# Hydronic radiant heat in ceiling



## CNM Design (May 5, 2009)

Placing hot water heat behind drywall in ceiling.
Can I use ultralite drywall or will the air in the plaster insulate the heat from getting radiated into the room underneath?

Floor joist/rafter then .5 in OSB then .75 foam board then drywall with screws through the drywall, foam board and into OSB.

If I can use Ultralite drywall, can I use 3/8th in. Ultralite drywall for ceiling?


Thanks


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## joecaption (Nov 30, 2011)

None of that makes any since to me.
Heat rises, why would try and heat from the ceiling?
This idea was tried long ago with electric heating in the ceiling and it never worked out.
You want to add 3 layers of items that all add R value, then try and defy physics and make the heat go down and through it all, HMM.


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## de-nagorg (Feb 23, 2014)

I agree with Joe, sounds like a dumb idea, 

As Scotty was prone to say, You can't change the laws of Physics.


ED


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## CNM Design (May 5, 2009)

joecaption said:


> None of that makes any since to me.
> Heat rises, why would try and heat from the ceiling?
> This idea was tried long ago with electric heating in the ceiling and it never worked out.
> You want to add 3 layers of items that all add R value, then try and defy physics and make the heat go down and through it all, HMM.


The only layer blocking would be the drywall.

https://www.doityourself.com/stry/effectiveness-of-radiant-ceiling-heat


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## CNM Design (May 5, 2009)

de-nagorg said:


> I agree with Joe, sounds like a dumb idea,
> 
> As Scotty was prone to say, You can't change the laws of Physics.
> 
> ...


The only layer blocking would be the drywall.

https://www.doityourself.com/stry/effectiveness-of-radiant-ceiling-heat


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## Missouri Bound (Apr 9, 2011)

Interesting. 
Are you using tubing with reflectors as used in floor heating?
The drywall will most likely be fine, but usually ceilings are 5/8".
Drywall, being 'sheetrock" will most likely act as a heat sink so it will take a while to warm up...and also take a while to cool down. If you can keep the heat from going up into the space above, whether attic or upper level it should work fine.
Many people think that heat automatically rises....well hot air does, but radiant heat does not. Infared heaters used in outdoor spaces and often garages are proof of that. I would be more concerned about the energy needed to get the drywall or for that matter any ceiling covering up to temperature. But once it is there you should be fine. I will assume that the drywall will be against the radiant surface and all insulating materials will be above the radiant heat to keep it from traveling up.


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## ClarenceBauer (Mar 4, 2005)

Hydronic Radiant heating was used in plaster ceilings for many years & some are still in use they work very well. When installed in the plaster ceilings the gypsum board was installed than a sanded plaster was used to cover the tubes.
As for todays radiant heat most use the cable system. In the newer systems the insulation is above the heat & will direct the heat into the Gypsum panel.
Provided the water temp. is not to high the Gypsum will hold the heat & release it over a period of time. Very high temps will break down the Gypsum product & cause failure of the Gypsum.


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## CNM Design (May 5, 2009)

Missouri Bound said:


> Interesting.
> Are you using tubing with reflectors as used in floor heating?
> The drywall will most likely be fine, but usually ceilings are 5/8".
> Drywall, being 'sheetrock" will most likely act as a heat sink so it will take a while to warm up...and also take a while to cool down. If you can keep the heat from going up into the space above, whether attic or upper level it should work fine.
> Many people think that heat automatically rises....well hot air does, but radiant heat does not. Infared heaters used in outdoor spaces and often garages are proof of that. I would be more concerned about the energy needed to get the drywall or for that matter any ceiling covering up to temperature. But once it is there you should be fine. I will assume that the drywall will be against the radiant surface and all insulating materials will be above the radiant heat to keep it from traveling up.


Reaction time for heat change is less then 10 min.
Water temp range inside pex 80-90 degrees. A water temp of 88 degrees can carry just over 55btus/sq. Most homes today require less than 30btus/sf.

https://www.pmmag.com/ext/resources/PM/2014/June/016-020-0614-Siggy.pdf


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## CNM Design (May 5, 2009)

ClarenceBauer said:


> Hydronic Radiant heating was used in plaster ceilings for many years & some are still in use they work very well. When installed in the plaster ceilings the gypsum board was installed than a sanded plaster was used to cover the tubes.
> As for todays radiant heat most use the cable system. In the newer systems the insulation is above the heat & will direct the heat into the Gypsum panel.
> Provided the water temp. is not to high the Gypsum will hold the heat & release it over a period of time. Very high temps will break down the Gypsum product & cause failure of the Gypsum.


