How much insulation do I really need?

[VietGnome]

Hey everyone,

Finally got plans stamped and am getting quotes for my pad.

Pad is a 30x40 floating in Atlantic Canada. Supporting a garage that won't be heated it's first winter, but will get a heatpump/insulation next year.

Plans call for full underslab insulation, vertical insulation, and wings extending 6'. However they specify you don't NEED insulation, it's just then on you if it heaves or cracks.

The quotes I'm getting are definitely putting me over budget, so I'm trying to cut where I can.

I'm strongly thinking of cutting the under slab insulation as that saves me nearly $2500, and I dont have radiant heat, and am not trying to keep the garage warm in the winter, more like 15C max.

For the wings Im obviously keeping vertical, but I don't know how much to cut down the horizontal wings? I was originally planning on 24", but don't want to risk any heave, especially in the first winter.

Im thinking just vertical and 24" wings should be sufficient, especially since the contractor quoted me for 6" of crushed rock (0-3/4, as per the plans), ontop of 1' of pit run. So realistically an 18" base.

Looking for any input. Id like to cut some costs and savings here, but don't want to underdo it and risk heave.

Thank!

 

[Overboost44]

Not experienced with Canada, but they say most of the heat is lost on the edges. I think I have seen 80%. I find that hard to believe but I would be sure to do the verticals and wings and maybe just run a 4'x 8' sheet around the perimeter and don't do the middle 22x32 that is leftover. Would that fit your budget?
This one has radiant in it, but take a look.
upgrading-a-shops-heating-system

 

[cgrutt]

I would advise keeping the underslab insulation if at all possible. That's one area you must do before its poured and $2500 probably isn't that much in overall scheme of build.

 

[JohnX14]

Insulation isn't going to prevent frost heaves. Footings below frost depth and well draining gravel below the slab will do that. vertical insulation isn't going to keep the cold temp from migrating under the slab over the course of the winter. And 2" insulation under the slab isn't going to keep frost from heaving both the insulation and the slab. I'd skip it all if it is a choice. The insulation is for energy efficiency, not frost damage prevention.

 

[The Cobbler]

the saying goes something like this... " pay for insulation once, pay for heat every time it comes on "

insulation will make it less expensive to heat , floor insulation will keep the floor somewhat warmer & as a result less prone to condensation. It probably won't stop heaving due to frost ( maybe marginally)

 

[VietGnome]

I would advise keeping the underslab insulation if at all possible. That's one area you must do before its poured and $2500 probably isn't that much in overall scheme of build.

The problem Im facing is that while 2500 isn't a huge amount, it's still noticeable.

But its all of those "not much's" together. Might as well spent 2.5k more on insulation, might as well spend another 1.5k for a metal roof, another $750 for raised heel trusses, another $1.2k for 240V, etc.

Snip

The vertical and wing insulation prevents heave though right? Thats like the entire point of it? I'm torn on the underslab and if it's in the budget, but im also interested in if the 6' if even work it on the flanks. I've never heard of it that big.

Snip

Isn't the entire purpose of vertical and the wings to stop cold from migrating under slab, and hold underneath the geothermal heat? Its like the definition of an FPSF slab?

 

[cgrutt]

Understood. My point being you only have one shot at insulating slab before you pour all the others you can do in future. There are other potential benefits in addition to R factor as well. Acts as isolation barrier resisting movements and preventing crack formation. Vapor barrier mitigates moisture prevents mold. Potentially helps against Radon intrusion. And others.

 

[VietGnome]

Understood. My point being you only have one shot at insulating slab before you pour all the others you can do in future. There are other potential benefits in addition to R factor as well. Acts as isolation barrier resisting movements and preventing crack formation. Vapor barrier mitigates moisture prevents mold. Potentially helps against Radon intrusion. And others.

Yeah I get what you're saying. I just feel it's the same way for everything. And I'm mostly unsure how much of a benefit I'll see. I think I'm going to rework the numbers for 2' wings and full underslab and see where I'm at.

