How much insulation under a radiant floor?

[arvidj]

I am having a 24 x 36 accessory building [can't call it a garage as the city would then require that I put in a driveway] that will have radiant heat in the floating slab. For reference, it is in the Minneapolis Minnesota area.

It is obviously cheaper to put the insulation in before the pour but the question is 'how much?' 2 inches seems to be the most popular but would there be any benefit to adding more? If so, how much, and what would be the con of such an investment be other than the cost?

I'll have to put the 'what kind of floor covering' in the appropriate forum.

Thanks for the advice,
Arvid

 

[yeldogt]

2" provides good return. The returns diminish as you go thicker -- so I would not put 4" under the slab and not spray foam due to overall insulation cost.

Make sure the edges are done as well.

 

[arvidj]

2" provides good return. The returns diminish as you go thicker -- so I would not put 4" under the slab and not spray foam due to overall insulation cost.

Make sure the edges are done as well.

Thanks for the feedback and the suggestion. A contractor will pour the floor and any suggestions on what to look for before they literally etch-the-heating-in-stone is appreciated.

 

[yeldogt]

Thanks for the feedback and the suggestion. A contractor will pour the floor and any suggestions on what to look for before they literally etch-the-heating-in-stone is appreciated.

Not sure of your question?

Everybody has a different idea of what they want and need -- you only get one shot at getting the insulation and the tubing correct. What are you doing for the rest of the building insulation?

I'm in the mid atlantic -- with 2" of foam under a slab in my area the slab is room temp. Every area is different.

I also like to have more tubing / closer together vs what many on here like to do.

 

[Radix2]

I did 2" over the whole floor and a second 2" layer around the perimeter (4'). Frost walls are ICF blocks for insulation down to the frost line.

Do you have a full foundation?

Obviously in MN, insulation pays off well.

 

[arvidj]

I did 2" over the whole floor and a second 2" layer around the perimeter (4'). Frost walls are ICF blocks for insulation down to the frost line.

Do you have a full foundation?

Obviously in MN, insulation pays off well.

It is slab on grade, no foundation.

I had not thought of a 4 inch for 4 feet perimeter with a 2 inch interior. Certainly an interesting thought.

 

[Radix2]

It is slab on grade, no foundation.

I had not thought of a 4 inch for 4 feet perimeter with a 2 inch interior. Certainly an interesting thought.

Do a google search and find the specs for a " frost protected shallow foundation" Basically you add foam outside the perimeter to prevent frost from getting under your floor and heaving it. It is used in the far north in places like Sweden. Slightly different designs for heated and unheated buildings. Would protect your structure and reduce heat losses.

Something to consider

 

[yeldogt]

How are you detailing the edge of the slab and building --- you don't want that edge to be uninsulated. With typical foundation, wall and slab the insulation is along the wall and wraps down under the slab.

Google insulated slab construction. Is this a mono - pour?

A typical basement requires about an R10 -- so you can get away with the Blue or Pink 2" board for R10. If it's heated you need to move up to the higher per inch R product. With heated floors it's recommended to have up to 50% more insulation -- so with the higher product you get R14. In my area with an insulated wall and the better 2" the slab stays room temp with the heat off.

You may want to bump up the permitter thickness.

 

[arvidj]

I have gotten an estimate for the accessory building. It will have a 4" slab on grade floor in Minnesota.

Both the contractor and the plumber are pushing back on my suggestion that it have 4" of insulation over their recommendation that it only needs 2" because "heat will have no way to go but up".

My understanding of thermodynamics is limited but somehow "no way to go but up" may be accurate for convection but not for conduction. And I am not so sure about how much convection there is in a concrete slab. But I though I would ask for more experiences opinions.

Also the plumber is suggesting 2 zones for a 24' x 36' slab. Any thoughts on this? Given that it is all "one big room" individual room zone heating is not an issue. My concern is related to proper looping, spacing and tubing length. I believe he is suggesting 3/4" pex but i would also like to hear suggestions related to what the 'correct' size and zoning should be.

Thanks for thinking about this,
Arvid

 

[Ed Devinney]

You're right about convection vs conduction, but many people don't get it.

2" vs 4" below the slab seems to depend on whether you have radiant heat or not. 4"/R-20 for radiant in your area seems prudent (https://buildingscience.com/documents/insights/bsi-059-slab-happy). Pay attention to the edges - 25%+ of the heat is lost there.. We used an insulated slab and stem wall detail similar to that article in our Virginia house and garage, with good results so far.

2 vs 1 zone: that size floor will need multiple balanced loops, but I'm not seeing the need for multiple controlled zones. Our garage is one zone, the downstairs is 2 because it's a large and a small room. That's probably overkill. We moved in last winter and so don't really know for sure how this will work in practice, but so far we keep the 2 in-house stats set the same and all is well.

 

[finn]

Sounds like the plumber is confusing zones and loops terminology.

 

[arvidj]

Thanks for the confirmation of convection vs conduction. Odd that I have a clue so early in the morning.