Reaction time for heat change is less then 10 min.
Water temp range inside pex 80-90 degrees. A water temp of 88 degrees can carry just over 55btus/sq. Most homes today require less than 30btus/sf.

https://www.pmmag.com/ext/resources/PM/2014/June/016-020-0614-Siggy.pdf


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## Missouri Bound (Apr 9, 2011)

It's pretty clear you have done your research and study.
I'm not sure coming to this chatroom is going to enhance your knowledge very much. It appears you are on the right track.
Good luck with your project.:vs_cool:


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## CNM Design (May 5, 2009)

Missouri Bound said:


> It's pretty clear you have done your research and study.
> I'm not sure coming to this chatroom is going to enhance your knowledge very much. It appears you are on the right track.
> Good luck with your project.:vs_cool:


Thanks

Just can't find anything anywhere regarding the R-value of Ultralight vs regular drywall. No regular drywall at Home Depot now, all Ultralight. Looks like it may even be a special order now.


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## de-nagorg (Feb 23, 2014)

There is a fella from Maine on here, that specializes in home weatherization, he probably knows the answer to that, If he don't chime in soon, I will PM him to take a look at your post.


ED


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## Missouri Bound (Apr 9, 2011)

CNM Design said:


> Thanks
> 
> Just can't find anything anywhere regarding the R-value of Ultralight vs regular drywall.


Go to the US Gypsum website. They have an 800 number to answer questions regarding their products. I would think you wouldn't be the first to ask about R value.


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## Missouri Bound (Apr 9, 2011)

Regular 1/2" drywall has an r value of .45.....fairly insignificant.
In my humble opinion, I would think the mass of the board would be a more important factor than any insulation factor for your purposes.


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## CNM Design (May 5, 2009)

Missouri Bound said:


> Regular 1/2" drywall has an r value of .45.....fairly insignificant.
> In my humble opinion, I would think the mass of the board would be a more important factor than any insulation factor for your purposes.


Thanks

Even the 5/8ths has a response time of less then 10 min. 
Theoretically, (IMO) seeing the Ultralight is wiped and has air bubbles in it, it would function more like a foam board and insulate more then regular drywall.
I will give them a call.


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## Bud9051 (Nov 11, 2015)

Virtually no difference. Residential Energy (the bible for energy auditing) does not list the ultralight but shows Gypsum or plasterboard at 0.9 r-value per inch. As a source of radiant energy the emissivity of the ceiling paint will have far more to do with the transfer of radiant heat. A reflective surface like aluminum foil would radiate almost zero energy while a black paint would be close to 100%. The color and characteristics of the paint you choose are important. Unfortunately that would require some research but I suspect the mfgs of the radiant system would have that answer.

Short answer, not enough difference to be concerned about.

As for installing a radiant ceiling, the added heat loss through the ceiling would also be a concern. All improvements to a house add or subtract to its value. I suspect this one subtracts more than it adds.

Bud


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## de-nagorg (Feb 23, 2014)

Bud9051 said:


> Virtually no difference. Residential Energy (the bible for energy auditing) does not list the ultralight but shows Gypsum or plasterboard at 0.9 r-value per inch. As a source of radiant energy the emissivity of the ceiling paint will have far more to do with the transfer of radiant heat. A reflective surface like aluminum foil would radiate almost zero energy while a black paint would be close to 100%. The color and characteristics of the paint you choose are important. Unfortunately that would require some research but I suspect the mfgs of the radiant system would have that answer.
> 
> Short answer, not enough difference to be concerned about.
> 
> ...


There you have it, from he who knows more than the rest of us.

About weatherization anyway.:devil3:


ED


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## Bud9051 (Nov 11, 2015)

Thanks ED, I think, a very limited specialty.

Bud


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## CNM Design (May 5, 2009)

Bud9051 said:


> Virtually no difference. Residential Energy (the bible for energy auditing) does not list the ultralight but shows Gypsum or plasterboard at 0.9 r-value per inch. As a source of radiant energy the emissivity of the ceiling paint will have far more to do with the transfer of radiant heat. A reflective surface like aluminum foil would radiate almost zero energy while a black paint would be close to 100%. The color and characteristics of the paint you choose are important. Unfortunately that would require some research but I suspect the mfgs of the radiant system would have that answer.
> 
> Short answer, not enough difference to be concerned about.
> 
> ...


