Insulating Slab on Grade?

[VietGnome]

Hey all,

I'm in the midst of planning my 30x40 shop/garage build. Im trying to navigate the slab, and keep the price as realistic as possible.

I'm looking at a slab on grade. 6" thick, 16" edges, with 2x 12" thick reinforced pads for a future lift.

I'm trying to determine how I want to insulate it, seeing as 2" XPS R-10 ain't cheap.

My options seem to be

1. Full slab insulation/FSPF. So full under slab insulation, vertical insulation around 16" edges, and 24" wings around perimeter. Most bomb proof, but most expensive.

2. Full under slab insulation, either with or without vertical edge insulation. Cheaper than 1, but insure about frost protection.

3. No under slab insulation, but vertical edge and wing insulation. (Still FPSF?). Cheapest option, good edge protection, but unsure about how no under slab insulation effects things?

I'm in Eastern Canada, with mostly clay around me. I plan for atleast 10" compacted gravel base. Trying to mitigate price as much as possible since this pad doesn't look like its going to be cheap, but frost heave is a definite concern.

Interested to hear experiences and opinions.

 

[billconner]

You could consider foamed glass aggregate, providing both your base and insulation. Works great for FPSF. Largely a question of availability, how far you are from a plant.

 

[ConCretin]

Presumably your clay soil is impervious to water and thus reasonably frost resistant. As long as your gravel base is free draining, it too should be frost resistant. Unless you are planning to heat the structure with radiant, no insulation is an option. If the soils under the slab are uniform, just let it float

I have an un-insulated 28x34 slab with a later 14x34 addition similar to what you're planning (mine slabs are 4" thick with 12" haunches). The addition is heated full time while the original structure is heated occasionally. I have never noticed any movement at all from frost. You can't even see the crack between the original slab and the subsequent addition or anywhere else for that matter

You don't mention steel reinforcing, which I'd recommend for a free-standing monolithic slab. It will help hold everything together if you do get some movement. Check out my Guide to Floor Slabs in the link below for some additional thoughts.

 

[VietGnome]

Presumably your clay soil is impervious to water and thus reasonably frost resistant. As long as your gravel base is free draining, it too should be frost resistant. Unless you are planning to heat the structure with radiant, no insulation is an option. If the soils under the slab are uniform, just let it float

I have an un-insulated 28x34 slab with a later 14x34 addition similar to what you're planning (mine slabs are 4" thick with 12" haunches). The addition is heated full time while the original structure is heated occasionally. I have never noticed any movement at all from frost. You can't even see the crack between the original slab and the subsequent addition or anywhere else for that matter

You don't mention steel reinforcing, which I'd recommend for a free-standing monolithic slab. It will help hold everything together if you do get some movement. Check out my Guide to Floor Slabs in the link below for some additional thoughts.

Thanks. I did read your post!

I'm planning on reinforcing with either 10M or 15M rebar (or 10M center, 15M edges, and 15M reinforcing in the lift pads. Price depending).

Is the frost bad in your area? It is quite wet in my area, but about half the garage will be on the end of my existing gravel driveway, and ill fill in the surrounding areas with fill/gravel.

If I can skip insulation altogether Id be happy to save the money. I'm just putting about $50k into the garage as a whole, so I dont want to take any shortcuts.

 

[ConCretin]

We get 4'+ of frost. The key is to allow the soil to drain above and below grade to keep the moisture in the slab low and consistent under the structure. I've never checked but I wouldn't be shocked if I got a little movement under my buildings but as long as it's somewhat uniform, the building doesn't care.

 

[jmdirk]

Also eastern Ontario here.

What do your plans say? For a shop that size, fairly sure that you'll need to have an engineered slab. You'll need to provide building/reaction drawings of your building in order to have the slab engineered.

I was required to do full under slab, vertical and skirt insulation out about 3 ft. All had to verified by building inspector.

 

[VietGnome]

Also eastern Ontario here.

