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Old 11-04-2019, 01:32 AM   #1
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Vapor barrier + floor insulation on shed/office


We've just purchased a home that has an unfinished 12x12' shed against the garage that I'm converting into an office space. The previous owner built the shed and insulated and hung drywall on the walls and ceiling. But there is no insulation under the floor.

I live in Bend Oregon where it gets warm in the summer months and we see snow in the winter. This shed is located on the north side of the house and in the shade of tall trees. So I expect it'll stay pretty cool in the summer. In fact, I'll likely have the doors and window open most of the day.

What concerns me is the air/moisture trying to get into the shed/office during the winter months when I have a heater/wood-burning stove running. From my research, it seems that there's a danger of the moisture coming up from the ground and into the shed and possibly condensing and pooling somewhere in the floor.

The shed currently has an OSB floor and I'm planning to install wood laminate that was leftover from repairs to the house before we purchased it.

Home Depot recommends R-30 insulation for insulating a crawlspace/floor.

QUESTIONS:
1.) How would I insulate the floor?
2.) Do I need to worry about a vapor barrier?

MORE DETAIL:
The back yard runs down a hill and the shed is on cement footers at the ground level on one side and on ~12" 4x4 treated posts (running up from cement footers) as the ground starts to slope down the hill. Meaning the floor joists are hanging above dirt and are also exposed to the elements on the downhill side.


Thoughts about this?

Thanks! -- Kurtis.

Last edited by kurtdurfey; 11-04-2019 at 01:34 AM. Reason: eliminating redundancies
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Old 11-04-2019, 02:36 AM   #2
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Re: Vapor barrier + floor insulation on shed/office


If it is on posts you have ventilation so moisture is not likely a problem. A solid plywood or OSB sub floor is considered the VP. You can just insulate with a rock wool. A bug screen and then a wire mesh under it would help hold it in place and keep insects and critters out of it.
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Old 11-04-2019, 10:16 AM   #3
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Re: Vapor barrier + floor insulation on shed/office


How high off the ground is this, room to get under it?
Going to be tough to come up with R-30 unless they used at least 2 X 8 joist.
https://www.reference.com/home-garde...124d9da743861d
What I've done before is just laid down 1-1/2 blue foam board, a layer of Advantec screwed down with 3" decking screws, then the flooring.
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Old 11-05-2019, 01:23 AM   #4
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Re: Vapor barrier + floor insulation on shed/office


I took a closer look at it tonight. The joists are 2x6s, so I think you're right about the R-30 being too thick.

And the joists are not resting on the ground, but they're in contact with the dirt. There's absolutely no way I'm going to get insulation in from underneath the shed.

I think I need to cut the floor our and remove some of the dirts to make sure the shed is completely off the ground. Then drop in some insulation and then re-lay the floor. But I'd only be able to get R-19 insulation (6.25") with 6" joists (plus a little room beneath the shed. R-30 insulation looks to be 8.25" and even the R-30 Rockwool is 7.25".

But I could maybe add some height to the joists by ripping down some 2x4s and nailing them on top to gain an extra 1.5" for the insulation...

Joe, does the foam board plus the Advantec together get an R-30 rating-ish? That sounds like the easiest solution.

Thanks, fellas.

-- Kurtis.
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Old 11-05-2019, 01:33 AM   #5
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Re: Vapor barrier + floor insulation on shed/office


... I could also compress the insulation. According to this chart (https://www.energyvanguard.com/sites...?itok=jq3gsGh8), R-30 fiberglass insulation compressed to 51/2" is still an R-21 rating. Good enough?

https://www.energyvanguard.com/blog/...-really-so-bad

Last edited by kurtdurfey; 11-05-2019 at 01:33 AM. Reason: include citation
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Old 11-05-2019, 05:17 AM   #6
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Re: Vapor barrier + floor insulation on shed/office


I would jack it up.
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Old 11-06-2019, 04:27 PM   #7
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Re: Vapor barrier + floor insulation on shed/office


Trusting somebody at Home Depot makes no sense at all. I trust them to sometimes know where something is located in the store and nothing more. For a floor in the northwest an R-value of 19 to 30 is recommended which is quite a range.

