How do people install slab-edge insulation?

[megachimp]

Hi all I recently completed a post-frame building on my property and I am getting ready to pour a 6" rebar-reinforced concrete slab on top of my compacted gravel pad. The energy code in my area states that I need to provide a minimum of 24" deep R-10 slab edge insulation. Since my slab will be 6" thick, that means I have to dig down 18". To my knowledge, nobody locally seems to be following that code in residential applications, but I'd like to follow it if at all possible for energy benefits.

My question is - how the heck do I actually install the 18" deep 2" wide trench adjacent to my building walls? A few ideas I've had:

1. Use a backhoe with a claw bucket and get as close as possible perpendicular to the exterior wall - it would work, but I'd take out much more material than needed, it would be hard to create spoil piles and there would be a big risk of damaging the siding.
2. Hand dig it with a pick-axe - it would take forever, but this might be the only option.
3. Use a trencher for most of the space between the poles, and hand dig right next to the poles. - It's a cool idea, but I would need to find a trencher that would let me work that close to the side of the building. The size of the trencher probably would make this not work.

 

[Jking24]

That is generally for foundation insulation and run on the exterior down below frost line some run it on the inside before backfiling. But all the pole barn stuff i have seen generally just gets under slab and perimeter slab/ pole insulation at grade

 

[megachimp]

That is generally for foundation insulation and run on the exterior down below frost line some run it on the inside before backfiling. But all the pole barn stuff i have seen generally just gets under slab and perimeter slab/ pole insulation at grade

Right. That's always what I see as well. I wonder whether it makes more sense (to simplify my work) to just rip off the top inch of the gravel, run 1" XPS board on it, and insulate the sides of the slab as you said.

The trouble is...the code says something different technically. And your best results come from slab edge insulation, not actually the insulation under the floor which is stupid expensive at current XPS pricing.

 

[Chuckster in NJ]

Are you heating or cooling the building?....... The energy code applies only to buildings with heat or cooling so that is why nobody in your neighborhood is doing it.

 

[megachimp]

Are you heating or cooling the building?....... The energy code applies only to buildings with heat or cooling so that is why nobody in your neighborhood is doing it.

Correct yes we will be heating and cooling it. You are correct the code only applies to "conditioned" spaces as defined by the code.

 

[dbldmnd]

I am in the middle of a similar planning exercise for the slab in a pole building that was constructed on my property last fall. In talking with the county building department and based on the 2015 IRC page 456 that the county provided, I need to have R10 with 4 feet depth because I am in climate zone 7/8. Because I am thinking of a radiant floor heat system, I need to go up another R5 for a heated slab which gives a total of R15. In speaking with the department supervisor we discussed what is reasonable for a pole building. He stated that the slab edge insulation requirement can be met using a combination of vertical and horizontal installations. We agreed that I can install a 12 inch section vertically attached to the skirt board and then the horizontal insulation I was planning under the full slab would meet the other 3 feet of the 4 foot total requirement.

So my plan is to install insulation 3" thick, and 12" tall between my posts around the interior perimeter of the building. Then install 3" insulation under the entire slab.

I haven't yet finished the final grading in the building, so I think I can dig along he skirt board by hand to get my 12" depth. At this point I'm just waiting for the snow to finish melting and the ground to unfreeze so I can start digging.

Good luck on your project.

 

[matt_i]

I think pickaxe/mattock is your best solution.

Digging with a mini-ex would be fast but huge trench and lots of dirt piles getting in the way. Digging with a trencher would likely be a wide offset you don't want.

Personally I would not do either of the three.

I would rake out the gravel and remove it. Then excavate deeper and replace the gravel pack then fully foam flat. Imo huge insulation gain right there in insulating the large surface area of the entire bottom of the slab vs the mini surface area of the sum of thin edges.

