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cattledog Member Posts: 60
I have seen the R values of 1.4/inch for typical soft woods. In your situation you have seven inches of wood between the hot cavity and the cold area for a rough R value of 10. You then need to figure out the exposed wood area to the cold. You will have two R values in parallel with the area of foam insulation with R=? and the area of the exposed beam with R=10. Analytically it is like an area of window in an area of wall.

Regarding response time, I think that the parasitic loss to the air trapped in the cavity and to the exposed wood of the joist is small compared to the heat delivered to and through the floor. The heat capacity of the air is small and the mass of the exposed wood and the heated beam would also be small compared to the floor.

I would think that the situation at the ends of the joist bays may more relevant than the three inches of exposed joist and the air pocket.


  • John Abbott
    John Abbott Member Posts: 356
    Insulation again

    Seeking help on a radiant installation I did recently.Lance Macey posted this question but I want to give details and hope get some insight into how bad this is screwed up if at all.
    The job is rather simple wirsbo joist trac plates 8" on center 1/2" hepex outdoor reset attepting to run low water temps because of 4" wide flooring above.
    Here is the the rub when the insulation estimator came to bid the job I told him to insulate right up to the plates,he questioned me on this saying other contractors wanted an airspace.I explained to him that was for staple up only different breed of cat.However either he didn't believe me or failed to convey this information to the foam crew.They installed bubble foil 3" below the plates and foamed the rest of the cavity about 7" with foam.
    I called wirsbo and they said the installation was improper but they had no data available to tell me how much hotter we would have to run the water to get the same out put.Should we have the void filled with foam or will it work at similar temps as if it had been done correctly in the first place?

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  • Seems ok to me...

    I do strap up in a similar fashion with no problems except I usually make the air gap 1.5". You would think the plates would make it that much better. All I can suggest is to try it at a low temp and increase accordingly if need be. Keep it below 135* though. I'm not fond of putting heat under hardwooda t all, but that's just me.
  • kpc_75
    kpc_75 Member Posts: 37

    that was very unprofessional of them. I did a job in Lee 1.5 years ago. and had the same situation. Uponor/wirsbo J Trac plates and 1/2 uponor tube. I worked w/ the contractor and we struck a reasonable compromise.... we put corregated cardboard right up to the plates then sprayed onto that... Hooked to a LP gas Mod Con. works great.
  • yea

    had a similar problem on my last job. I only wanted 3" of concrete and the idjits must be so used to doing 4" that they ended up at like 4.25" instead of 3". Cost me an extra 5 yds of concrete, among other things.
  • Bob Bona_4
    Bob Bona_4 Member Posts: 2,083
    response time

    will be longer, that's a given. You know it's incorrect, and if it's not corrected I would print up a disclaimer as to state performance is not guaranteed becuase of such and such. I would run the ADS for heat emission plates (not JoistTrac), split the difference maybe in supply water temp..Biggest thing is you don't exceed 82 on the floor.

    Slab sensing stat is crucial.
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Joist Trak

    Wirsbo Joist track proper installation is to have the insulation in contact with the plate. The reason for a gap with other plates or no plates is that you are trying to cause as much convection in the bay as possible to get heat transfer to the floor. With the joist trak plates you don't want convection you want heat transfer directly from the plate to the floor.

    Here is what I would do in your case today. I would do nothing. Now is not the greatest time to try and trouble shoot a water temp issue. It's not cold enough. Unfortunatly you are going to have to wait until is gets cold out. You can add in more insulation now. If there is one thing that is the life blood of radiant it's insulation, the more the better. What does your design state that you design water temp should be? I can give you a simple formula to help trouble shoot the floor. It will atleast tell you how many btus' per sqft your giving off.

    TI-TO divided by RV = Btu sqft

    TI = Water temp in tubing
    TO = Floor surface temp
    RV = R-Value of the floor

    Example 120 (supply temp) - 78 degrees (floor surface temp = 42 divded by floor R-value 1.38 (5/8 sub w/3/4 oak is about a 1.38) = 30btus sqft. Now multiple that by total sqft of the floor (100 sqft) total but's = 3000 btus.

    Now plug in your numbers and then compare that to the heat loss. Now you can make your adjustments as needed. If you don't have the heat loss get one done. Just make sure it's a radiant loss not a hydronic loss.

    Another simple formula for a radiant loss. I only use this for budget quotes /basic ideas.

    Sqft x 25btus x .65 = Radiant Loss. It's pretty accurrate based on doing comparisons from my radiant losses with my program.

    Hope this helps you out. Best regards

    PS. I never run 1/2" tubing in joist heating applications. Run 3/8". It's a whole hell of alot easier to pull, you get the same btu out-put and it's less money. The only thing 1/2" gives you is a 50' more loop length which 90 percent of the time is not needed.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • hot_rod
    hot_rod Member Posts: 21,862
    It will work fine

    I predict, maybe need a few more degrees on the supply.

    The main thing is the amount of insulation under the plates, but most importantly the insulation around the rim joist. You now have some heated air space in the bay, and need to have a good air tight high R-value between the end of the bay and outside temperatures. If you have 6-7" of spray foam in that area I suspect it will heat just fine.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • John Abbott
    John Abbott Member Posts: 356
    Thanks guys...

    I really appreciate the input. Can anyone give me a rough Idea of how much higher water temp required based on the circumstances I described.By the way I do have floor sensors installed I consider them a must with hardwood.One of my concerns is with 3"of floor joist exposed to higher temps and the exposure is significant.My math is 3" exp X 2 sides of joist X 19' the length of the joist X 36 the number of joists I come up with 342 square feet.Considering heat flows to cold and we are over an unheated crawl space I just wonder what the energy losses are going to be? Please feel free to correct my math and or thouht process.



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  • kpc_75
    kpc_75 Member Posts: 37
    7\" of foam....

    there will not be much. That is about an r-40. and NO air infiltration... Which as you know is the majority of heatloss.If they put the bubble foil junk up there there is a good chance that the foam pushed up the foil closer to the JT than 3".
  • Seems strange

    that if it's exposed to ODT they wouldn't at least give each joist a quick coat of foam. More insulation under there wouldn't be a bad thing. Unless there's some issue with adding more that I'm not aware of. When you tell the GC or whoever the story mention that. Maybe the foam company should spring for more foam to correct their error.
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