Edit: I know theoretically the others can be changed, but nobody is going to change their trusses, and it obviously seems wasteful to do shingles only to change them to steel before their life is up. But maybe ill just do shingles to save a few.

 

[ConCretin]

Unless you are heating with in-slab radiant, you likely won't see a return on your insulation investment in your lifetime. Not only is heat loss in a conventionally heated space minimal down through the slab, one thing people fail to consider is you are also blocking the natural heat from the ground that would otherwise find its way into the structure. I have an unheated garage with no under slab insulation. The interior of the structure stays above freezing for a significant period after outside temps drop below freezing.

With regard to frost, if you build on uniform and reasonably free draining soils, there won't be enough moisture to cause frost issues. A well constructed mono-slab will float on whatever minimal movement there is. I have several un-insulated mono-slabs myself and have constructed hundreds more and have never seen an issue.

 

[VietGnome]

Unless you are heating with in-slab radiant, you likely won't see a return on your insulation investment in your lifetime. Not only is heat loss in a conventionally heated space minimal down through the slab, one thing people fail to consider is you are also blocking the natural heat from the ground that would otherwise find its way into the structure. I have an unheated garage with no under slab insulation. The interior of the structure stays above freezing for a significant period after outside temps drop below freezing.

With regard to frost, if you build on uniform and reasonably free draining soils, there won't be enough moisture to cause frost issues. A well constructed mono-slab will float on whatever minimal movement there is. I have several un-insulated mono-slabs myself and have constructed hundreds more and have never seen an issue.

Thank you, this is the answer I'm looking for.

I live in quite a wet area, and soil is clay. Like I said the contractor is planning on 1' of pit run, then the 6" granular A, so it should be a good base.

I'll likely keep the edge and do 2' wings since that's only like $1100 give or take in insulation, and it gives me some protection and good piece of mind.

I wasn't sure how much I would actually save in heating costs by doing R10 under the slab.

Thanks!

 

[JohnX14]

Isn't the entire purpose of vertical and the wings to stop cold from migrating under slab, and hold underneath the geothermal heat? Its like the definition of an FPSF slab?

I guess I misread yuor OP. I thought you were constructing a foundation to a depth below the frost level. And was considering insulating the exterior foundation walls along with the slab. If you're doing a FPSF, you have to insulate as designed.

 

[VietGnome]

I guess I misread yuor OP. I thought you were constructing a foundation to a depth below the frost level. And was considering insulating the exterior foundation walls along with the slab. If you're doing a FPSF, you have to insulate as designed.

My plans show insulation but specify if insulation is not used then the client/contractor accepts the potential of cracking/heaving.

 

[rust in the eye]

2" rigid foam will give you R10 and 1200 sf of it will cost around $1900(shudders) where I'm at. Sounds like a $600 savings if you lay this stuff down yourself. Insulating the slab from the foundation perimeter is also a good idea.

 

[VietGnome]

2" rigid foam will give you R10 and 1200 sf of it will cost around $1900(shudders) where I'm at. Sounds like a $600 savings if you lay this stuff down yourself. Insulating the slab from the foundation perimeter is also a good idea.

Converted to USD, taxes in, it comes to just under $1800USD

 

[rust in the eye]

Converted to USD, taxes in, it comes to just under $1800USD

I don't know about you but I'd be happy to put down that foam board to save 6 or $700 bucks. So roughly $.65/sq ft to DIY insulate. An old bread knife works very well for cutting.
Your saving could go to running pex tube to allow for radiant later.

 

[VietGnome]

I don't know about you but I'd be happy to put down that foam board to save 6 or $700 bucks. So roughly $.65/sq ft to DIY insulate. An old bread knife works very well for cutting.
Your saving could go to running pex tube to allow for radiant later.

I dont think you're understanding. The 2500 has nothing to do with labor. Its 2500CAD, Which is 1800USD.

Im looking at 1800USD complete for concrete labor.

Laying insulation myself will definitely not save me $600.