What do your plans say? For a shop that size, fairly sure that you'll need to have an engineered slab. You'll need to provide building/reaction drawings of your building in order to have the slab engineered.

I was required to do full under slab, vertical and skirt insulation out about 3 ft. All had to verified by building inspector.

Good point. Im in NB. I'm still in the initial planning process. I'm trying to get a good idea of how much its all going to cost to make sure I can swing it before I commit.

I just found out the other day that I need stamped plans for a pad. I reached out to a local engineer to see how much plans would cost but haven't heard back.

I also live semi rural, so I know people haven't pulled permits, or have recommended skirting rules in some sense. I don't want to risk that though.

I also wasn't sure how much flex you had when it came to plans. If you had different options, or if you just told the engineer your pad size and they have you one option take it or leave it.

EDIT: I should include im trying to DIY as much of this as possible, so I'm learning the process as I go through.

 

[jmdirk]

Good point. Im in NB. I'm still in the initial planning process. I'm trying to get a good idea of how much its all going to cost to make sure I can swing it before I commit.

I just found out the other day that I need stamped plans for a pad. I reached out to a local engineer to see how much plans would cost but haven't heard back.

I also live semi rural, so I know people haven't pulled permits, or have recommended skirting rules in some sense. I don't want to risk that though.

I also wasn't sure how much flex you had when it came to plans. If you had different options, or if you just told the engineer your pad size and they have you one option take it or leave it.

EDIT: I should include im trying to DIY as much of this as possible, so I'm learning the process as I go through.

So my shop is the same size, 30 x 40 and I installed a cold rolled c/z-channel metal building. In order to get my slab plans, I needed the engineering plans from the building manufacturer which includes what they called the reaction drawings. This basically gives all the details about what the design parameters of the building for things like, live and dead loads for the each column, snow/rain/wind loading (which is all based on the location) etc.

Now, as I understand, there's two ways to go about getting the slab plans:
1) Have the engineer come out, take test the soil bearing capacity etc and then using the drawings etc., come up with a slab design. This certainly takes more of the engineers time etc., and will likely cost you more, or;
2) You give the engineer all the drawings and they just come up with the design based on worst case scenario for your area. I think here in Ontario, you can get this from some online engineering firm for maybe $500 or so. The downside here is that you're likely going to end up with an over engineered slab, because it's designed for the worst case soil conditions.

Slab design will/should include any rebar, wire mesh, tie rod and anything else.

 

[VietGnome]

So my shop is the same size, 30 x 40 and I installed a cold rolled c/z-channel metal building. In order to get my slab plans, I needed the engineering plans from the building manufacturer which includes what they called the reaction drawings. This basically gives all the details about what the design parameters of the building for things like, live and dead loads for the each column, snow/rain/wind loading (which is all based on the location) etc.

Now, as I understand, there's two ways to go about getting the slab plans:
1) Have the engineer come out, take test the soil bearing capacity etc and then using the drawings etc., come up with a slab design. This certainly takes more of the engineers time etc., and will likely cost you more, or;
2) You give the engineer all the drawings and they just come up with the design based on worst case scenario for your area. I think here in Ontario, you can get this from some online engineering firm for maybe $500 or so. The downside here is that you're likely going to end up with an over engineered slab, because it's designed for the worst case soil conditions.

Slab design will/should include any rebar, wire mesh, tie rod and anything else.

Thanks. That's a lot of clarity.

I guess I'll see what the places I reached out to have to say. I have a detailed scale flooring plan, and electrical plan, and joist layouts but nothing beyond that.

Hopefully I can add wall and roof plans and that will suffice, since I'm not having it built.

 

[VietGnome]

So my shop is the same size, 30 x 40 and I installed a cold rolled c/z-channel metal building. In order to get my slab plans, I needed the engineering plans from the building manufacturer which includes what they called the reaction drawings. This basically gives all the details about what the design parameters of the building for things like, live and dead loads for the each column, snow/rain/wind loading (which is all based on the location) etc.