In a 30x40, you'd have (30+30+40+40) * 4/12 = 140/3 = 46 ft^2 on the edges vs 1200 ft^2 on the bottom of the slab. Comparing areas almost 24x more heat loss thru the bottom. If you want to hit the edges just do it vertically, on the thickness, inside the skirt board before you pour.

If you used F-250 foamular, it has a higher bearing (higher comrpession strength) than the average soil. Obviously can be dented by point loads but that's why you need the concrete to distribute concentrated loads.

 

[megachimp]

I am in the middle of a similar planning exercise for the slab in a pole building that was constructed on my property last fall. In talking with the county building department and based on the 2015 IRC page 456 that the county provided, I need to have R10 with 4 feet depth because I am in climate zone 7/8. Because I am thinking of a radiant floor heat system, I need to go up another R5 for a heated slab which gives a total of R15. In speaking with the department supervisor we discussed what is reasonable for a pole building. He stated that the slab edge insulation requirement can be met using a combination of vertical and horizontal installations. We agreed that I can install a 12 inch section vertically attached to the skirt board and then the horizontal insulation I was planning under the full slab would meet the other 3 feet of the 4 foot total requirement.

So my plan is to install insulation 3" thick, and 12" tall between my posts around the interior perimeter of the building. Then install 3" insulation under the entire slab.

I haven't yet finished the final grading in the building, so I think I can dig along he skirt board by hand to get my 12" depth. At this point I'm just waiting for the snow to finish melting and the ground to unfreeze so I can start digging.

Good luck on your project.

Wow. Yup that's correct for CZ 7/8 and really sums up how ridiculous this can be. For a pole building that puts you almost at the bottom of the poles, which means you could be compromising the integrity of the structure. And good luck ever digging up that much material without major disruption of your pad. It's good that your AHJ is being reasonable.

Have you considered installing a grade board on the inside of your poles and running the insulation up on that grade board? That would give you a nice flat surface to attach to and maintain a constant depth on the vertical XPS. It would also let you run continuous around the entire structure, so no bridging from the poles in the vertical insulation assemblies. That's what I'm planning for our pole barn and then we'll thermal scan it (have a FLIR gun) to see how it's performing. We already have all vertical assemblies spray foamed.

3" XPS under the slab is just nuts. Once again, I get that it's what the code says, but it's just so expensive to implement.

Are you planning to run the in-floor heating system continuously? We put a lot of thought into going with the in-floor approach but decided that we just don't ever use the building enough to create value in running in-floor heat all the time.

 

[96PSD]

I did 2'x8' insulation panels all around the perimeter of my 40x40 PB. I used a trench shovel and pick-ax around the whole perimeter. I used great stuff foam to fill any voids around post beams as well. Then backfilled the dirt and did my base of 6" clean 3/4" gravel and then poured a 5" slab.

 

[megachimp]

I think pickaxe/mattock is your best solution.

Digging with a mini-ex would be fast but huge trench and lots of dirt piles getting in the way. Digging with a trencher would likely be a wide offset you don't want.

Personally I would not do either of the three.

I would rake out the gravel and remove it. Then excavate deeper and replace the gravel pack then fully foam flat. Imo huge insulation gain right there in insulating the large surface area of the entire bottom of the slab vs the mini surface area of the sum of thin edges.

In a 30x40, you'd have (30+30+40+40) * 4/12 = 140/3 = 46 ft^2 on the edges vs 1200 ft^2 on the bottom of the slab. Comparing areas almost 24x more heat loss thru the bottom. If you want to hit the edges just do it vertically, on the thickness, inside the skirt board before you pour.

If you used F-250 foamular, it has a higher bearing (higher comrpession strength) than the average soil. Obviously can be dented by point loads but that's why you need the concrete to distribute concentrated loads.

Good thoughts! To be clear we have a backhoe so we'd probably be purchasing something like this or this. Nothing bad like a normal bucket, but still not ideal.



