Radiant Slab/Insulation Questions

[mroczka30]

I am in the process of prepping my pole building for concrete. I am going to be installing pex for radiant heat and insulation under the slab and I had a few questions. first a little bit about my building, 30x40x10 with scissor trusses located in NE PA. I was planning on 5" of concrete but I had a contractor come in and say that 4" would be plenty. 1) I'm using 2" EPS for the outside perimeter and underneath the slab, should I wrap all of the posts with 2" also? or could I use 3/4" or 1" to go around the posts? 2) where the overhead doors are, does any one have a suggestion on what they have done here? do you put insulation on the perimeter? no insulation? 3) for laying the pex out, is there any advantages/disadvantages to stapling to the insulation vs. tieing to mesh? also, any suggestions on pex size and spacing? Thanks in advance for any help provided.

 

[derkperk]

I don't think there is much advantage to wrapping the post with insulation. I have heard of people digging down a foot or 2 and placing 2" vertically to keep the heat "under" the building.
As for the doors, I didn't insulate anything other than under the slab right up to the edge. I figured a little heat radiating out for snow melt would be just fine.
I prefer to secure the tube right to the insulation with pex staples.

Sent from my XT907 using Tapatalk

 

[Rookie2]

I built my 40x60 with a floating slab inside the 6x6 posts. There is less heat loss to the outside and a lot of work insulating and sealing that dead space between the posts. 2" x24" verticle insulation also. I built a block wall under my 12' garage door with 2" insulation between the top of the block and the slab. Recess your garage door opening down about 3/4" to keep the rain out.

 

[kj_mustang]

I put foam around my posts to separate them from the slab and to add insulation.

 

[mopar440_6]

After the sub-contractor my GC brought in tried to screw me over with the roll type insulation for my 50'x60' shop, I pulled everything out and started over. First we put down a 6 mil vapor barrier on the 3/8 crushed stone up to the top of the band board on all sides and taped the seams with duct tape. Then we laid down Owens-Corning Foamular 150 2" R-10 4'x8' sheets over the entire floor tight to the band board and flush with the outer edges of the overhead door openings. Then we ripped the remaining sheets so that they would sit on top of the floor sheets and stop at the top of the band board. We also used short pieces of 2" on the sides of the posts and cut some 1" foam for the front faces of the posts because the 2" would have stuck out past my interior walls. These were also even with the top of the band board. The foam around the perimeter and posts should allow the slab to "float." Then we taped all the seams with the tin-foil HVAC tape (GC supplied it, if I had to do it over I'd just use duct tape). After that we snapped chalk lines on the foam, laid out the 1/2" PEX on 12" centers, stapled directly to the foam, and pressurized the system to 30 PSI. Finally, we put the wire mesh on top and put rebar "E"s at each overhead door opening, all on 2-1/2" rebar chairs. I chose to staple direct to the foam for a couple of reasons; 1, it was a heck of a lot faster and 2, it allowed me to sawcut control joints and install anchors in the slab without fear of damaging the PEX. This is with the understanding that the response time will be reduced with the 5-3/4" slab in my building but I plan to hold a constant temperature for the entire winter (once the heating system is finished next fall).

You can see more detail and pictures in my build thread >HERE<.

TL;DR
1: You can use 3/4" or 1" foam around the posts, I used 2" on the sides of the posts because we had extra and 1" on the front faces because the 2" would have stuck out past my interior walls. Wrapping the posts should allow the slab to "float."

2: Run insulation underneath the slab to the very edge of the overhead door opening, losses will be minimal. Rookie2 makes a good point to slope the edge of the slab away from the doors.

3: Stapling is faster and keeps the PEX at the bottom of the slab, allowing sawcuts and (short) anchors without fear of damaging the tubing but will slightly reduce response time of the heating system.

4: 1/2" PEX is probably sufficient but 5/8" will have less head loss if you plan to run antifreeze in the system. Do a layout in LoopCAD to figure out your spacing.

Suggestions: Put a vapor barrier down under the foam and download the trial version of LoopCAD to do your PEX layout.

 

[Gooch]

After the sub-contractor my GC brought in tried to screw me over with the roll type insulation for my 50'x60' shop, I pulled everything out and started over. First we put down a 6 mil vapor barrier on the 3/8 crushed stone up to the top of the band board on all sides and taped the seams with duct tape. Then we laid down Owens-Corning Foamular 150 2" R-10 4'x8' sheets over the entire floor tight to the band board and flush with the outer edges of the overhead door openings. Then we ripped the remaining sheets so that they would sit on top of the floor sheets and stop at the top of the band board. We also used short pieces of 2" on the sides of the posts and cut some 1" foam for the front faces of the posts because the 2" would have stuck out past my interior walls. These were also even with the top of the band board. The foam around the perimeter and posts should allow the slab to "float." Then we taped all the seams with the tin-foil HVAC tape (GC supplied it, if I had to do it over I'd just use duct tape). After that we snapped chalk lines on the foam, laid out the 1/2" PEX on 12" centers, stapled directly to the foam, and pressurized the system to 30 PSI. Finally, we put the wire mesh on top and put rebar "E"s at each overhead door opening, all on 2-1/2" rebar chairs. I chose to staple direct to the foam for a couple of reasons; 1, it was a heck of a lot faster and 2, it allowed me to sawcut control joints and install anchors in the slab without fear of damaging the PEX. This is with the understanding that the response time will be reduced with the 5-3/4" slab in my building but I plan to hold a constant temperature for the entire winter (once the heating system is finished next fall).

