New concrete slab pour 60x100

[sol]

I’m currently building an addition on to my shop. Total size 60x100. I have decided to do infloor heating and one installer is suggesting spray foam 2inch thick before the concrete pour of 6” thickness. My question is spray foam better to use then foam boards? Also the spray foam is closed cell and gives a 25 psi compressive strength rating. Will this be good enough under the 6” concrete thickness ? Any advice if tips greatly appreciated. Any advantages / disadvantages to use spray foam ? I like the idea of 100% sealed. Thanks in advance. I have attachef some photos of the final packed grading before we do the insulation of the floor then concrete pour comes next.

 

[juddspaintballs]

If you stick to foam board, you can lie them down, tape them together (or not), and then lay out the PEX and staple it onto the foam board. That is, if you're doing PEX...

 

[ConCretin]

I'm not an expert on the subject of spray foam but I've got a little experience and seen it done under slabs we've placed. From what I understand there are advantages, the most notable of which is spray foams ability to act as a vapor barrier and seal more completely. I don't know if it's permeance rating is as high as a true vapor barrier such as Stegowrap with taped seams but that shouldn't be an issue in most circumstances. I'd look into it further if you are considering an adhered floor covering but I suspect you'd be fine.

The compressive strength of the foam is not an issue. It's at least as strong as traditional board insulation and isn't subject to voids when used over an uneven or haunched base.

The primary disadvantage seems to be cost. I'm sure it varies regionally and by contractor but I understand spray foam is often more expensive than rigid boards. I'm not sure the increase in performance is enough to justify the cost. I'm not even sure it would be noticeable. If it were me, I'd base the decision on cost and convenience.

Check out my Guide to Floor Slabs in the link below for some additional thoughts on vapor barriers and slab construction in general. Congrats and good luck with your project.

 

[sol]

Llwillysfan thank you for the reply. Yeah the price for spray foam is more money but I feel for the difference it’s worth it. I’m glad to hear I have zero issues with 25 psi rating under the slab.

And yes I have read your Guides to floor slabs a few times. So much great information. My pour is for the 17th of this month so I’m praying I get a good pour. Even after reading your guide a few times seems like so much info and things to watch out for a bit over my head. Thanks for the info.

I was told on my 6inch thick pour nothing goes in no rebar or anything. Does that sound right ?

 

[ConCretin]

I was told on my 6inch thick pour nothing goes in no rebar or anything. Does that sound right ?

Unfortunately, It's not quite as simple as that but let's think it through so you can make an informed decision. The important thing to note when it comes to reinforcing is that it doesn't prevent cracks, it simply holds them together once they occur. In fact, reinforcing can increase the risk of shrinkage cracks so there is a benefit to not using it.

So how likely is your floor to crack AND how likely are those cracks to want to pull apart? A 6" thick floor on a solid base is very stout and unlikely to experience structural cracks from imposed loads. However, with a floor that large, you will likely get numerous shrinkage cracks. With some effort, you can limit them but that's a big floor and you probably can't prevent them.

Presumably you will be cutting some control joints to hide the shrinkage cracks? If you cut them in a timely manner, it's likely that you will get multiple cracks that relieve the tension from drying shrinkage and the cracks won't be very wide. In that case, the aggregate at the cracks will remain interlocked and prevent any vertical displacement. Not to mention that a solid base will prevent any forces that want to create that vertical displacement in the first place.

So I wouldn't say it's automatic that a 6" floor doesn't need rebar. A lot of people will put it in as insurance against unforeseen conditions. But it's also reasonable to avoid the cost and go without. If the base is solid, water is kept to a minimum, control joints are cut immediately after finishing to the proper depth (1 1/2" in your case) and the slab is cured properly, rebar isn't necessary.

You could also consider adding fibermesh, which will limit the formation of shrinkage cracks. It won't prevent them completely on a floor that large but it will provide a little margin for error. I hope my painfully lengthy response is helpful.

 

[sol]

Unfortunately, It's not quite as simple as that but let's think it through so you can make an informed decision. The important thing to note when it comes to reinforcing is that it doesn't prevent cracks, it simply holds them together once they occur. In fact, reinforcing can increase the risk of shrinkage cracks so there is a benefit to not using it.

So how likely is your floor to crack AND how likely are those cracks to want to pull apart? A 6" thick floor on a solid base is very stout and unlikely to experience structural cracks from imposed loads. However, with a floor that large, you will likely get numerous shrinkage cracks. With some effort, you can limit them but that's a big floor and you probably can't prevent them.

Presumably you will be cutting some control joints to hide the shrinkage cracks? If you cut them in a timely manner, it's likely that you will get multiple cracks that relieve the tension from drying shrinkage and the cracks won't be very wide. In that case, the aggregate at the cracks will remain interlocked and prevent any vertical displacement. Not to mention that a solid base will prevent any forces that want to create that vertical displacement in the first place.

So I wouldn't say it's automatic that a 6" floor doesn't need rebar. A lot of people will put it in as insurance against unforeseen conditions. But it's also reasonable to avoid the cost and go without. If the base is solid, water is kept to a minimum, control joints are cut immediately after finishing to the proper depth (1 1/2" in your case) and the slab is cured properly, rebar isn't necessary.

You could also consider adding fibermesh, which will limit the formation of shrinkage cracks. It won't prevent them completely on a floor that large but it will provide a little margin for error. I hope my painfully lengthy response is helpful.

Amazing reply in information. Thank you for taking the time. As per compaction it was done very well. The majority of it was done in December and the photos above show what they did the last week. The final 12 inches or so was very good fill. It was slightly wet and they tamped it down very hard with Jack plate machine and the huge rolling machine which vibrates and rolled at once ? We also had compression testing done and the company said it’s much higher then they expected. Also as the guys lay the plumbing pipe now they had a ***** digging the trenching and used a jackhammer machine with a spade end in it to break up the new compressed fill. So I guess I’m hoping it’s done well and will 2 inch spray foam closed cell I better my odds of a good concrete pour. The contracter is subbing out the floor and apparently using a very high end company that uses all lazers and high end equipment whatever that means. My pour is to happen on the 17th of this month so wish me luck. Thanks again for your time and wealth of information.