Replies

1. 
   user-1137156 | Jul 27, 2014 01:27am | #1

   Mark,
   Double stud walls and 6 - 7" of Roxul doesn't add up 2 2x4 walls is 7" cavity. The benefit of double walls is that there can be insulation between the walls. There are mineral wool bats that are 2 1/2" thick and using them between the 2 walls adds r 10 + to the assembly. "Is a concrete slab the best way to go?" Best is very subjective but for what you describe it is certainly very suitable and should be reasonably cost effective. What is your frost depth? The choice of foundation type is strongly dependent on local soil conditions. If your goal is minimum heat loss and your soil allows it, you should consider a " Permenant Wood Foundation" aka PWF. And more than 2" of sub slab insulation, PWFs are heavily pressure treated wood walls that usually sit on gravel that is well drained and extends below frost depth. A 2x8 "pony wall with plywood on both sides and Roxul bats in the spaces between studs will give very good thermal isolation of your slab edge (where most of the heat loss occurs) and serve as the form for pouring the slab. The PWF should cost significantly less than the other alternatives. You need to prevent frost heave of the slab. This means very good drainage under the insulation ideally gravel to below frost depth.

2. GBA Editor
   Martin Holladay | Jul 27, 2014 05:28am | #2

   Mark,
   Most workshops of the type you describe have a concrete floor. But if there is any reason that you don't like working on concrete, it's always possible to build a building with a wood floor. My guess is that the concrete slab will cost less.

   Concerning the foundation details, you should talk to your concrete contractor. If you place the concrete on a layer of crushed stone, I see no reason why you can't install a thickened-edge slab (a monolithic slab). While this type of foundation may be unusual in cold climates, it works just fine for garages and outbuildings, as long as the slab is placed over crushed stone in a well-drained location.

   Your reference to "SM insulation" had me scratching my head, so I looked it up. It turns out that SM is a brand name for a type of XPS insulation sold by Dow in Canada. But I guess you know that.

   If your local building inspector insists on a vapor barrier for your walls, you might consider installing a smart retarder like MemBrain. It will cost a little more, but it's a better choice in your climate, and will allow more flexible use of your building in the future if the use of the building ever changes. (Remember, the climate is changing.)

3. 
   markgodfrey | Jul 28, 2014 04:31pm | #3

   Thank you for your comments.
   - Frost depth here is assumed to be 48", so all footing is expected to begin at or below that depth.
   - Is more than 2" of sub slab insulation really cost effective? This isn't a Passivhaus, just a workshop.
   - I like the idea of Roxul surrounding slab perimeter, certainly more environmentally friendly than the
   XPS, I do have concerns about the chemicals in pressure treated plywood, is there another
   method of encapsulating Roxul on or under the soil?
   - I won't be excavating 48" of soil and replacing with gravel, will confine it to removing organic soil,
   laying about 12" of coarse gravel, packing to form base for the slab. I will install Big O drainpipe
   at 48" depth around the perimeter and cover with same coarse gravel. This will drain out the side
   of the rise. Note, must remember to block open end with small stones so local critters don't set
   up house in the drain.
   I would be quite content to use monolithic slab foundation/floor but; I will have a 30'x14' room on the
   side of the shop. It will be a roofed, unheated, uninsulated, gravel floor work/storage area. It's
   roof will be a continuation (same slope) of the workshop roof. I want to support the end beam on
   4 posts set on concrete pilings. The inspector will not allow the 2 types of foundation...monolithic
   slab (shallow depth) and piling (48" depth) in the same building. So I believe I will go with the
   concrete pilings all round and pour a slab floor. I am spacing pilings 10' apart, OK with the
   inspector as long as I use tube without the bigfoot ( apparently tube plus bigfoot combination was
   tested at 7 1/2 foot spacing, so anything greater not approved). I've just discovered Footing Tubes
   they seem to make more sense than sonotube. I will see what he makes of that. Sometimes
   convincing building officials a frustration....I have several friends in this neck of the woods who
   have built strawbale houses....wonderful buildings yet building officials remain skeptical.
   Thanks for the MemBrain suggestion, looks promising and you reminded me of the possibility of
   a change in use...I hope this building will remain hale and hearty long after I go.