Now, as I understand, there's two ways to go about getting the slab plans:
1) Have the engineer come out, take test the soil bearing capacity etc and then using the drawings etc., come up with a slab design. This certainly takes more of the engineers time etc., and will likely cost you more, or;
2) You give the engineer all the drawings and they just come up with the design based on worst case scenario for your area. I think here in Ontario, you can get this from some online engineering firm for maybe $500 or so. The downside here is that you're likely going to end up with an over engineered slab, because it's designed for the worst case soil conditions.

Slab design will/should include any rebar, wire mesh, tie rod and anything else.

Only been able to receive one quote so far. $3600 all in.

Said they only will do plans in house, and that I need stamped structural plans before they'll do pad plans.

 

[jmdirk]

Only been able to receive one quote so far. $3600 all in.

Said they only will do plans in house, and that I need stamped structural plans before they'll do pad plans.

That's pretty common, they need to know what they're designing for and need a paper trail as there is individual liability for the engineer who stamps the drawings.

$3600 sounds steep. I paid $1500 back in '21 and it was done by the supplier of my building.

I do know that here in Ontario, you do not absolutely need stamped signed drawings for the building or foundation IF you follow all the specifications/requirements laid out in the Ontario Building Code. However, slab foundations aren't covered in the Code. So if you want a slab, you pretty much need to get everything engineered.

Are you planning on DIY stick build?

Honestly, there's a reason people in rural areas do unpermitted work, the bureaucracy makes it very challenging. I had a significant unexpected expense because the City of Ottawa demanded that I get a 'drainage plan'. Which required a couple extra grand to have a surveying company come out, measure the grade slope of the property and then proceed to give me a plan that said to slope the grade away from the building. It should be noted that I am basically on the city limits boundary and surrounded by acres of woods and fields - yet had to abide by the same requirements as someone in an urban neighborhood.

 

[VietGnome]

That's pretty common, they need to know what they're designing for and need a paper trail as there is individual liability for the engineer who stamps the drawings.

$3600 sounds steep. I paid $1500 back in '21 and it was done by the supplier of my building.

I do know that here in Ontario, you do not absolutely need stamped signed drawings for the building or foundation IF you follow all the specifications/requirements laid out in the Ontario Building Code. However, slab foundations aren't covered in the Code. So if you want a slab, you pretty much need to get everything engineered.

Are you planning on DIY stick build?

Honestly, there's a reason people in rural areas do unpermitted work, the bureaucracy makes it very challenging. I had a significant unexpected expense because the City of Ottawa demanded that I get a 'drainage plan'. Which required a couple extra grand to have a surveying company come out, measure the grade slope of the property and then proceed to give me a plan that said to slope the grade away from the building. It should be noted that I am basically on the city limits boundary and surrounded by acres of woods and fields - yet had to abide by the same requirements as someone in an urban neighborhood.

Very interesting.
The building permit application for my area seems to specifically specify that I either need the frost wall/footing details, OR an engineered slab plan.

To be completely honest, the unpermitted route sounds more enticing. We have a friend who is on the other side of town, but very similar situation (just outside of town, 2-4ish acre lots, wooded, etc) who build a 30x40 with no permits, inspections, etc. It sounds simple, and seems fairly common in the province. The only concern is I don't know the process "if" something happens. What happens when/if someone finds out years down the road? Can I not sell, get fined, etc? If I get "frozen" from building halfway through can they make me tear the pad out? etc. Lots of unknowns and I can't exactly ask the planning division lol.

I was told by the engineer firm that I needed structural plans, THEN slab plans. I was told by a friends coworker who's an engineer that the price seemed "on par", but the structural plans should not be required at all, since I'm using trusses. So that should cut the price almost in half, theoretically.

EDIT: I am planning a DIY stick build.