But the payback is going to be difficult to calculate and it is likely to be more than 10 years at best. I replaced the insulation in my attic and in terms of energy savings the breakeven point will be in roughly 20 years at current energy rates.

I would avoid real wood laminate on the floor and use either linoleum or hard tile (with a decoupling membrane) that has a wood grain affect. If you go with tile and the membrane then moisture from below will not be a concern.
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Old 11-17-2019, 10:58 AM   #8
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Re: Vapor barrier + floor insulation on shed/office


My apologies to the original poster but I have a similar project, hoping replies to my questions help him as well. My shed/addition is accessible, (barely) from underneath. I'm more than 18" off the ground and I am able to get underneath since I already added a dryer vent. I have 2x6 joist. What's the best material to use? I'd love to use Fiberglass but should I use unfaced or faced? Or is foam or rock wool a better choice. If fiberglass I am going to cover with metal to keep out mice, but I'm worried about mold issues. Should I cover dirt with plastic? I'm in Philadelphia area.

Thanks,

Jay
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Old 11-17-2019, 11:22 AM   #9
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Re: Vapor barrier + floor insulation on shed/office


Quote:
Originally Posted by alexnook1 View Post
My apologies to the original poster but I have a similar project, hoping replies to my questions help him as well. My shed/addition is accessible, (barely) from underneath. I'm more than 18" off the ground and I am able to get underneath since I already added a dryer vent. I have 2x6 joist. What's the best material to use? I'd love to use Fiberglass but should I use unfaced or faced? Or is foam or rock wool a better choice. If fiberglass I am going to cover with metal to keep out mice, but I'm worried about mold issues. Should I cover dirt with plastic? I'm in Philadelphia area.

Thanks,

Jay
shape the dirt so water will not sit under there and cover the dirt with 10 mill poly. no paper the floor is the vapour barrier. Batts are fine and rock wool can handle some moisture better than FG. We just put up a wire mesh to hold it in place and keep critters out and insect screen to keep the insects out,
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Old 11-18-2019, 06:49 AM   #10
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Re: Vapor barrier + floor insulation on shed/office


Don't insulate the floor. Shed standing on ground will see a LOT of moisture and the insulation will trap that moisture. If the shed is attached to the garage, that's another problem since they are likely to move separately. Wood product flooring will have problems also with that much moisture. Even if engineered, wider boards may cup. I think you should use vinyl.

As nealtw said, I also think you should raise it away from the dirt if possible or live with it but I wouldn't put too much money into it trying to make it into an office. Another flooring that works would be astroturf. It breathes. Carpet may trap moisture also and start floor sheathing rot or mold.
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Old 11-25-2019, 07:38 PM   #11
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Re: Vapor barrier + floor insulation on shed/office


Hey, everybody. For what it's worth: here's the approach we landed on after looking at this feedback and several other conversations with other people:

1.) I'm not going to run insulation under the shed. Instead, I'm going to follow @joecaption 's advice and run 1" 5-R foam insulation on TOP of the OSB.
2.) I'm going to add 1" plywood for extra rigidity, and
3.) I'll lay the wood laminate on top of the ply.

So this question remains:

Do I need to add a vapor barrier between the OSB and the insulation? Or is the OSB sufficient knowing that the shed is off the ground (but not by much — the joists are just over the dirt in some places)?

Or will the OSB serve as a sufficient vapor barrier on its own?
@alexnook1 , I'm convinced now that fiberglass insulation beneath a shed will likely absorb moisture and will eventually fall down. Hence my move to foam.

But my research suggests that you would use faced fiberglass and the paper side would go on TOP with the tabs on the paper stapled to your floor joists. Then the paper is your vapor barrier.

Is that accurate, fellas?

Thanks! -- Kurtis.

Last edited by kurtdurfey; 11-25-2019 at 07:40 PM. Reason: Corrected tag/mention
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