 

[landstuhltaylor]

Depends on how warm you will heat it and what the ground temp is. I only want to keep my shop at 45F in the winter and the ground temp doesn't get below 30F for all but a few months of the year, so I didn't bother. I probably should have done the vertical edges of the slab but I'm not losing sleep over it. Canada should lean to more insulation but those are the factors I would be looking at.

 

[dscheidt]

one thing people fail to consider is you are also blocking the natural heat from the ground that would otherwise find its way into the structure. I have an unheated garage with no under slab insulation.

No, you're not. Heat flows from warmer objects to cooler ones, and if you're heating at all, your structure will be warmer than the ground, so the net flow is into the ground, not out.

 

[bdbecker]

The problem Im facing is that while 2500 isn't a huge amount, it's still noticeable.

But its all of those "not much's" together. Might as well spent 2.5k more on insulation, might as well spend another 1.5k for a metal roof, another $750 for raised heel trusses, another $1.2k for 240V, etc.

This is called 'scope creep'. While there is certainly merit to the 'buy once, cry once' mentality, the reality is that sometimes what you want and what you can afford are not the same thing. The solution is to lay the groundwork now by focusing your dollars on the areas where it truly matters. Don't compromise on the trusses or foundation, instead, try to find expenses that can be deferred to a later time when money might not be as tight.

...and it obviously seems wasteful to do shingles only to change them to steel before their life is up. But maybe ill just do shingles to save a few...

Why would you replace a perfectly good roof before the life is up? Get your money out of the shingles over the next 15-20 years, and then replace with metal when the time comes. If 240v power is a 'nice to have' not a 'need to have' right now, have a couple of spare conduits installed in the trench now so you can have the wire pulled in the future. I would assume there is an overhead door involved with this build - do you need a powered opener, or can you get by with manually opening it for now?

If you sit down and take a look at where your money is going on this project, I bet you can find other ways to defer costs. Sure, it might take a few more years before you are truly done, but at least you didn't have to cut corners in the process.

 

[ConCretin]

No, you're not. Heat flows from warmer objects to cooler ones, and if you're heating at all, your structure will be warmer than the ground, so the net flow is into the ground, not out.

I was obviously referring to unheated structures. Under slab insulation will provide less temperature differential than an uninsulated slab but how much savings in actual heating costs will result. I believe the savings are minimal and unlikely to exceed the initial cost for many years.

 

[Kaizen]

I cut insulation out of my budget and regret it every winter. Remember heat transfers........or is it cold transfers?....either way no insulation and that slab is like a huge block of ice. I just had some water get in to mine as we had a big melt here and although air temp was 40s for several days I had ice on my slab. I did cut it and pex as I doubted I would want to keep it heated in winter due to the cost but I really wish I had kept it. Even if i never used the pex it was an option if i wanted.

 

[TurnipTruck]

During design and construction of my 40x60 shop, the local natural gas provider was yet again threatening us with another winter of potential gas shortages. With that in mind, I went with just wing foam instead of pex with underslab insulation in case we ever had to limit gas usage and have to let the shop go cold. The wing foam would help prevent frost heave but the constant 42*F ground temperature might keep the uninsulated floor from freezing too hard. Kind of apocalyptic I know, but that was the winter of discontent when I was designing.


I was also kind of on a cost cutting bent when I fired the ******* who first did my dirtwork had gone $8k over our agreed-upon stopping point and I had to bring in a different guy for ANOTHER $8k to pull the 4’ of gravel out and actually compact it in foot lifts which pushed the concrete back a year.

But all that extra money and delay was worth it when the well-drained gravel doesn’t freeze under the shop, while the dirt/clay/gravel mix ten feet away from the shop heaves two inches plus every Fall and Spring, as seen in front of my pup:

 

[VietGnome]

This is called 'scope creep'. While there is certainly merit to the 'buy once, cry once' mentality, the reality is that sometimes what you want and what you can afford are not the same thing. The solution is to lay the groundwork now by focusing your dollars on the areas where it truly matters. Don't compromise on the trusses or foundation, instead, try to find expenses that can be deferred to a later time when money might not be as tight.