Unfortunately I put a lot of effort into building a compacted slab with fabric, etc so complete removal isn't an option. But it's a good idea

I guess I'll use the pickaxe, lol.

The vertical vs horizontal insulation thing is an interesting topic. My understanding of building thermal envelope is that vertical slab edge insulation is actually far more effective than horizontal (and cheaper!). Basically, you have a big thermal mass under the structure and you want to stop heat from migrating out of said mass. The heat is going to want to move up/out instead of down although it does move down as well to a lesser extent. That's why IECC prioritizes slab-edge insulation and doesn't even require horizontal unless you have a heated slab. Of course, all of this is nothing compared to infiltration, roof, and walls.

It has been a while since I played with this in simulation software, and it's confusing because of F-factor, but basically your best "bang-for-buck" is vertical slab edge.

 

[andyvh1959]

I'm in a similar situation. I have a 4" thick lock edge slab under my 24x28 garage, set on top of 2" foam sheet and 6mil vapor barrier. I laid a grid of PEX tubing for eventual floor heat. On top of the slab are two rows of 8" block to suit the grade. So I plan to dig around the slab to install vertical sheets of 2x8 pink foam sheets or dig deeper and install 4x8 sheets. After the sheets are in place I'll backfill to near the top of the sheets as i plan to have raised grade beds around the garage to complete the landscaping.

But I have an option to much more easily dig down to the bottom edge of the slab, and lay 2" foam sheet out two feet at a slight angle. Then vertically against the slab edge and up to near the top of the 8" block and then backfill and grade to finish. Would one versus the other be much more effective? If there would be little difference I'll go the less digging option.

 

[matt_i]

The vertical vs horizontal insulation thing is an interesting topic. My understanding of building thermal envelope is that vertical slab edge insulation is actually far more effective than horizontal (and cheaper!). Basically, you have a big thermal mass under the structure and you want to stop heat from migrating out of said mass. The heat is going to want to move up/out instead of down although it does move down as well to a lesser extent. That's why IECC prioritizes slab-edge insulation and doesn't even require horizontal unless you have a heated slab. Of course, all of this is nothing compared to infiltration, roof, and walls.

Suppose you did a simple conduction model using R-values.

R has units of (sqft*degF*hr)/BTU which isn't that useful but for U = 1/R with units of BTU/(hr*sqft*degF) some easy calcs can be done.

Suppose you are trying to keep 70deg F interior heat from the 50degF earth/ground with R value = 1 for concrete.

1 BTU/(hr*sqft*degF) * 1200sqft * 20deg-difference = 24000 BTU/hr

Now suppose you are trying to keep the heat from escaping from the slab edges with 20 below zero exterior.

1 BTU/(hr*sqft*degF) * 46sqft * 90deg-difference = 4140 BTU/hr

You'd still be losing 6x as much heat thru the bottom of the slab...

 

[ConCretin]

I'd go down far enough to get a little cover and then out.

 

[like2wheel]

I asked the same question a couple years ago & got crickets.

So I bought a pick axe & a trenching shovel, ate a bowl of wheaties, and handed them to my 20 something athlete son.
He hated it. I did a bunch & it really did ****. I had 2 year packed big crusher run over heavy clay. It wasn't bad till about 12" then it becomes hard to get the spoils out without bumping the sides. A garden hoe was very helpful; faster to just drag out the trench than scoop it out with the shovel.

Got down 18-24" on 3 sides. The other side was out of the ground a bunch, so I got about 3' on a 45°. Covered everything with dirt & called it good.

Everybody around here just seems to put 2" under the slab & up the sides.

 

[like2wheel]

https://www.minitrencher.ca/product/georipper-gr-20-powered-makita-4-stroke-engine/

 

[Bclinehand]

Different construction but.........under my five inch slab I used 3” XP’s and on the outside of my frost walls I used 4” from the top of the frost wall to 32” below grade



Sent from my iPad using The Garage Journal mobile app

 

[dbldmnd]

Wow. Yup that's correct for CZ 7/8 and really sums up how ridiculous this can be. For a pole building that puts you almost at the bottom of the poles, which means you could be compromising the integrity of the structure. And good luck ever digging up that much material without major disruption of your pad. It's good that your AHJ is being reasonable.