 

[jmdirk]

Very interesting.
The building permit application for my area seems to specifically specify that I either need the frost wall/footing details, OR an engineered slab plan.

To be completely honest, the unpermitted route sounds more enticing. We have a friend who is on the other side of town, but very similar situation (just outside of town, 2-4ish acre lots, wooded, etc) who build a 30x40 with no permits, inspections, etc. It sounds simple, and seems fairly common in the province. The only concern is I don't know the process "if" something happens. What happens when/if someone finds out years down the road? Can I not sell, get fined, etc? If I get "frozen" from building halfway through can they make me tear the pad out? etc. Lots of unknowns and I can't exactly ask the planning division lol.

I was told by the engineer firm that I needed structural plans, THEN slab plans. I was told by a friends coworker who's an engineer that the price seemed "on par", but the structural plans should not be required at all, since I'm using trusses. So that should cut the price almost in half, theoretically.

EDIT: I am planning a DIY stick build.

Yeah, that's basically the same here. The specs for the frost wall etc. would be in the building code. For example depth would vary depending on which part of Ontario you're in, but as long as you follow that, you can get a permit for a garage fairly easy. A house would be a different story.

The risks of building without a permit? Well, again, I'm not familiar with the rules in NB. I'm sure it happens a lot in rural areas here as well. I mean, you could have a busy body neighbor snitch on you. (Had that happen to a friend of mine here for a small shed he was building. He didn't even need a permit based on the size he was building. But it didn't stop one of his neighbors from calling bylaw). You could have an over-zealous building inspector drive by and notice the work going on and decide to check. They could order you to stop work, they could fine you, they could order you to dig up the slab and expose a bunch of framing to make sure it complies with local code, they could order you to tear it down. Here in Ontario when you sell a property, you're often asking to complete a property information form which asks a whole bunch of things, but specifically if any renovations have been done and if a permit was obtained.

I think you just need to find a foundation design firm that's a bit more friendly to the DIY kind of builder. I mean, you're going to build a basic structure using common techniques and engineered trusses. You should be able to get/make basic plans for your structure, they shouldn't necessarily have to be engineered.

I think a friend used these guys for a house build, their website says they are licensed for NB.

Floating Slabs

Our engineers have been specializing in floating slab design for 10 years. We focus on providing economical and practical solutions to construction. Our engineers are licensed in Ontario, British...

www.waddelleng.com

I'd also check with your municipalities building dept, some cities will actually have sample plans of what they expect you to submit for certain types of structures

 

[Innovate1]

Can't really add much advice. I'm in a somewhat warmer climate and did under slab 2" foam and inside the foundation walls (standard foundation for stick frame). But that was just before prices went crazy during covid. Also glad I didn't have to jump through all the engineering hoops. Residential here the owner can do their own plans and they only want minimal detail.

 

[VietGnome]

Yeah, that's basically the same here. The specs for the frost wall etc. would be in the building code. For example depth would vary depending on which part of Ontario you're in, but as long as you follow that, you can get a permit for a garage fairly easy. A house would be a different story.

The risks of building without a permit? Well, again, I'm not familiar with the rules in NB. I'm sure it happens a lot in rural areas here as well. I mean, you could have a busy body neighbor snitch on you. (Had that happen to a friend of mine here for a small shed he was building. He didn't even need a permit based on the size he was building. But it didn't stop one of his neighbors from calling bylaw). You could have an over-zealous building inspector drive by and notice the work going on and decide to check. They could order you to stop work, they could fine you, they could order you to dig up the slab and expose a bunch of framing to make sure it complies with local code, they could order you to tear it down. Here in Ontario when you sell a property, you're often asking to complete a property information form which asks a whole bunch of things, but specifically if any renovations have been done and if a permit was obtained.

I think you just need to find a foundation design firm that's a bit more friendly to the DIY kind of builder. I mean, you're going to build a basic structure using common techniques and engineered trusses. You should be able to get/make basic plans for your structure, they shouldn't necessarily have to be engineered.