Thanks for your input.

Looking more at radiant heat coming into the room, not leaving.

https://www.pmmag.com/ext/resources/...0614-Siggy.pdf


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## Bud9051 (Nov 11, 2015)

Radiant heat transfer is line of sight and all surfaces emit it. So everything in the room below the radiant ceiling is radiating towards the ceiling while the ceiling is radiating back down. It is an exchange process that is constantly trying to equalize the temperature of the two surfaces. The greater the temperature difference in conjunction with the surface properties determines the net transfer of energy.

The r-value of the drywall or other materials below the radiant tubes determines how fast the heat loss can be replenished. 

Bud


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## ClarenceBauer (Mar 4, 2005)

CNM Design said:


> Thanks for your input.
> 
> Looking more at radiant heat coming into the room, not leaving.
> 
> https://www.pmmag.com/ext/resources/...0614-Siggy.pdf


Calling USG for the info on Ultralite is your best bet.
I would think that being Ultralite is NON_Fire rated it would be a better product for radiant heat than regular gypsum board.


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## CNM Design (May 5, 2009)

ClarenceBauer said:


> Calling USG for the info on Ultralite is your best bet.
> I would think that being Ultralite is NON_Fire rated it would be a better product for radiant heat than regular gypsum board.


Thanks

Gave them a call, Tec looked in three different places and could not come up with an answer. Waiting on an email from their Product manager.


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## A Squared (Dec 19, 2005)

Missouri Bound said:


> Many people think that heat automatically rises....well hot air does, but radiant heat does not.


That's true, as far as it goes, but radiant heating elements aren't a conduction free zone, and there *will* be conduction of to surrounding air, and convection *will* tend carry heat away from the living space to be heated. How much, and how well that can be inhibited of course depends on a lot of variables, but it's not like consideration of convection is moot because it's a radiant system.


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## CNM Design (May 5, 2009)

A Squared said:


> That's true, as far as it goes, but radiant heating elements aren't a conduction free zone, and there *will* be conduction of to surrounding air, and convection *will* tend carry heat away from the living space to be heated. How much, and how well that can be inhibited of course depends on a lot of variables, but it's not like consideration of convection is moot because it's a radiant system.


Yes, that is true as well. However in this system 99% of the convection will come from items (floor, chairs, tables) heated by the radiant ceiling system not the radiant heat emitter itself. It is not a fin or CI baseboard radiator system.

Any heat heading up from the basement ceiling will take the chill off the tile floor above. Any heat heading up from the first floor ceiling into the attic must pass through .75 foam board .50 OSB and 13 in of insulation.

Thanks

https://www.pmmag.com/ext/resources/PM/2014/June/016-020-0614-Siggy.pdf


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## ClarenceBauer (Mar 4, 2005)

CNM did you see the post in this section this date about the bath room ceiling heat.
Read the post on Wall to Ceiling Joint.


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## Missouri Bound (Apr 9, 2011)

A Squared said:


> That's true, as far as it goes, but radiant heating elements aren't a conduction free zone.


Well there are instances where that may be true.
But when all the heat is coming from the ceiling there won't be any convection, just stratification. But radiant heat is not affected by stratification.
Radiant heat warms objects, not the air. Now granted there will warming of the air due to the heated objects in that space but that's not convection. It is still radiation and stratification.


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## CNM Design (May 5, 2009)

ClarenceBauer said:


> CNM did you see the post in this section this date about the bath room ceiling heat.
> Read the post on Wall to Ceiling Joint.


Cut strips of 1/4 in. wire mesh just a little wider (1/4in) then the gap so it fits in tight, put a nail or screw in the starter end (If you can get to it easy use roofing nails) stretch tight, nail or screw the other end to the last available stud or ledger then nail each stud. Do this all the way around. It's ok if it over laps, then fill it in.

Suggest using sheetrock 45 as your filler. This stuff will shrink very little if at all, compared to most stuff at the Big Box Stores and is designed for large fills. I think it comes in 20-45 and 90 min. set up times using cold water. The hotter the water the faster it will set up, not allowing the time stated on the bag.

Start with mixing 1 1/2 cups of powder to 1/2 cup of water. This will give you a chance to see what you are working with and how fast it sets up.
After that, just mix 3 parts powder to one part water. Add water a little at a time and stir.

lots of other ways, this is just how I would do it.


