Slab questions for Maryland

[MerlinsBeard]

I'm in Maryland working with a concrete contractor and we hashed out most of the details for a 10x16 slab for a detached shed to be a workshop. The concrete contractor has quoted for a 6 mil vapor barrier, but I'm wondering if I should push for a thicker value.

Is there a significant difference between 6 and 10 mil or higher if installed correctly?

The second question is whether I should insulate the slab. I don't plan to have in-slab heating, but after a year I may decide to invest to heat/cool the shop.

I'm in Maryland, so it's not like things get super cold here, but if I don't do in-slab heating, am I still getting value from insulating the slab?

 

[thammel]

Insulating the slab will certainly reduce heat loss and the downside is the slight additional cost. I think the only difference between 6 and 10 mil thick barrier is resistance to puncture during concrete work. Again, if it's not a big cost issue, go thicker.

 

[ConCretin]

A vapor barrier's primary purpose is limiting water vapor transmission that could interfere with the bonding of adhered flooring. Limiting moisture in a structure is generally a good thing but a vapor barrier really isn't a necessity.

There is very little difference between a 6 and 10 mil polyethylene vapor barrier, which are technically vapor retarders. A true vapor barrier will be 15 mils and much more expensive. A 10 mil poly mil poly will resist damage a little better but either is fine.

You're unlikely to live long enough to see the payback from under slab insulation even if you eventually heat the structure. Keep in mind, insulation works both ways. If you insulate under the slab, you'll lose the benefit of the natural heat from the earth. I have a 32 x 28 un-insulated garage up here in Maine and liquids don't freeze until well after freezing temps set in outside.

I hope this helps. Congrats on the new shop and good luck with construction.

 

[MerlinsBeard]

A vapor barrier's primary purpose is limiting water vapor transmission that could interfere with the bonding of adhered flooring. Limiting moisture in a structure is generally a good thing but a vapor barrier really isn't a necessity.

There is very little difference between a 6 and 10 mil polyethylene vapor barrier, which are technically vapor retarders. A true vapor barrier will be 15 mils and much more expensive. A 10 mil poly mil poly will resist damage a little better but either is fine.

You're unlikely to live long enough to see the payback from under slab insulation even if you eventually heat the structure. Keep in mind, insulation works both ways. If you insulate under the slab, you'll lose the benefit of the natural heat from the earth. I have a 32 x 28 un-insulated garage up here in Maine and liquids don't freeze until well after freezing temps set in outside.

I hope this helps. Congrats on the new shop and good luck with construction.

Thanks for the feedback, and for your sticky guide! I used that to help clarify the details over email since the slab quote I got wasn't very detailed. I feel a lot more confident about what I'm getting now.

I do have one last question and that's about how best to attach the shed to the concrete. I believe they'll use treated wood for the frame in contact with the concrete with concrete anchors, but I'm not sure what additional methods to help seal that barrier to protect the wood longer.

I'm hoping to convince the shed builder to not finish the siding down low until delivery and use some kind of black asphalt coating that they use to help waterproof my basement walls, staple some kind of house wrap over that, then siding over that.

I just don't know the proper materials terminology to communicate this. I don't know if this is going overboard, but I want this shed to last best I can.

 

[ConCretin]

A concrete or masonry curb on top of the slab is a great way to lift the wood framing above the exterior grade and protect it from moisture. Absent that, make sure the wall framing/sheathing extends slightly beyond the edge of the slab to avoid a lip that will convey water into the building.

You ideally want a pressure treated sill plate under the wall framing. Sill seal is a good idea as well. There are a number of methods of attaching the sill plate to the concrete. Cast in place anchor bolts are probably the most common method but there are a number of post installed methods that are fine too. These include expansion bolts, epoxy anchors and Tapcon style self tapping concrete anchors. Just make sure the wall framing is adequately anchored to the sill plate or you've wasted your time anchoring it to the concrete.