I think a friend used these guys for a house build, their website says they are licensed for NB.

Floating Slabs

Our engineers have been specializing in floating slab design for 10 years. We focus on providing economical and practical solutions to construction. Our engineers are licensed in Ontario, British...

www.waddelleng.com

I'd also check with your municipalities building dept, some cities will actually have sample plans of what they expect you to submit for certain types of structures

Wow thanks man, this changes the game. I haven't been able to find anywhere local that doesn't advertise more towards industry/large scale work. I think that was why my quote was so high.

This place looks vetted by the province and should be good to go, and its only $300 with 24hr turnaround. You're a true godsend.

 

[NWOhioChevyGuy]

I'm in MI so similar climate.

I'm gonna do 2" under slab, 25# in areas with light loading / storage, 40# in areas where there is potential for heavier loads (16x32) area in from main doorway.
Then 18"-24" down on the perimeter / slab edge.
Pole build here, so a little different beast.

25# has gone up $10/sheet since my garage build during covid
40# is $10 more per sheet than the 25# currently

 

[VietGnome]

I'm in MI so similar climate.

I'm gonna do 2" under slab, 25# in areas with light loading / storage, 40# in areas where there is potential for heavier loads (16x32) area in from main doorway.
Then 18"-24" down on the perimeter / slab edge.
Pole build here, so a little different beast.

25# has gone up $10/sheet since my garage build during covid
40# is $10 more per sheet than the 25# currently

What do you mean by "#"?

 

[NWOhioChevyGuy]

What do you mean by "#"?

The foam has a # rating (pound)
25# for normal slabs / traffic
40# for heavier loading

 

[VietGnome]

The foam has a # rating (pound)
25# for normal slabs / traffic
40# for heavier loading

Ah I wasn't sure you meant literally lbs.

I didn't even realize the XPS I was looking at was 20psi rated. What would you classify as "heavy loading"?

Im looking at $80/sheet for 2" XPS

 

[NWOhioChevyGuy]

Ah I wasn't sure you meant literally lbs.

I didn't even realize the XPS I was looking at was 20psi rated. What would you classify as "heavy loading"?

Im looking at $80/sheet for 2" XPS

Local supplier here has
2" 25# $32.52/sheet
and
2" 40# $42.15/sheet
These are US $ values

Heavy loading in our conversation was large farm equipment &/or loaded truck / semi / dump truck

 

[billconner]

Price and environment is just why I'm going to foamed glass.

 

[crashmtb]

To be completely honest, the unpermitted route sounds more enticing. We have a friend who is on the other side of town, but very similar situation (just outside of town, 2-4ish acre lots, wooded, etc) who build a 30x40 with no permits, inspections, etc. It sounds simple, and seems fairly common in the province. The only concern is I don't know the process "if" something happens. What happens when/if someone finds out years down the road? Can I not sell, get fined, etc? If I get "frozen" from building halfway through can they make me tear the pad out? etc. Lots of unknowns and I can't exactly ask the planning division lol.

Do it the right way. Pull permits. Makes things easier later, and there's no chance of your city/municipality ordering you to remove the structure at some point.

Get some quotes from companies that do ICF foundations, they will likely have engineers they work with regularly

 

[jblnut]

To be completely honest, the unpermitted route sounds more enticing.

Some quick backstory for my area ..... Building permits are required. Electrical permits/inspections are required. Septic permits/inspections are required. That's it. Never had to get a dirtwork, plumbing, concrete, or any sort of other inspection that's been mentioned in this thread.

That being said the permits and inspections that are required are enforced. I've a neighbor that started to build an addition on his house and was given a stop order and was told to get an "after the fact permit" to continue construction. He didn't meet a few setback requirements and they did not issues him the permit. They made him tear the entire thing off. The addition he was adding was a mere 1' over the setback and at the meeting they told him if he'd have gone through the proper process he'd have had the permit without issues.