Why would you replace a perfectly good roof before the life is up? Get your money out of the shingles over the next 15-20 years, and then replace with metal when the time comes. If 240v power is a 'nice to have' not a 'need to have' right now, have a couple of spare conduits installed in the trench now so you can have the wire pulled in the future. I would assume there is an overhead door involved with this build - do you need a powered opener, or can you get by with manually opening it for now?

If you sit down and take a look at where your money is going on this project, I bet you can find other ways to defer costs. Sure, it might take a few more years before you are truly done, but at least you didn't have to cut corners in the process.

Yeah fair, I was just rambling I guess.

Well the price between metal and shingles seems to be less than $1k, so its hard to justify shingles.

3 doors, 2 will need motors and I can't really skip on that.

I'm pretty sure the inital price I got for vinyl from hone depot was disgustingly high. I found a store that prices it so it allegedly should be half to less than half. That and cutting underslab puts me $2k under my self imposed budget.

Im not heating or insulating right away most likely, so I might refrain on running 240V everywhere until later on. Or just run conduit into the ceiling, finish when I can, and I'll just add 240V circuits as required.

Even if in the end underslab puts me at 61k instead of 60k ill probably just send it.

 

[472scout]

If you'll be using the shop for 20+ years you'll regret this.

I sure wish my slab was insulated

 

[spudley]

I built a new woodworking shop in NE Wisconsin in 2018 and was planning 6-8 weeks of wintering in Florida so I skipped the under slab insulation and pex. Plans changed and now I'm on a cold floor. I did put heavy plastic under the concrete so the floor doesn't sweat like the old garage but its cold until August. As ConCretin said, with heat off the space never gets below freezing, so there is some gain and I did install a Modine type 75K hanging heater (free) that gets the temp up pretty quick. That said, I wish I'd have floor heat as that fan forced heat system blows a lot of dust around.

 

[Bert_]

No, you're not. Heat flows from warmer objects to cooler ones, and if you're heating at all, your structure will be warmer than the ground, so the net flow is into the ground, not out.

If you insulate the footings the ground under the building will stay mid 40's. That's barely below the 50* that most guys around here keep the shop at.

If you turn your heat off the ground will help keep the building above freezing

 

[jblnut]

Insulate it. No more to be said. You will wish you had later.

I debated on how much insulation to put under my slab and I decided to go 2' out, 3' down, 4" on slab edge itself, 4" for the outside 8' and 2" for the rest of the interior. I also put 1-1/2" blue foamboard between all exterior perlins and on the inside of all the posts. It's not quite spray foam tight but was a whole lot less expensive. On a -25f windy day it'll use around 5gal of LP to keep it 62f. At 54x72x18 with an office kept at 68f I don't think that's just too shabby.

You'll never regret more insulation. You can add other things later or wait to build until you can afford it or borrow a few more dollars or stretch the loan out or or or or ..... there are likely better options than cutting insulation.

"You buy insulation once, you buy heat every day"

 

[Sumboodie]

Insulation isn't going to prevent frost heaves. Footings below frost depth and well draining gravel below the slab will do that. vertical insulation isn't going to keep the cold temp from migrating under the slab over the course of the winter. And 2" insulation under the slab isn't going to keep frost from heaving both the insulation and the slab. I'd skip it all if it is a choice. The insulation is for energy efficiency, not frost damage prevention.

Footing below frost would be big money!

 

[Sumboodie]

Yeah fair, I was just rambling I guess.

Well the price between metal and shingles seems to be less than $1k, so its hard to justify shingles.

3 doors, 2 will need motors and I can't really skip on that.

I'm pretty sure the inital price I got for vinyl from hone depot was disgustingly high. I found a store that prices it so it allegedly should be half to less than half. That and cutting underslab puts me $2k under my self imposed budget.

Im not heating or insulating right away most likely, so I might refrain on running 240V everywhere until later on. Or just run conduit into the ceiling, finish when I can, and I'll just add 240V circuits as required.

