New shop, floor questions

[KPack]

Hi guys,

Complete newbie here. I've searched for an answer, but haven't found anything that gives me a firm answer.

New shop is hopefully going up in the next month or so. 36'x72' with 14' walls pole building. For the floor I'm doing 6" concrete slab, 2" foam around the outside perimeter as a frost barrier, 15 mil vapor barrier underneath, and rebar on 24" centers throughout the floor. Front half of the shop will be garage space with a lift, back half will be hobby/game/etc, no living space. Not planning on in-floor radiant heat. I'm planning on running natural gas to the shop and doing some sort of shop heater with that (have gas to the house, about a 200' run).

Location is Pacific Northwest, high desert. Cold in the winter, hot in the summer. Dry-ish climate, ground is moist.

My question: do I need to do 2" insulation under the entire slab? Or is that only for in-floor radiant heat? Adding the foam is going to increase the cost significantly....and the shop is already way more expensive than it should be.

 

[Sumboodie]

It helps have a warmer floor.

I've worked in a shop without floor heat and it *****. I wouldn't consider building a heated shop without in this area.
Freezing while near the floor under equipment, sweating while working at the bench with feet freezing.
Especially worse if the doors were opened or cold equipment was brought in.

Even with ceiling fans, there was easily a 40* temp differential between the floor and ceiling on cold days (-20*)

 

[KPack]

I definitely don't deal with the extreme temps that you do. We do get down to the teens and single digits occasionally, but that's only for a week or so. I doubt I'll be doing much work in the shop at that point. The builder (one of those companies that does pole buildings only) says they never do insulation underneath the slab unless it is living space or heated floors.

Separate question:
What thickness of concrete is appropriate? I'm having them do rebar throughout. Some guys I talk to says do 6" all the way through, some say that 4" is plenty. Front half of the shop will be garage space with a lift. Private vehicles, boat, etc. No RV's or campers. Am I overspending on 6"?

 

[ConCretin]

There is no need to put rigid under an unheated floor.

As for thickness, 4" is more than adequate for most uses because it's the ground under the slab that supports the weight you put on the slab. While it's true that a 6" slab is significantly stronger than a 4" slab, you'd have to a lot thicker still to make up for an inadequate base underneath.

With that said an extra inch or two is relatively inexpensive for a little peace of mind. In your case, its about 8 cy or $800+/- for each additional inch of concrete. I think 5" makes sense because it gives you a little margin of error for the inevitable variations in base and top of slab elevations that could conspire to reduce your slab thickness by 1/2" or more.

Check out my Guide to Floor Slabs in the link below for some additional thoughts, Congrats and good luck with your project.

 

[haugy]

While I know you don't want to hear this, I'd seriously consider getting in-floor heat. Or at least running it. While there is an additional cost up-front, you don't have to add heat until later, if ever. But if you spend now, and run the PEX, and it's there and you realize, holy balls this floor is cold. You have everything run to hook up heat. And with Natural Gas available, you're primed for an easy installation of a gas boiler to heat the floor.

I live in TN. Nowhere near as cold as you get. My last shop floor was unheated, uninsulated underneath. It absolutely sucked to get down on the floor. In winter I had to lay something down, in summer, if it was humid and I had opened the door, the floors would be sweating from the temperature variance. Each spring my shop turned into a rainforest because the shop floor hadn't gotten up to warmer temps yet, so it was still cold. But the air and humidity had gone up. Open a door, and everything was coated in moisture, floor, tools, everything.

I'm building now, and while it does **** to add additional foam and PEX, on top of the other expenses, it should cost me about $1200 to run what I need for in-floor heat. Later if you want it, and I'm betting you would if you had it run, you can do heat. No more ice cold floors to lay on, or fighting the floor when trying to heat the shop, or condensation during temp swings. And with your humidity and moisture, that's a lot of moisture.

I just wanted to share some perspective. I can't wait to have a shop with in-floor heat that doesn't make my toes cold in winter while working. As well as keeping my temps more balanced so I can protect the items that are parked inside that don't like the cold too much.

 

[KPack]

LLWillysfan - Thanks for the info and for the links. I've been doing a lot of reading through those the past day.

Haugy - I'm on the east side of the Cascades, which means I'm basically in desert. No humidity here. So I won't be dealing with the condensation issues that you have struggled with. I haven't seen a single shop around here have anything like that happen. As far as the winter goes, the floor will definitely be cold I'm sure. Will it be colder than the garage floor in my house? I don't know. I can honestly say I haven't noticed how cold the floor is in there during the winter. It's for sure uninsulated (besides maybe a frost barrier) and the floor is always so dirty from dust, snow melt, and general grime during the winter that I throw a towel down before crawling under a vehicle anyways. I guess because of that I've never felt that it was overly cold. I'll consider adding PEX and insulation, but I'm very concerned about the cost.