Foam insulation on TOP of concrete slab
Last Post 30 Jan 2022 06:42 PM by sailawayrb. 11 Replies.
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IdahoanUser is Offline
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07 Jun 2021 04:41 PM
All,

I am new to this radiant heat thing, and am looking for some advice: I have a shed I built which I am converting into a woodworking workshop.
1) What kind of foam would you recommend that I lay down on top of the concrete slab (which was not insulated)? Why? [I think all I can get away with is about 1" thick foam.]
2) Vapor barrier between foam and concrete? Or between foam and subfloor?

Details in case it matters:
- The workshop is approximately 10' x 16'
- Flooring will be 3/4" white oak
- I intend on using pex/aluminum fins/subfloor

Thank you!
smartwallUser is Offline
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07 Jun 2021 11:40 PM
I would go with an under slab foam made for installing pex. Then pour a thin slab over the top. Skip the alum diffusers and the oak floor. Less work and less costly while improving your radiant system.
sailawayrbUser is Offline
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08 Jun 2021 12:46 AM
Smartwall provided good advice.

You likely need at least 2" thick EPS insulation or too much of you heat will go to ground. Plates are expensive and don't extract the heat out of the PEX as well as a concrete or gypsum slab. So a slab makes good sense.

You could also use a product like Warmboard which is nearly as good as a slab for extracting the heat out of the PEX, but it is way more expensive than plates. The one advantage it might give you though is that it wouldn't be as thick as a slab.

Skip the hardwood floor as it puts insulation above the PEX which will send heat to the ground. Tile would be good choice if it works for what will be in this shop. I have seen folks use steel sheets over Warmboard for some applications.

There are some DIY hydronic radiant floor heating design calculators on our website that you might find useful too.
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IdahoanUser is Offline
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08 Jun 2021 11:16 PM
Thank you for this feedback and advice. As much as I want to skip the hardwood floor, it serves two very important functions:
1) It protects valuable tools (a $150 chisel or $400 hand plane, for example). Landing on oak will preserve the tool whereas landing on concrete won't.
2) It protects my body from the cruel tyranny of solid concrete. Wood floors are simply kinder to the human body.
I'll look at all the options you mentioned. If I may ask a rookie question: What makes Warmboard so much more efficient than aluminum plates (I'm looking at Radiant Design & Supply)? Also, were is the best place to learn about gypsum slabs?
I appreciate the help!
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09 Jun 2021 10:54 AM
I do my heated floors with concrete, I just have them mix it with pea gravel.

You could do an inch and a half concrete over osb over the insulation....

I calculate you need 20 sqft of concrete(which is 33 bags of 80lb'ers- if you want to mix by hand).... A yard of concrete is 27 sqft....Sand topping mix is the type if you do it by hand.

Then attach to your wood floor to concrete....I found a great glue we have been using for the concrete if you are interested (Bona is the manufacture) I can get the model number if needed.....It cleans off the wood floor easy even a month later, and sticks like made to concrete...

I don't know why you would be doing heated floors for this....seems like a huge waste....they take hours to heatup and you still need A/C....Just put in a mini split and be done with it in a day and ~$2000....

Good Luck.

"Never argue with an idiot. They will only bring you down to their level and beat you with experience." George Carlins
IdahoanUser is Offline
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09 Jun 2021 02:32 PM
It looks like to do radiant correctly, I'll need to raise the floor level to the point where I'll either need a shorter door or I'll move the framing to compensate. Neither are impossible. I was hoping to avoid the step up into the shop, but physics has the final say in such matters. However, after looking at these mini-split systems - I think I may have found a wonderful solution! Keep the advice coming in ;)
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09 Jun 2021 11:04 PM
Yes, mini split would be great solution if you need AC. If you don’t need AC, cove electric radiant heat would be great too.