Even if in the end underslab puts me at 61k instead of 60k ill probably just send it.

30x40 with slab for 60k?

How tall?

I have 30x42x14 and to rebuild my insurance guy says it'd be around 175k

 

[American Locomotive]

I'm sorry, but this seems crazy to me. This isn't even about heat - it's about the basic structure of your building.

Your engineer/architect is basically saying "We will guarantee if you build it like this, it will not heave and crack. You CAN build in a different way, but if it fail, we will not help you". The insulation below the slab works both ways: If the building is unheated and is below freezing temperatures, it stops the ground heat from leaking out through the slab and freezing. If the building is heated, it keeps your slab warmer and makes the shop marginally easier to heat.

The problem Im facing is that while 2500 isn't a huge amount, it's still noticeable.

But its all of those "not much's" together. Might as well spent 2.5k more on insulation, might as well spend another 1.5k for a metal roof, another $750 for raised heel trusses, another $1.2k for 240V, etc.

That is a fallacy. Your plans call for the insulation. Your plans don't call for any of the other stuff. A roof, wiring, electrical can all be added later. You can't add insulation under your slab, later.

I would feel pretty stupid if the slab in my $60,000 or whatever building heaved and cracked because I cheaped out on 2k worth of insulation.

 

[Dig Doug]

Living in Southern California and having a Desert Lake home in Arizona where it’s gets HOT

As a Builder, I find it interesting to read about the building challenges that you COLDER climate guys face

I read -25 like OMG that’s COLD !

and
Im sure 118 -122 temps during summer are just as shocking to you

@VietGnome

thank you for sharing

 

[Bert_]

Footing below frost would be big money!

Trencher makes a cheap footing

 

[shaune]

10 years ago I had my garage added on at the back and there was no insulation at all. i Heat mine almost the entire winter (Canadian winter at that) and while the floor isn’t warm to sit on I have not had any issues with cracking or heaving. I like the idea of adding wing insulation but that’s too late now unless I want to do mom work.

 

[VietGnome]

Snip

30x40x12. Except for the slab the entire build will be DIY, besides maybe door install.

Insulate it. No more to be said.

I mean fair enough. Reason I was debating scrapping the floor was it seems like wing and edge do lots of the work. But I'm leaning towards cutting the wings down to 2-3' and doing full underslab for the comfort.

It just SEEMS that the difference isn't huge. Especially if I'm heating with a heatpump.

Snip

The thing is, from my understanding underslab doesn't seem to prevent frost heave. It's for comfort, and like you said, marginally improve heating costs. The edge/wing is what prevents heave, and I'm not getting rid of that. So I can't consider a plausible scenario where cutting underslab would make the slab heave and crack. If anything the base is more likely the result, and my planned base will be exceptionally better than what plans call for.

I also really don't know how realistic it would be to get the firm to cover it if it DID heave. I feel it would require the demo of the pad, inspection underneath, soil testing, full inspection of the pad specs, etc. A complete nightmare.

Snip

Those summer temps would cook us alive. We get pretty big swings in my neck of the woods. Sometimes as low as -30C (-22F) before windchill in the winter. Rarely it'll hit -40 at night with windchill.

Summer here can get to 40C (104f) maybe a little higher with the humidity. But we more likely sit around 30-35C.

Trencher makes a cheap footing

I've entertained the idea of a frost footing, however in both materials and labor the price is significantly more expensive

 

[jblnut]

doing full underslab for the comfort.

With everything there is a balance between how much to spend and what it'll get you. Having worked a lot of hours on cold slabs and warm slabs I assure you that you won't regret having the floor warmer. You can get a heatpump that'll play with radiant floor heat .....