For zones vs loops, I agree that only one zone would be needed. So it now becomes a "how many loops" would be best. Assuming he is transposing zones for loops, would two loops of ??? sized pex make sense for that size floor?

 

[howpeculiar]

I am also doing a radiant floor in MN, and just got all my PEX down. I used 2" Foamular 250. I have 2' sections to bury outside yet, they will help keep the frost from getting under the building. I have 2 zones, one for the room with the big door and the 2 post, and one for the other room with the three doors and the 4 posts.

Youtube video:

 

[howpeculiar]

I used 1/2" PEX, and no loops longer than 275'. In the bay with three doors, it is 28x38 and has 4 loops, and in the bay with the large door, it is 28x32 and I used 3 loops. Most of the places you buy the tubing from will design the layout for you once you buy the tube. I think the company that sells at Menards will also, though I used BlueRidge.

 

[garage guy1of38]

I used R-10 Amvic Pex Panels.

More expensive initially but the time savings are significant when it comes time to lay the pipe.

 

[like2wheel]

I used R-10 Amvic Pex Panels.

More expensive initially but the time savings are significant when it comes time to lay the pipe.

Isn't the tubing supposed to be close to the midpoint the slab?

 

[garage guy1of38]

Isn't the tubing supposed to be close to the midpoint the slab?

Yep. That's what these panels provide with consistency.

http://www.amvicsystem.com/ampex

 

[FANTM58]

I'm about to install mine in the attached garages 1000 sq' and the detached 2400 sq'
Shop. I'll be using 1 lb-11psi sheet ESP . We have a foam fabrication facility here.
$11.00 a sheet. I'm told this will add plenty of support for my intentions.
4" in the garages , daily driver parking and 5" in the shop , lifts ETC.
I'm gonna use 6x6 mesh on top of the foam to fasten my loops
Then rebar on 24" centers for strength.

 

[Radiantec Guy]

It's very common to go with 2" of rigid foam or an R-10. Be sure to check your local building codes because some codes now dictate that you put 3" (R-15) below your slab if you will be heating it.

Also, be sure the material you use is designed for below grade or under slab use and that the foam is of the right compressive strength for the job. A standard slab with normal use (4" slab for parking cars for example) will typically use a 25 psi whereas a thick slab for heavy equipment will want to use a 40 psi product.

Stay away from the blanket style of insulation as many of these products claim high r-value based on reflectivity studies but their true r-value is much lower. Once you pour concrete over this material, you will lose any of the reflective benefits.

Be sure to insulate the side edge of the slab. Either between the slab and your foundation or the outside of your foundation all the way down to the footing.

http://www.radiantec.com/about-radiant-heating/tubing-installation-methods/#concreteslabinstallation

 

[theoldwizard1]

If you use 2 layers of rigid foam and tape and stagger all joints, do you still need a vapor barrier ?

 

[arvidj]

Well this has been an interesting adventure.

Recap: A 24 x 36 slab-on-grade accessory building near Minneapolis, Minnesota.

The concrete team finally showed up, put down a lot of sand and then put down 4 inches of foam under the main part of the slab. They taped the seams ... suggesting that would work as a vapor barrier ... and did not put anything under the footings. I was not happy with the idea but 'they seemed to know what they were doing'.

The next day the plumber showed up to put in the PEX, saw that there was nothing under the footings, said "That will never do", and left. I was not home at the time so it was 100% his evaluation that not having the foam under the footings was not acceptable. This seemed to validate my original thoughts.

So Day 3 the concrete team came back and put down 2 inches of foam in the footings but did not tape anything between the slab and the footing insulation.

At this point I called the General Contractor and asked the he stop out so we could talk. I indicated that I really did want a poly vapor barrier under everything and would like 4 inches of foam under the footings, plus two inches vertical on the inside of the footing from the slab down to the footing plus the two inches on the outside of the footing that they had originally been planned.

He indicated that he would be here on Monday morning to get it right the second time.

Any thoughts on what I am requesting? Especially the 4 inches under the footings?

Thanks,
Arvid

 

[yeldogt]

Can't say I'm following ..... exactly. I do the foam under the slab to the walls and on the inside wall down past the foam under the slab. This way I have insulated the slab and the inside of the block/concrete wall. I don't insulate the outside of the wall.

But .....If it's a mono setup with the slab and footings together -- they have to insulate the outside of the footing and around to the underside of the slab -- otherwise the heat is going right out the side of the slab.

You need the VB -- tape on the foam will not work if you get water under the slab.

Concrete guys don't like the VB as it stops the water from leaving the slab (makes the job longer) But -- they will tell you that tape on the seams is a VB. When I said to the last concrete guy -- but, if tape is as good as a poly VB where is all the water going ?

Most people have a boiler that's oversized -- even with little insulation a radiant setup can get you a hot slab ... since it's impossible to compare one with the other ... most people don't understand they are wasting energy when they have substandard insulation /VB ... they just know it works.

Your concrete guys don't care if you spend more to heat your space -- most people don't get the concept.