Attaching Sub-Slab Insulation

[rnpatrick]

Well after 20 years of waiting today I got my building permit for my garage!

My floor will be on 6' of virgin fill and I am going to have to put in a structural slab with grade beams. I am also wanting to put radiant heat in the slab which will require insulating under the slab.

The fill dirt will only be compacted enough to be stable enough to pour the slab. After that, it will settle and create an air gap - hence the grade beams to hold the slab.

But if I just put down XPS insulation, then poly, then the slab, as the dirt settles, the insulation will go with it and create an air pocket which will defeat the purpose. I need some way to create a mechanical connection to hold the XPS to the bottom of the slab.

Surely I am not the first person to solve this problem but my searching has not turned up any elegant solution.

Anybody else had this problem and how did you solve it?

Thanks,
RP

 

[kbs2244]

Will not the air pockets be sealed against the outside?
If so, I would think they would act as insulation.

 

[mcbane]

Just an outside the box idea: What about compacting that fill and use the grade beam $$$ to buy more XPS rather than try to create the small air void?

 

[scottydosnntkno]

XPS even taped at the seams doesn’t count as a vapor barrier/replacement for the poly right?

The simplest of ways would be after everything’s ready to pour and inspected, you walk around shoving hundreds of 16d nails into the insulation and poly. Ahead of time, use a vice just open enough to slip the pointy end in and use a short pipe to bend the nail 45-90 degrees and throw in a bucket. Then take that bucket and walk around shoving them into the foam at an angle.

Sure, some of them will get stepped on during the pour or bent over, but that’s what the bends for to help hold them in the foam and why you use so many. The heads will be imbedded in the concrete, and the other side will hold the foam up.

 

[ConCretin]

I am familiar with the concept of anticipated settlement under a structural slab but am wondering why you think the soils under the grade beams aren't going to settle just as much as the soils under the slab on a 6' fill. I think I'd be shooting for as close to 100% compaction as I could.. Disregard the question if your grade beams are 6' deep and bear on native soil.

With regard to your specific question, I'd staple the pex to the rigid insulation in the hope that the concrete would encapsulate the pex and hold the insulation tight if the base subsides.

The vapor barrier is a little more complicated. You don't want to put it between the pex and the rigid because all the staples will reduce it's effectiveness. I'd probably place it first and let it float between the base and the rigid. It will work just fine as long as its taped up enough to keep it intact.

I hope you'll post pictures as you go. Sounds like an interesting build.

 

[matt_i]

Something stable like a piece of HDPE milk jug could be used with a stainless wire under the foam. Have to prep sheets of foam first.

Tie the wire to the rebar which you will place to organize your PEX tubing.

Why are you unwilling to compact the subgrade in steps and instead rely solely on the grade beams? Is there some issue with the soils?

 

[joe--h]

The fill dirt will only be compacted enough to be stable enough to pour the slab. After that, it will settle and create an air gap - hence the grade beams to hold the slab.

Where the hell did you get an idea like this?

Joe H

 

[dfiler2]

If all the soil settles away from the bottom of your slab, the insulation will be the least of your worries.

 

[wssix99]

Lot's of great comments and questions above.

Well after 20 years of waiting today I got my building permit for my garage!

Where in Northern Virginia? I don't recall that this sort of thing would be permitted in Fairfax or Arlington.

My floor will be on 6' of virgin fill and I am going to have to put in a structural slab with grade beams./QUOTE]

Which type of slab and structure are you looking at: (it's not clear how you are using the "beams") https://theconstructor.org/practical-guide/concrete-slab-construction-cost/28153/

^ Some of these are more and less complicated with radiant...

The fill dirt will only be compacted enough to be stable enough to pour the slab.

This doesn't sound right. Compacting the fill is where you get all the strength. Grade beams sit on this fill and if you don't compact it WELL, then you could end up with a leaning structure. It's happened before:

 

[matt_i]

I can almost think of worse where the entire thing settles 1-2" and suddenly the garage floor is a shallow bathtub from which no water can escape, not with a squeegee, only with something like a shop vac.

 

[ConCretin]

Unlike a typical slab on grade, a structural slab by definition is self supporting and doesn't rely on the underlying soil for support. They are common in elevated and on grade conditions. It's not uncommon to place structural slabs on an unstable base with the expectation that it will settle or subside. As was mentioned, they can even be placed on cardboard void form if frost or expansive soils are present. They can span relatively long distances with the proper design. This by itself isn't necessarily a concern.