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## CNM Design (May 5, 2009)

ClarenceBauer said:


> CNM did you see the post in this section this date about the bath room ceiling heat.
> Read the post on Wall to Ceiling Joint.


OOPs, posted on wrong site. :surprise:


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## CNM Design (May 5, 2009)

Bud9051 said:


> Virtually no difference. Residential Energy (the bible for energy auditing) does not list the ultralight but shows Gypsum or plasterboard at 0.9 r-value per inch. As a source of radiant energy the emissivity of the ceiling paint will have far more to do with the transfer of radiant heat. A reflective surface like aluminum foil would radiate almost zero energy while a black paint would be close to 100%. The color and characteristics of the paint you choose are important. Unfortunately that would require some research but I suspect the mfgs of the radiant system would have that answer.
> 
> Short answer, not enough difference to be concerned about.
> 
> ...


Just got this answer from USG

Subject: RE: 1/2 ultralight r value/case# 00208465 [ ref:_00Dd0hbhG._5000W1bPS4Q:ref ]

0.45 °F x ft2 x h/Btu (0.08 K x m2/W)


Kenneth W. Wagner
Product Manager, Wallboard

United States Gypsum Company


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## Missouri Bound (Apr 9, 2011)

lain:
So what the hell does that mean?


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## Bud9051 (Nov 11, 2015)

LOL, typical engineer who doesn't want to answer the question.

BTW what was your question to him?

Bud


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## CNM Design (May 5, 2009)

Bud9051 said:


> LOL, typical engineer who doesn't want to answer the question.
> 
> BTW what was your question to him?
> 
> Bud


Q: What is the R-value of 1/2in Ultralight drywall?

A: 0.45 °F x ft2 x h/Btu (0.08 K x m2/W)

No Engineers on site?


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## de-nagorg (Feb 23, 2014)

CNM Design said:


> Q: What is the R-value of 1/2in Ultralight drywall?
> 
> A: 0.45 °F x ft2 x h/Btu (0.08 K x m2/W)
> 
> No Engineers on site?


It's an algebraic formula, where you plug in the variables.

0.45 of the degrees F that you want to achieve X the sq. footage X h / Btu rating of the drywall?

or .08 kelvin X meters square / W.

Still need to know what some of the things are though.

And yes there is at least one engineer that visits, maybe they will try to help in a little while.


ED


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## Missouri Bound (Apr 9, 2011)

I have an answer for the OP:
"It doesn't matter".
I suppose you could do your own research with a piece of each and some heat source. I doubt that the difference will be noticeable.


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## CNM Design (May 5, 2009)

Sorry, this Thread has gone longer than needed.

Just wanted to know if the new Ultralight drywall with air bubbles in it, had a higher R-value then the old heavy drywall with no air bubbles.

My guess is yes, but not enough to make a difference in this application.

Admin, you can close this Thread if you wish.

Thanks to all that replied.


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## Missouri Bound (Apr 9, 2011)

CNM Design said:


> Admin, you can close this Thread if you wish.
> 
> Thanks to all that replied.


These threads never die.....in a year or two someone will add a post thinking it is a current thread......and so what? Information never gets old so let it ride.


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## Edwinder (Sep 30, 2021)

Bud9051 said:


> Radiant heat transfer is line of sight and all surfaces emit it. So everything in the room below the radiant ceiling is radiating towards the ceiling while the ceiling is radiating back down. It is an exchange process that is constantly trying to equalize the temperature of the two surfaces. The greater the temperature difference in conjunction with the surface properties determines the net transfer of energy.
> 
> The r-value of the drywall or other materials below the radiant tubes determines how fast the heat loss can be replenished.
> 
> Bud


Does anyone know if there's a recommended paint on such ceilings?


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## Bud9051 (Nov 11, 2015)

The color and characteristics of the paint do affect its emissivity, ability to radiate heat. Extreme example would be a foil covering would radiate almost zero heat. Black has good emissivity but probably not a high choice for ceiling paint. 

Not a pro on the best choice but should be something you can search. Radiant heat mfgs should have guidance.

Bud


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## GCTony (Jul 21, 2021)

My uncle, a builder built his own home,a brick rancher back in the mid 60's and he had electric heat in his plaster ceilings. I don't know anything about the system or how it worked but they had the most comfortable home in those cold PA winters.


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