I know you said heatpump for heat and you likely said it and I likely read it and forgot or missed it BUT are you planning on putting PEX in the floor ? There is truly nothing that compares to the feeling of a nice warm floor. When I built my shop having a heated floor was a must have. I didn't price out other heating options because I knew my tight *** would likely cheap out and not do the in floor system. The PEX isn't all the expensive and is easy to lay by yourself. The pumps, boiler and other stuff wasn't all that bad either. It's quiet and damn comfortable in the shop at all times. I put pipes every 6" around the outside 3', 8" the next 32", 12" the next 48" and 16" apart for the rest of the building. I did 8" spaced loops in my office as I wanted to make sure it was warm being in the NW corner with my desk in the NW corner of the room.

If you're insulating the floor putting PEX in really isn't that expensive and it'll give you, or the next owner, the option to have nice warm feet. Lots of hate on here for in floor and how fast/slow it heats up and how expensive it is and blah blah blah BUT it is the most comfortable type of heat that exists in a working shop. I'll die on that hill but I'll have nice warm feet.

 

[American Locomotive]

The thing is, from my understanding underslab doesn't seem to prevent frost heave. It's for comfort, and like you said, marginally improve heating costs. The edge/wing is what prevents heave, and I'm not getting rid of that. So I can't consider a plausible scenario where cutting underslab would make the slab heave and crack. If anything the base is more likely the result, and my planned base will be exceptionally better than what plans call for.

I also really don't know how realistic it would be to get the firm to cover it if it DID heave. I feel it would require the demo of the pad, inspection underneath, soil testing, full inspection of the pad specs, etc. A complete nightmare.

It seems like you're just kind of looking for people to agree with you and give you the okay to omit the foam.

The degreed, licensed and insured engineer that drew your plans said it's "on you" if the slab heaves and cracks without that insulation. That kind of warning seems to come from experience. In my opinion, there is one person you should be asking these questions, and it's the guy whose name is stamped on the plans. Call them/the company, ask what purpose the insulation under the middle of the slab serves, and what are the consequences for omitting it.

You've got a lot of money on the line.

 

[VietGnome]

It seems like you're just kind of looking for people to agree with you and give you the okay to omit the foam.

The degreed, licensed and insured engineer that drew your plans said it's "on you" if the slab heaves and cracks without that insulation. That kind of warning seems to come from experience. In my opinion, there is one person you should be asking these questions, and it's the guy whose name is stamped on the plans. Call them/the company, ask what purpose the insulation under the middle of the slab serves, and what are the consequences for omitting it.

You've got a lot of money on the line.

I'm not looking for people to agree with me, because I'm on the fence.

Sure I can ask the company buti understand what the foam does, how it works, and what you get by omitting it.

The engineer also isn't going to stamp a plan that compromises the structural integrity of the pad. You also have to acknowledge that the pad it floating and can experience movement before they'll send you any plans.

 

[VietGnome]

With everything there is a balance between how much to spend and what it'll get you. Having worked a lot of hours on cold slabs and warm slabs I assure you that you won't regret having the floor warmer. You can get a heatpump that'll play with radiant floor heat .....

I know you said heatpump for heat and you likely said it and I likely read it and forgot or missed it BUT are you planning on putting PEX in the floor ? There is truly nothing that compares to the feeling of a nice warm floor. When I built my shop having a heated floor was a must have. I didn't price out other heating options because I knew my tight *** would likely cheap out and not do the in floor system. The PEX isn't all the expensive and is easy to lay by yourself. The pumps, boiler and other stuff wasn't all that bad either. It's quiet and damn comfortable in the shop at all times. I put pipes every 6" around the outside 3', 8" the next 32", 12" the next 48" and 16" apart for the rest of the building. I did 8" spaced loops in my office as I wanted to make sure it was warm being in the NW corner with my desk in the NW corner of the room.

If you're insulating the floor putting PEX in really isn't that expensive and it'll give you, or the next owner, the option to have nice warm feet. Lots of hate on here for in floor and how fast/slow it heats up and how expensive it is and blah blah blah BUT it is the most comfortable type of heat that exists in a working shop. I'll die on that hill but I'll have nice warm feet.

I mean I might look into in floor for giggles, but I'm pretty sure it's completely out of my price range.