to agree with a temperature around 150 degrees for a continous, safe working temperature for wood. Above that, Im told, the wood cells starts to suffer damage.

    Of course, like anything, it depends on which expert you ask!

    The Wirsbo Design Books chart shows 145 degree supply with a R-2 floor covering (3/4" subfloor plus 3/4" hardwood) at a 20 degree delta T, expect an output of about 15 btus per square foot. Quite a bit shy of the 25 BTUs / sq. ft. that you require. Even at 180 supply you don't hit 20 BTU/ sq. ft. Way beyond my comfort range for joist bay temperatures.

    I do quite a bit of joist bay work, BUT it is for floor warming in forced air homes (tile warming) generally in the 10 BTU/ square foot range, or less.

    Radiant Engineering specs show 8" OC plates at that temperature (145)would provide right around 20 BTUs per square foot. I feel they are a bit conservitive with their output numbers. I'll bet 150 degree supply with 8" OC plates would meet your load. You do have some wiggle room their in regards to temperatures on design days. An outdoor reset would allow you to put a limit on the supply temperature.

    Keep in mind the design you have (25 BTU/ sq. ft.)is for the coldest design day. (I'm guessing without seeing the actual calcs and building specs))How often do you hit that temperature in your area?

    Your price per square foot seems really high for plates! Are you talking an installed system price? The published list price for Thermofin in small quantities is around $1.66/ per foot

    hot rod

    To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"
This discussion has been closed.