Question re insulating the floor in my workshop

[Ed Weiser]

Frank--
It all depends how you will cover the subfloor. If you plan resilient tile or wood flooring, the vapor barrier goes beneath the subfloor. For masonry products it can go directly beneath the product.

Ed

[Frank Pellow]

Frank--
It all depends how you will cover the subfloor. If you plan resilient tile or wood flooring, the vapor barrier goes beneath the subfloor. For masonry products it can go directly beneath the product.

Ed

Ed, my floor is going to be wood so I guess the barrier goes below. the sub-floor.

You did not answer my question about the fact that I am planning to use pressure treated plywood in the sub-floor. Is that a mistake?

[Ed Weiser]

Frank--
Since the vapor barrier is beneath the subfloor, ordinary T & G 3/4" subfloor-rated plywood is fine. The additional expense of pressure treated plywood is unnecessary. If you are still worried about drifting snow up against the insulation, you could also use housewrap-type product (e.g. Tyvek, etc.) which is moisture permeable to offer some additional protection beneath the joists. It is not particularly expensive.

Ed

[Rob Russell]

Frank,

You want to find a way to keep ground moisture from bleeding up into the fiberglass batt insulation. Water vapor that seeps into the batt insulation will greatly reduce the effectiveness of the insulation. You might need a layer of housewrap on the bottom of your joists.

I agree that, with a vapor barrier just beneath the plywood, you don't P/T plywood. I'd use a minimum of 3/4". If you want a really stiff floor, put a layer of 1/2" over the 3/4". Stagger the joints.

I'm still working on the DC schematics - had to create a bunch of standard symbols for the various electrical components (switches, contacts, coils, etc.)

Rob

[Gary Sutherland]

Awhile back, I ran some speaker lines for an elderly friend who lives in a mobile home. It had a very tough black 'fabric' (plastic? It was dark under there!) of some sort stapled under the joists, with fibreglas insulation above that, which I had to get the wires through.

It seems to work very well and hold up; try giving a mobile home mfg. or dealer a call and see what they use.

Good luck!

gary

[Andy London]

Andy, I had not seen your response when I posted the above reply. Do you think that enclosing the bottom with plywood will solve the problem?

Also, you suggest a vapour barrier on the outside. I guess that I could line the plywodd with plastic before installing it. If I did this, do you think that I also need a vapour barrier above the pressure treated plywood sub-floor?

Frank I think for the most part that yes plywood on the bottom will solve the problem. I think for the effort you are going through rigid insulation with 1 X 3 strapping would be easier and less of a problem down the road.

I had my joists sprayed for the most part, due to the way the grade runs on bedrock, the back 20 feet of the shop has 2" rigid. Then to help things out a bit I built a knee wall and insulated it.

If you go with fiberglass, house wrap or plastic will work, you do not require air movement under a floor like you do in an attic where there is drastic temp changes.

The other option Frank, give the inspectors a call and ask what they would do, they will give you a quick answer, I've built quite a few homes/buildings here in Halifax and know what I suggested will pass in our area.

Have fun!!

[Frank Pellow]

Ed, I did not make it clear but the reason I asked whether it was a mistake to use the 3/4 inch pressure treated plywood above the plastic is that I already have the plywood. I can't return it. Do you know if there will be problems if I use it? Just how bad is it if I put the plastic above the plywood sub-floor and below the plywood floor?

Ed and Rob, I will check into what types of housewrap are available here starting with Tyvek (which I have never heard of). I am definitely going to put something solid at the bottom of the joists.

Rob, I am planning to put 1/2" plywood over the 3/4" plywood sub-floor and I going to place it so that the joints are in different places.

Gary, I will see if I can find fabric of the type you mention.

Andy, I am relieved to hear from you that "you do not require air movement under a floor like you do in an attic where there is drastic temp changes". That is what I suspected but it is good the hear it from someone else.

Andy, I spoke to the inspector assigned to my building and he did not seem to know much (in fact, he did not seem to know anything) about floor insulation under open spaces. Everything I suggested he seemed to go along with and say that I should try it and see how it works out.

Rob, thanks for letting me know about the DC schematics. I hope that you are not just doing the symbols for me and that you will be able to use them elsewhere. I really really appreciate the electrical advise that you have given to me.

[Jim Becker]

One point..."house wrap" (Tyvek, etc.) does restrict air flow, but it is not a vapor barrier. It is a moisture permeable material. That is on purpose to avoid a double barrier and cause moisture retention in any wall/floor materials, including insulation.

Were I building this floor-on-piers design, I'd put a layer of 1/2" Ext plywood below the deck, construct the floor structure and insulate in the bays with the vapor barrier on the floor side. Given the northern climate, I'd use R30 batts for fiberglass which are also self-supporting in the bays due to the slightly wider width and stiffness they come in.

[Frank Pellow]

Jim, I would put the house wrap below the fiberglass so I would not want it to be a vapour barrier. I want it to at least slow down "critters" who want to nest in the insulation. I am also going to see about using galvinized 1/4" mesh if I can find some with . I would use both with the hope that this combination would actually stop mice.

I will think about R30 but I expect that I will still go with the R21.5 from Rexul. When you add in the R value of tyhe two floors, that brings me up to R23 and the government recomendation for floors over unherated spaces in this part of Canada is R25. Also, I expect that R30 would be too thick. The R21.5 from Roxul is only 5.5 inches thick and they say that it is mosture resistant.

[JayStPeter]

After thinking about it last night, I was prepared to make the same arguments against my previous statement.
I agree w/Jim that Tyvec is a wind barrier, not moisture. There is a very good water resistant versions of osb out there these days. Local Lowes has a piece in a jar of water as a demo. I can't remember the name of the stuff, something with "tech" in it I think. I think the price was relatively in-line with other osb, but am not sure how it would fit into your budget/plan.

Jay

[Frank Pellow]

After thinking about it last night, I was prepared to make the same arguments against my previous statement.
I agree w/Jim that Tyvec is a wind barrier, not moisture. There is a very good water resistant versions of osb out there these days. Local Lowes has a piece in a jar of water as a demo. I can't remember the name of the stuff, something with "tech" in it I think. I think the price was relatively in-line with other osb, but am not sure how it would fit into your budget/plan.

Jay

Jay, what does OSB stand for?

[JayStPeter]

oriented strand board

It's the stuff that looks sorta like particle board, but made with larger chips of wood.

Jay

[Frank Pellow]

oriented strand board

It's the stuff that looks sorta like particle board, but made with larger chips of wood.

Jay

Sounds like what I would call "aspenite".

[Chris Padilla]

OSB at the local orange box is nearly $20/sheet for 7/16" thick stuff. I was kinda surprised at the cost. It must have had some kind of chemical treatment because it was blue. I don't recall any other name attached to it but I wasn't really paying attention to it.

[Jim Becker]

OSB at the local orange box is nearly $20/sheet for 7/16" thick stuff. I was kinda surprised at the cost.

OSB "used to be" about $7.95 per sheet...as recent as two years ago when I did my shop expansion.