The question in my mind would be the grade beams that provide the support. Unless these grade beams are placed on well compacted or undisturbed soils, the whole structure is likely to settle and all bets are off. Maybe the OP can wade back in with more info.

 

[dfiler2]

Thanks for that info, I've never heard of that.

 

[spudley]

When I built my house, my lot was three feet lower than the neighboring lots. I had to match grades so I was looking at bringing in many yards of fill. Instead I fully excavated 8' below my 24' x 34' attached garage and used spancrete decking. Then I used that excavated fill elsewhere on the lot and had a large bonus room.

Maybe the OP could do the same and skip the grade beams and concern on his garage settling. Plus he'll have twice the storage space.

 

[larry4406]

Unlike a typical slab on grade, a structural slab by definition is self supporting and doesn't rely on the underlying soil for support. They are common in elevated and on grade conditions. It's not uncommon to place structural slabs on an unstable base with the expectation that it will settle or subside. As was mentioned, they can even be placed on cardboard void form if frost or expansive soils are present. They can span relatively long distances with the proper design. This by itself isn't necessarily a concern.

The question in my mind would be the grade beams that provide the support. Unless these grade beams are placed on well compacted or undisturbed soils, the whole structure is likely to settle and all bets are off. Maybe the OP can wade back in with more info.

New construction guy here - Northern VA and MD.

OP - Unfortunately I can't offer any suggestions to address your insulation retention question but I can provide some insight to structural slabs. Perhaps the information below can be helpful to you in finding a solution.

Structural slabs are done all the time in ours and others new builds. In our case, the grade beams engage with notches in the poured side walls. The side walls in turn stand on footings which bear on proper soil with proper bearing strength. This method is done routinely as the desired finished slab elevation rarely matches the virgin grade elevation (think cut and fill subdivisions where rolling hills are panned off and depressions filled...). Seven to eight feet of loose backfill inside the garage area is not uncommon where I build.

For these types of grade conditions (7-8' fill) one would loose their shirts $-wise on compacting the entire garage area in 6-inch lifts and have it continuous geotech monitored so that they would prepare and stamp a report certifying it suitably compacted for slab bearing. Several days would be consumed doing this and one has to start first with suitable soil (often in subdivisions native soils are lousy and costs to import suitable compactable fill are high). In these conditions, construction costs and time saved tip favorably towards the structural slab. When the elevation change is small (couple of feet) one could simply fill the area with stone (#57 in my area) to obtain suitable bearing (which is what I did on my barn project).

Here are links to some of the attached garage foundations on the houses I build showing the foundations, slab ledge, and structural slab grade beam details.
https://www.garagejournal.com/forum/showpost.php?p=5170926&postcount=2
https://www.garagejournal.com/forum/showpost.php?p=5190670&postcount=8
https://www.garagejournal.com/forum/showpost.php?p=5215371&postcount=10
https://www.garagejournal.com/forum/showpost.php?p=5501758&postcount=19

My barn I am building has a 24x26 slab on grade. There was about a 30" grade difference to accommodate. I poured the walls and footings in one shot and filled the interior with #57 stone then poured a typical slab on grade with the slab sitting on top of the walls at the perimeter and on a central pier. In my case, only 2 loads of stone where used so not too expensive. Using this technique for 8-0' of fill would be cost prohibitive.

 

[wssix99]

Structural slabs are done all the time in ours and others new builds. In our case, the grade beams engage with notches in the poured side walls. The side walls in turn stand on footings which bear on proper soil with proper bearing strength. This method is done routinely as the desired finished slab elevation rarely matches the virgin grade elevation (think cut and fill subdivisions where rolling hills are panned off and depressions filled...).

It looks like you are describing a monolithic slab: https://www.a-concrete.com/monolithic-slab

These are grade beams: https://en.wikipedia.org/wiki/Grade_beam

There's a big difference and it would be helpful to understand what the OP is talking about precisely in order to address the situation appropriately.

For example: I would expect a true grade beam foundation to use an independent floating slab.

 

[larry4406]

It looks like you are describing a monolithic slab: https://www.a-concrete.com/monolithic-slab

These are grade beams: https://en.wikipedia.org/wiki/Grade_beam

There's a big difference and it would be helpful to understand what the OP is talking about precisely in order to address the situation appropriately.

