New member to the forum and first post.
Searching the forum still has not addressed my growing concern, so please accept my apologies if this has been answered before.

As I live in Canada, the cold winter needs to be addressed to make my workshop more accessible in the winter.

I have a 16x24 workshop built on a slight slope on concrete piers. At one end I have about a 6 inch clearance from grade while at the other end it is about 16 inches. The reason that I mention this is that I need to increase the R value of the floor to make it somewhat more comfortable.

As I have an inward swinging door I only have about 1.5 inches to play with to try to address the insulation issue as getting underneath the floor may be to difficult.

I currently have 3/4 inch Norbord OSB as the floor and that is it. In the winter this stays very cold regardless of the heat source. I was looking at adding XPS then covering it with 5/8 OSB.
In the opinion of members of the forum is 3/4 XPS going to give me any bank for my buck.
A contractor I discussed this with said no and he recommended just adding another layer of 3/4 OSB. He also suggested a a vapour barrier between the two sheets of OSB. Is that a good suggestion?

The temperature that we are talking about during the winter could be as low as minus 10-20 degree C. The wind blows underneath the floor but there are no air leaks.
Sorry for the long post, any help would be appreciated.
Thanks
Martin

How do you heat the shop now? What type soil is it built over? One option that might help without effecting floor height is to insulate the perimeter from floor height down below grade. This would turn the space below the floor into a buffer from those minus outdoor temps. You may have to do something to deal with moisture below the floor though... You could also seal the earth and even force heat into the space for warm shop floors

If you don't want to do anything from underneath, I'd add 1" of XPS, tape the joints and then put 7/16" or 3/8" of OSB on top. Hopefully this shop isn't heated all the time...

Thank you for your replies. The shop isn't heated all the time as I'm only in there on my days off. Was there a particular reason that you mention heating it all the time other than the cost effectiveness.
Also is 3/8 osb thick enough. What about compression of the XPS, or will it need strapping every 16 inch or so?

Is there any benefit from doubling up on the OSB alone as one contractor mentioned? He also mentioned a vapour barrier between the two layers?

Just heating cost on the "all the time" comment. I put 1/4" OSB over cheap EPS (no strapping) and it was fine for walking (but probably not a whole lot more). For any heavy machine, you could put some much thicker (or steel) pads under the feet to further distribute the weight. You could also use higher density XPS (evidently common practice for any overlay thickness).

XPS is one of the least green insulation products out there, due to the amount of HFC134a blowing agent used in it's manufacture. HFC134a has a 100 year global warming potential of nearly 1400x CO2.

"Type-II" EPS (= 1.5lbs per cubic foot nominal density) works fine sandwiched between a 3/4" subfloor and half-inch OSB. EPS is the same polymer as XPS, but it's blown with pentane (~7x CO2) , the vast majority of which is typically recaptured on-site and burned for process heat (turning it into CO2 and water.) In a handful of decades as the HFCs bleed out of XPS it's performance drops to that of EPS.

If there is a finish floor going on top of the sandwich fastener retention becomes an issue at 3/8" and thinner, but most flooring works fine with 7/16" or half-inch. From a foam compression point of view, as long as the seams of the foam overlaps those of the subfloor by a foot or so, and the seams of the top OSB overlaps those of the foam by a foot or so, the dynamic and static loads are fairly evenly spread. If you use foam-board adhesive on both sides of the foam it'll be even less of an issue, since it will stiffen the sandwich beyond what mere nails would deliver.

The vapor permeance of 3/4" OSB meets the Canadian code definition of a vapour barrier when dry, and if it ever becomes damp you'd like it to be able to dry in both directions. There is no advantage to inserting 6 mil poly.

Is there any insulation between, or sheathing on the bottom side of the floor joists?

Thank you very much for your response, Sorry for the late reply

The Norbord osb is directly on the joist with no insulation whatsoever, on top, below or between.

My intention is to create the most cost effective way to keep the floor from robbing the rest of the workshops heat.

Doubling up on OSB seems to be the cheapest option however it may not accomplish the task at hand.

Adding XPS or EPS will obviously increase the over all cost but will the 1/2 sheathing on top allow for the wight of a cast iron table saw as well as a cast iron 14 inch band say...these are the issues that I'm dealing with

thanks again

I wouldn't bother with doubling the OSB without insulation. It would help a little, but insulation is so much better. You will certainly be OK if you use 12x12x1.5" wood pads or 12x12x1/4" steel pads under the legs. Without pads - I don't know how to calculate the exact depression you will get with a given overlay, foam density, foot area, equipment weight and time. You could do a test for a week and measure it - sounds like your equipment isn't that heavy (no 400 ton press).

You could box in the joists and fill with cellulose.

Rather than doubling the OSB, is it possible to pull up the OSB and put insulated sheathing (not foil-faced) underneath? That still has some vapor permeability, but will put the OSB on the inside of the condensation line when you're heating in cold weather. I have a 12x14 shop that we put OSB in as interior cladding, and it still makes me nervous with moisture risk. My grain of salt - I tend to be leery of OSB, and I'm definitely not a contractor - just dealing with the fallout from moisture and mold in the house I'm trying my best to get out of.

Make a double entry, cut your inswinging door off a little to afford you room to add substantial insulation and OSB topping. Consider sealing off your exterior below ground. You said you are on piers. Is your shop open to atmosphere underneath it?

Thanks to all that have replied, just got back in town.

After careful consideration, base on the feedback received on this forum, I've decided to go with the 1 inch rigid insulation with OSB on top.

To those who asked, yes the floor is on joist that sit on piers open to the air. Getting under the floor is not an option to do any significant amount of work.

I will be gluing the rigid insulation to the floor then gluing to OSB on top. The seems will be taped and the osb staggered hiding those seems.

The only question left (unless someone mentions another potential issue) is that about strapping. Should the osb have strapping along the edges to allow it to be screwed to the existing floor to make the edges more secure to avoid an uneven floor due to compression of the insultion along the edges of the osb. I just don't want any issues due to the fact that gluing the insulation to the osb will make redoing things out of the question.

So basically, if anyone has any feedback about how to properly install 1 inch rigid insulation to the existing osb and then gluing 1/2 inch osb to that, I'd appreciate it.

Again, thank you to all who have contributed to my questions.

Martin

With a glued/ laminated stackup through-screwing the top OSB to the original subfloor at the corners, and on a 24" o.c. grid would be sufficient to keep "potato chipping" curl from getting started. It doesn't really need strapping at the edges.

Thanks for the feedback Dana 1, was hoping that strapping would not be needed.
Regards

I question the need/wisdom of gluing the top sheet of OSB to the insulation. Since that surface will be seeing the most wear, I would expect that occasional replacement may be needed. Perhaps not an issue depending on use, but a glued interface doesn't lend itself to easy replacement. Would not the through screws be adequate?

With half-inch OSB screws alone are not going to cut it.

Do you think if he upgraded to 3/4" sheet goods the screws alone would be adequate? T&G 3/4" OSB adds about $0.21/sq. ft. to the cost. Going to a 3/4" plywood product about $0.60. I'd be concerned about the longevity of thin OSB in a work shop. Heavy equipment dragging across the floor, point load denting, spills of any kind. Understand cost is an issue; but penny wise ....?