Long story short .... we cowboy some things out here but if there is a process in place for permits, inspections and such it's best to follow it. All it takes is one person to squeak and the whole things comes crumbling down. The person that complained was a neighbor that was turned down for a garage addition and felt a little slighted that the other neighbor got to build and he didn't. He called in and the county lookers came out and the mess started.

 

[VietGnome]

Some quick backstory for my area ..... Building permits are required. Electrical permits/inspections are required. Septic permits/inspections are required. That's it. Never had to get a dirtwork, plumbing, concrete, or any sort of other inspection that's been mentioned in this thread.

That being said the permits and inspections that are required are enforced. I've a neighbor that started to build an addition on his house and was given a stop order and was told to get an "after the fact permit" to continue construction. He didn't meet a few setback requirements and they did not issues him the permit. They made him tear the entire thing off. The addition he was adding was a mere 1' over the setback and at the meeting they told him if he'd have gone through the proper process he'd have had the permit without issues.

Long story short .... we cowboy some things out here but if there is a process in place for permits, inspections and such it's best to follow it. All it takes is one person to squeak and the whole things comes crumbling down. The person that complained was a neighbor that was turned down for a garage addition and felt a little slighted that the other neighbor got to build and he didn't. He called in and the county lookers came out and the mess started.

Do it the right way. Pull permits. Makes things easier later, and there's no chance of your city/municipality ordering you to remove the structure at some point.

Get some quotes from companies that do ICF foundations, they will likely have engineers they work with regularly

Yeahhh all good advice. I wasn't going to go that route more than likely since the risks were too high on something so expensive.

Now that I found a company that does stamped plans for $300, things are sitting a lot better.

 

[Briandel]

Eastern Canada + clay + heated shop = don’t cheap out on frost protection.

If you’re planning to heat it even occasionally, full under-slab insulation with vertical edge insulation is the safest play. The heat loss at the slab perimeter is where most problems start, especially in clay.

Option 3 (no under-slab insulation) works fine for unheated buildings, but once you introduce heat, you’re driving frost deeper around the edges and increasing heave risk.

The 24" wings are what make it true FPSF and help the most with frost protection. That said, a properly compacted 10" granular base and good drainage matter just as much as foam thickness.

If it were mine:

• Heated shop → full under-slab + vertical edge.
• Unheated shop → edge + wing insulation only is usually fine.

I wouldn’t skip edge insulation in your climate.

 

[VietGnome]

Eastern Canada + clay + heated shop = don’t cheap out on frost protection.

If you’re planning to heat it even occasionally, full under-slab insulation with vertical edge insulation is the safest play. The heat loss at the slab perimeter is where most problems start, especially in clay.

Option 3 (no under-slab insulation) works fine for unheated buildings, but once you introduce heat, you’re driving frost deeper around the edges and increasing heave risk.

The 24" wings are what make it true FPSF and help the most with frost protection. That said, a properly compacted 10" granular base and good drainage matter just as much as foam thickness.

If it were mine:

• Heated shop → full under-slab + vertical edge.
• Unheated shop → edge + wing insulation only is usually fine.

I wouldn’t skip edge insulation in your climate.

Thanks for the good reply. I'll need stamped so I think that will ultimately be the deciding factor.

I was also under the impression that vertical and wing insulation was what prevented frost heave, and the full under slab was more for heat retention.

Im looking into some other options but I fully plan on R-10 perimeter and wings, whether I do a cheaper option to complete the full uneerslab is undecided.

I've also recently seen adds advertising R12 Polyicocyanurate pieces for a rather decent price.

 

[billconner]

I was also under the impression that vertical and wing insulation was what prevented frost heave, and the full under slab was more for heat retention.

In heated buildings yes. In unheated frost protection also depends on full under slab.

While "vertical" insulation may make sense for slab edge, for frost protection it could all just be horizontal.