Thermal heat transfer is all about conducting heat from one place to another place efficiently. PEX embedded in a slab results in 100% of the PEX surface being in contact with the concrete allowing the concrete to extract the heat from the PEX at maximum efficiency. Warmboard provides more contact to the PEX surface than plates can provide, but not as much as a concrete slab provides.
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14 Aug 2021 07:16 PM
Unless your dust control is better than mine, a mini-split would require twice weekly filter cleanings, and truth be told, those filters aren't that good. And unless you want the shed heated continuously, a radiant floor will require more warm-up time and hence more forethought than a plain ole woodstove. Consider a rocket stove: very efficient and it runs on scrap wood. This one is sufficient for a small shed. https://www.amazon.com/SHTFandGO-Bullet-Gasifier-Rocket-Gravity/dp/B07KB7ZP6M/ref=sr_1_17?dchild=1&keywords=rocket+stove&qid=1628968304&sr=8-17
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14 Aug 2021 07:16 PM
Unless your dust control is better than mine, a mini-split would require twice weekly filter cleanings, and truth be told, those filters aren't that good. And unless you want the shed heated continuously, a radiant floor will require more warm-up time and hence more forethought than a plain ole woodstove. Consider a rocket stove: very efficient and it runs on scrap wood. This one is sufficient for a small shed. https://www.amazon.com/SHTFandGO-Bullet-Gasifier-Rocket-Gravity/dp/B07KB7ZP6M/ref=sr_1_17?dchild=1&keywords=rocket+stove&qid=1628968304&sr=8-17
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15 Aug 2021 02:39 PM
A central vacuum system that exhausts to outside of the house will eliminate any need to ever dust again in pretty short order. It essentially turns your house into a "clean room" environment. Unlike other vacuums that forever exhaust smaller and smaller dust particles into the house, a central vacuum system removes all dust particles from the house permanently.

For a woodworking shop where indoor dust is created all the time, I agree with Toddm that this could be problematic for a mini split. However, I would be leery about having any exposed flame heating device in a dusty indoor space as an explosion could result. This has happened during wood floor sanding in homes where the contractor forgot to turn off the gas pilot light on fireplace or water heater.
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30 Jan 2022 05:09 PM
Posted By Idahoan on 07 Jun 2021 04:41 PM
All,

I am new to this radiant heat thing, and am looking for some advice: I have a shed I built which I am converting into a woodworking workshop.
1) What kind of foam would you recommend that I lay down on top of the concrete slab (which was not insulated)? Why? [I think all I can get away with is about 1" thick foam.]
2) Vapor barrier between foam and concrete? Or between foam and subfloor?

Details in case it matters:
- The workshop is approximately 10' x 16'
- Flooring will be 3/4" white oak
- I intend on using pex/aluminum fins/subfloor

Thank you!


Hi, I lived for a few years in apartments with radiant heating of Pex pipe over the foam and with the engineered oak or bamboo wood floor on top, and it was all good. No cracking noise no nothing. I also lived on a loft apt with a concrete floor and radiant heating, although it worked fine I would not recommend it: hard on your joints and feet, no better performance compared to the wood flooring, and after a while, you get tired of looking at the industrial type of flooring. On top of it if you have an issue with your piping you will find out faster on the wood floor and you will be able to remove and fix it with little expense and no jack hammer...
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30 Jan 2022 06:42 PM
We absolutely love the stamped concrete floors in our place! We don't stand around on our floors in many places and we use area rugs where desired too. But this is a personal choice and opinion thing. Photos of our place can be found on our website.

If you don't damage the PEX when pouring your slabs, it should last for a couple centuries. A concrete slab emitter has by far the best heat transfer performance of any emitter. Plate systems are only about 30% as efficient at best and when you start putting wood above the PEX (sub floor or finished floor), performance quickly degrades even further. You can improve this lousy performance some by placing lots of insulation below the PEX, but this just adds to the crazy expense of plate systems. This is a well-documented physics thing...see John Siegenthaler's Modern Hydronic Heating if you have any doubts.

Hydronic radiant floor heating makes the most sense (performance and economics) when you are pouring concrete slabs anyhow and you don't need AC. Otherwise, think about using mini splits...
Borst Engineering & Construction LLC - Competence, Integrity and Professionalism are integral to all that we do!
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