You can see more detail and pictures in my build thread >HERE<.

TL;DR
1: You can use 3/4" or 1" foam around the posts, I used 2" on the sides of the posts because we had extra and 1" on the front faces because the 2" would have stuck out past my interior walls. Wrapping the posts should allow the slab to "float."

2: Run insulation underneath the slab to the very edge of the overhead door opening, losses will be minimal. Rookie2 makes a good point to slope the edge of the slab away from the doors.

3: Stapling is faster and keeps the PEX at the bottom of the slab, allowing sawcuts and (short) anchors without fear of damaging the tubing but will slightly reduce response time of the heating system.

4: 1/2" PEX is probably sufficient but 5/8" will have less head loss if you plan to run antifreeze in the system. Do a layout in LoopCAD to figure out your spacing.

Suggestions: Put a vapor barrier down under the foam and download the trial version of LoopCAD to do your PEX layout.

You should really be using the 250 board for under-slab applications.

 

[mopar440_6]

You should really be using the 250 board for under-slab applications.

While 250+ would be ideal, 150 is what was available locally without a ridiculous shipping charge to have it trucked in. I'm a mechanical engineer by trade and I did the load calculations. Assuming the concrete does not distribute any of the force, at most the insulation would see 14 PSI with my 2 post lift fully loaded to MAX.

 

[pstnbly]

EPS or did you mean XPS. EPS is beadboard Styrofoam like a coffee cup. You need XPS.

 

[Gooch]

While 250+ would be ideal, 150 is what was available locally without a ridiculous shipping charge to have it trucked in. I'm a mechanical engineer by trade and I did the load calculations. Assuming the concrete does not distribute any of the force, at most the insulation would see 14 PSI with my 2 post lift fully loaded to MAX.

strange, My local menards stocks the 250 and it costs $1 more than 150.

 

[mopar440_6]

strange, My local menards stocks the 250 and it costs $1 more than 150.

The closest Menards is 5 hours away in OH. Lowe's and HD only had the 150 and even the local contractor supply place quoted almost double the price and a 2 week lead time for 250 board. You guys in the mid-west seem to have a better selection as far as building supplies go...

 

[rburke65]

Gooch....why does he need the 250? The 250 will withstand 3,600 pounds per sq. foot where the 150 would support 2,160 pounds per sq. ft. You must be storing 55 gallon drums of lead to necessitate the 250. I dont understand the need.

 

[Gooch]

Gooch....why does he need the 250? The 250 will withstand 3,600 pounds per sq. foot where the 150 would support 2,160 pounds per sq. ft. You must be storing 55 gallon drums of lead to necessitate the 250. I dont understand the need.

Look up the Manufacture specifications. 150 isn't intended for underslab use. It might be more than just a weight issue.

 

[mopar440_6]

Look up the Manufacture specifications. 150 isn't intended for underslab use. It might be more than just a weight issue.

 

[Gooch]

strange the sheets my supplier gave me clearly states 150 is not intended for underslab use.

 

[Crazy68Dart]

150 is fine. I used 400, way, way overkill because I got lucky and a guy on Craigslist locally had the stuff from a tear-out.

The detail at the overhead door is debatable. I think it really depends on your setup/construction style, and overall opinion. My garage is stem/footer wall, with 8" block. I put 2" XPS 400 from the top of the slab to the footer the whole way around. At the overhead doors I trimmed it to the top of the stem wall. It meets flush to the 2" XPS along the entire floor. I wanted to retain the strength of the slab over the wall, so that is one reason I didn't bring the 2" the whole way up and otherwise create a floating slab. You can do things like tie in with rebar, etc across to the wall, but my OHDs face North, and I figured 8" of exposed concrete would be nice to melt off, and otherwise isn't going to be that big of a deal.

I did however make sure anywhere the slab meets any other concrete/block/etc, to create a barrier either with the XPS or with vapor barrier. Otherwise the slab is going to want to "grab" and when it shrinks during curing has a higher likelihood of cracking.

I went a little against the grain on the vapor barrier placement. Most put it under the XPS, however I compacted/leveled the base, then 2" XPS, then vapor barrier. I think used number 8 or 10 6x6 wire mesh, and tied the PEX @ 12" OC. We then used number 6 6x6 on top of PEX before the pour. All while having the system pressurized.