 

[VietGnome]

In heated buildings yes. In unheated frost protection also depends on full under slab.

While "vertical" insulation may make sense for slab edge, for frost protection it could all just be horizontal.

Ok. It will definitely be heated, but probably not until next spring. I'll price out some R5 or a cheaper alternative to the floor and try to decide if its worth it, or if I'll just do the R10 vertical and wings.

Adding R10 XPS to the entire floor is another $3.5k-4k in just materials.

 

[billconner]

Ok. It will definitely be heated, but probably not until next spring. I'll price out some R5 or a cheaper alternative to the floor and try to decide if its worth it, or if I'll just do the R10 vertical and wings.

Adding R10 XPS to the entire floor is another $3.5k-4k in just materials.

Which is why I'm drawn to foamed glass aggregate - both insulation and base - for an albeit smaller house addition. I found it less expensive the new foam. I havn't priced used foam. (And that was R20 with with the foamed glass aggregate versus R10 with EPS.)

 

[VietGnome]

Which is why I'm drawn to foamed glass aggregate - both insulation and base - for an albeit smaller house addition. I found it less expensive the new foam. I havn't priced used foam. (And that was R20 with with the foamed glass aggregate versus R10 with EPS.)

That's very interesting. Ill look into it but Id say its very likely that it's not available in my province.

 

[billconner]

I'm looking at Barrington VT. Think there's tarrifs on it? I find a company in Ottawa. I don't know what is considered Eastern Canada. Newfoundland? (I'd be envious anywhere from Quebec City east.)

 

[VietGnome]

I'm looking at Barrington VT. Think there's tarrifs on it? I find a company in Ottawa. I don't know what is considered Eastern Canada. Newfoundland? (I'd be envious anywhere from Quebec City east.)

I'm in New Brunswick. I would assume the volume I would need to do a 30x40 pad would mean that anything not local isn't even remotely an option?

 

[billconner]

I'm in New Brunswick. I would assume the volume I would need to do a 30x40 pad would mean that anything not local isn't even remotely an option?

Fair enough.

EPS or XPS or polyiso? Rockwool is also used except under footing.

I like the Glavel because of environmental concern, and simplicity of base and insulation in one.

I do believe whichever foam you use, designing it with a good gravel base and drainage to free air is very desirable. And cover it with a peel and stick or edpm around the exterior.

 

[VietGnome]

Fair enough.

EPS or XPS or polyiso? Rockwool is also used except under footing.

I like the Glavel because of environmental concern, and simplicity of base and insulation in one.

I do believe whichever foam you use, designing it with a good gravel base and drainage to free air is very desirable. And cover it with a peel and stick or edpm around the exterior.

Unsure. I was looking at XPS, but I've seen people selling polyiso panels on FB for cheaper.

I might mix and do something cheaper like EPS under pad of price permits, and something better on the edges.

First im hearing of rockwool for a pad. Same with the peel and stick. You would cover the entire vertical outter edge, outside of the insilation with the peel and stick? I've read it should be a cement board.

 

[billconner]

For slab on grade, is might use peel and stick on stem wall or just a liquid coating like Henry's. (I was thinking peel and stick for PWF.) Not all EPS is the same. Some less dense is more likely to absorb water.

I think the cement board - one manufactured for ground contact - makes best sense. I learned on GBA about using an EPDM (pond liner) skirt that starts at top of stem wall, vertical behind cement board, and then out several feet at a slight downward angle, at least 8 to 10 inches below grade. That will keep foam apron drier.

If I don't use foamed glass aggregate I'm thinking XPS outside stem wall and EPS under slab. (My current planning is a concrete free slab.) Prices could affect that. Don't know what I'll find used when I'm ready to buy. I do think polyiso tends to absorb water and loose R value, but maybe fine under slab.

 

[ConCretin]

XPS and EPS are tested in accordance with ASTM C578 for under slab use. I'm not sure if other materials are as well but that would be a good place to start.