For example: I would expect a true grade beam foundation to use an independent floating slab.

Yes we pour the slab and grade beams in one shot single pour. Stamped approved plans label them as grade beams so that's the term we use.

 

[ConCretin]

The lines get blurred because the various parts often look similar. It depends whether the component is imparting loads directly to the ground below (spread footing) or carrying those loads back to another foundation element (beam).

A monolithic slab is best described as a slab with an integral spread footing. Instead of placing a footing, slab and wall separately, the footing is incorporated into the slab in the form of a thickened edge.

The plans larry posted show similar thickened slab sections but since these are designed to carry the load back to the foundation walls they are better described as beams. Since they are placed using the soil as a bottom form rather than falsework, they are referred to as grade beams.

The barn foundation larry posted doesn't technically utilize grade beams. There is a perimeter wall and an interior pier that bear directly on the ground. Even though he placed them separately, they perform the same function as the thickened sections of a monolithic slab.

 

[ford33]

This is interesting and I will be following along. I learned something new today. Thanks for posting.

 

[GMCGarage]

Well after 20 years of waiting today I got my building permit for my garage!

My floor will be on 6' of virgin fill and I am going to have to put in a structural slab with grade beams. I am also wanting to put radiant heat in the slab which will require insulating under the slab.

The fill dirt will only be compacted enough to be stable enough to pour the slab. After that, it will settle and create an air gap - hence the grade beams to hold the slab.

But if I just put down XPS insulation, then poly, then the slab, as the dirt settles, the insulation will go with it and create an air pocket which will defeat the purpose. I need some way to create a mechanical connection to hold the XPS to the bottom of the slab.

Surely I am not the first person to solve this problem but my searching has not turned up any elegant solution.

Anybody else had this problem and how did you solve it?

Thanks,
RP

Use thermomass connectors. https://thermomass.com/

 

[ConCretin]

Use thermomass connectors. https://thermomass.com/

With all due respect GMC, Thermomass is designed to position a rigid insulation board within a concrete wall, not a slab. I used this product in the Concrete Underground and while it worked great in it's intended application, I don't think it would be much use to the OP.

 

[wssix99]

The lines get blurred because the various parts often look similar. It depends whether the component is imparting loads directly to the ground below (spread footing) or carrying those loads back to another foundation element (beam).

A monolithic slab is best described as a slab with an integral spread footing. Instead of placing a footing, slab and wall separately, the footing is incorporated into the slab in the form of a thickened edge.

Excellent explanation.

Yes we pour the slab and grade beams in one shot single pour. Stamped approved plans label them as grade beams so that's the term we use.

As long as the inspectors think they know what they are talking about and everything is designed safely, all is probably OK.

The plans larry posted show similar thickened slab sections but since these are designed to carry the load back to the foundation walls they are better described as beams. Since they are placed using the soil as a bottom form rather than falsework, they are referred to as grade beams.

I found this interesting. It looks like any other one-way reinforced slab, but it appears that the open space was filled in? This puzzles me - why? With all that structure, I wouldn't think the fill would be needed for anything other than a form for the floor. Why not skip the fill and just have a basement or storage underneath? If the fill was necessary, why not use a less expensive floating slab?

 

[SGKent]

I'd ask the engineer who designed it how to insulate it. If you didn't use an engineer... . well...

 

[larry4406]

Excellent explanation.




As long as the inspectors think they know what they are talking about and everything is designed safely, all is probably OK.




I found this interesting. It looks like any other one-way reinforced slab, but it appears that the open space was filled in? This puzzles me - why? With all that structure, I wouldn't think the fill would be needed for anything other than a form for the floor. Why not skip the fill and just have a basement or storage underneath? If the fill was necessary, why not use a less expensive floating slab?

Yes - The loose dirt is only form work. One could invest in shoring to support the forms for later removal, use metal decking forms, use span Crete , etc all to create a room underneath. It’s only money....

 

[wssix99]

Yes - The loose dirt is only form work. One could invest in shoring to support the forms for later removal, use metal decking forms, use span Crete , etc all to create a room underneath. It’s only money....

If one doesn't need a special operator and already has equipment mobilized on-site, the money could work.

Why not go for a floating slab? Is all the effort associated with the proper compaction more than a structural slab?

With those really deep foundations, that might be a good situation for a true grade beam sitting on piers. (I expect that would save a ton of cash.)