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asphalt laminated kraft paper and staple up radiant floor heat

I am looking for advice on whether to expect to have a smell or off-gasing problem from asphalt contained in the 1930 floor underlayment due to installing a radiant floor heating system using Onix tubing. The details are as follows:

1. 1930 house with 2.25 inch white oak floors now has Onix staple-up radiant floor heating installed under first and second floors (although new boiler not yet operational);

2. just discovered that underlayment is asphalt laminated kraft paper (with some fiber reinforcement). Appears to be original with the floors, which are from 1930 as far as we know. To be clear this is NOT #15 or #30 asphalt impregnated roofing paper. Aquabar B seems most similar current product equivalent. The upshot is that there is some asphalt in the middle layer between the two kraft sheets, but not the higher amounts that are found in saturated roofing paper;

3. sub-floors are 3/4 inch boards with some variable gaps between the planks of up to 1/4 inch;

4. maximum design temp for water in the onix tubes is 120 degrees. Tube installation is 8" on center as standard (so 2 tubes per joist bay), except near some large doors / windows there are double the runs in some adjacent areas (but small square footage overall of this higher density installation);

5. house is located in New England.

We are at a cross-roads on project progress. We can not yet run test of the radiant as boiler not yet operational and we are awaiting gas service. Would hate to move forward on adding interior finishes if we ultimately have to take up the current floors to remove the asphalt paper. But if we wait, we may end up with no place to live for a while.

I would greatly appreciate hearing if anyone has experience with similar circumstances -- retrofit of staple-up radiant floor heating under solid wood floors with old asphalt laminated kraft paper. Once the heat comes on, does it smell like an old tire factory, none at all or somewhere in between?

Many thanks


  • Larry Weingarten
    Larry Weingarten Member Posts: 2,245
    Is there any way...

    ... you can hook up an electric water heater to the tubing in a room and heat it up?  That would give you an answer pretty fast.

    Yours,  Larry
  • Gordy
    Gordy Member Posts: 9,514
    Floor total thickness

    Is it 2.25, or 3". . I'm thinking the 2.25 is including the .75 of subfloor.

    Being the Kraft paper is that old, and design temps are 120 likely no gassing. But like Larry suggested that's a sure way to tell.

    Are you using plates?
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,185
    Are you sure....

    Are you sure that your..." just discovered that underlayment is asphalt laminated kraft paper (with some fiber reinforcement)."..isn't asbestos? Pretty popular back in that era. If so, just wondering if that would/could act as an insulator.
  • house_renov
    house_renov Member Posts: 3
    Many thanks

    Larry, Gordy and Steve, thanks for your fast suggestions. Will look into them all. Gordy, I think I miscommunicated -- the sub-floor is 3/4" thick, as is the oak. The width of the boards is 2 and 1/4.

    Steve if it were asbestos, what appearance or telltale indicators would I look for to determine that?

  • house_renov
    house_renov Member Posts: 3
    No plates

    Gordy, no plates. Watts does not recommend them for Onix and the tubing is too wide for pex plates. Thanks.
  • Zman
    Zman Member Posts: 6,787

    If your design temps are truly 120, I think you should be ok. Has a proper heat loss been done? My concern is that if you end up needing  more like150 degree water you will start to get off gassing. Regardless of what onix advertises, I don't think the assembly you describe will produce more than 10-15 btu/ft. This seems like an unlikely heat loss for a 1930's NE home. Transfer platse make a huge difference in the reduction of water temps.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rod
    hot_rod Member Posts: 14,868
    At about 160F

    it will start to outgas and smell, from my one experience. Many years ago my brother in law installed a rubber tube staple up in his 40's vintage home in SLC.

    3/4 diagonal plank subfloor with hardwood above, asphalt felt paper under the hardwood. On cold days the leaky old home would ramp up to 160•F on it's reset control and the smell was noticeable. Even to a heavy smoker :)

    One weekend he went camping and cranked the supply to 180F, windows open, the smell finally went away?

    But you really don't want to run much over 150F to the bottom of a wood floor, especially ply or wafer as the glue starts to squirm. If you need to run that warm, add some transfer plates and drop the temperature 20 degrees or more.

    i thought the new Watts graphite plates fit around the Onix?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Gordy
    Gordy Member Posts: 9,514
    The truth about Kraft Paper

    Is you really dont need it.

    It was used back in the day actually to keep the dust in check while laying the actual finished product. Otherwise it really serves no purpose. Felt paper is actually a pain in the A$$ when laying hardwood as it wants to bunch up in the T&G while slaming them together. Rosin paper well its about the same not as sticky though.

    As for whats there in your case, lord only knows. In the 30's asbestos was in a lot of building products. ITs coated so thats a plus. So long as its not disturbed it should be fine.
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