Have you considered installing a grade board on the inside of your poles and running the insulation up on that grade board? That would give you a nice flat surface to attach to and maintain a constant depth on the vertical XPS. It would also let you run continuous around the entire structure, so no bridging from the poles in the vertical insulation assemblies. That's what I'm planning for our pole barn and then we'll thermal scan it (have a FLIR gun) to see how it's performing. We already have all vertical assemblies spray foamed.

3" XPS under the slab is just nuts. Once again, I get that it's what the code says, but it's just so expensive to implement.

Are you planning to run the in-floor heating system continuously? We put a lot of thought into going with the in-floor approach but decided that we just don't ever use the building enough to create value in running in-floor heat all the time.

I've thought about adding a grade board on the inside, but it would start encroaching on the interior space and a flat wall to build against, not alot but with 1.5" and 3" of insulation per wall you would loose 9" in each dimension. I've got a 2x8 skirt board on the exterior and it is designed so the concrete surface sits 1 3/4" below the top of the board. Since I need to have 12" of vertical insulation, I am planning on just digging down 6" along the edge of the skirt board and installing the insulation on the inside of the board between the posts. I think that should work.

For thermal bridging, I was going to install 1 or 1.5" XPS on the edges of the posts for insulation between the posts and the slab.

As for the building heat, I'm still working on it. I do plan on installing the pex tubing for the radiant system and go from there. I've got no access to natural gas or propane, so I'm limited to electric heat. Given this, I'm concentrating on insulation as I finish the building and then work through the heat system options.

 

[megachimp]

I've thought about adding a grade board on the inside, but it would start encroaching on the interior space and a flat wall to build against, not alot but with 1.5" and 3" of insulation per wall you would loose 9" in each dimension. I've got a 2x8 skirt board on the exterior and it is designed so the concrete surface sits 1 3/4" below the top of the board. Since I need to have 12" of vertical insulation, I am planning on just digging down 6" along the edge of the skirt board and installing the insulation on the inside of the board between the posts. I think that should work.

For thermal bridging, I was going to install 1 or 1.5" XPS on the edges of the posts for insulation between the posts and the slab.

As for the building heat, I'm still working on it. I do plan on installing the pex tubing for the radiant system and go from there. I've got no access to natural gas or propane, so I'm limited to electric heat. Given this, I'm concentrating on insulation as I finish the building and then work through the heat system options.

Makes sense.

Just an idea on heating - electric tube heating is going to get super-expensive super-fast if you have to run it a lot. You might want to look into putting that money into a heat pump setup - air source or geothermal. For air source, you'd have a condensing unit outside and a unit in the building. For water source (geothermal) you might hang a water-air heat pump from the ceiling and duct out with fabric duct. Run piping down the wall to a header, and a pump (can be wall mounted) and then out the side of the building. People are under the impression that geothermal is expensive but if you are comfortable running a mini-ex and you're at this stage of construction there's a lot you can do cheaply and it's not much more expensive than in-floor radiant. For 5 tons I'm looking at $5000-7000 unit, $1000-2000 piping, and $1000-2000 equipment rental to do the job before incentives. After incentives it's a <$5000 job.

I think I'm probably going to take that approach with our barn.

 

[TurnipTruck]

I'd go down far enough to get a little cover and then out.

I scraped the gravel away from my monolithic slab with a back blade on a tractor, deep enough to expose just the slab edge. That allowed a 12” rip of R10 xps to sit vertical. Then I laid 2 to 3 feet of foam horizontally and backbladed the gravel back up to the slab. The foam was then covered with custom Z flashing.