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Old 08-19-2020, 03:13 AM   #1
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Shed roof insulation options, on a budget . . .


Greetings Neighbors! Browsed a bit and learned a lot, but still need help with a decision on insulation. I recently bought a 12 x 24 wooden shed on 4x6 skids with a gambrel roof. Originally I planned to use the shed just for storage, so I gave no thought to things like ridge vents and soffit vents. But now it'll also be a man-cave and tinker-shop, so I'll be installing a/c and heat and will need to insulate the building. I live in the boonies of middle-Georgia, so I'm not concerned with code issues. I'll use R-13 kraft-faced fiberglass for the walls (2x4 on 16-inch centers), but the roof has me stumped because I want to avoid condensation and other blah-blah (and still be solvent). The roof deck is metal over OSB. The roof joists are also 2 x 4, but on 24-inch centers. There is an 8-inch by 8-inch vent near the top of both gambrel ends, but no ridge vent and no soffit vents. The building has a 12'x12' loft (2x6 on 24-inch centers with OSB decking) covering half of the structure.

Spray foam is cost-prohibitive. I see two options:

1. Just add R-15 (or R-13?) kraft-faced fiberglass in the roof bays without further venting and start saving money to replace the roof framing and OSB in a few years. I realize R-15/R-13 isn't enough, but it's all that will fit in the rafter bays. (Alternative might be some flavor of XPS).
2. Frame out/extend the loft over the 2nd half of the building (effectively creating a ceiling/attic situation), add R-19 kraft-faced fiberglass to the "ceiling" joists (loft floor), and figure out how to add soffit and ridge venting to the newly-formed "attic" area.

I'm leaning toward option 1. Grateful for any guidance you can give. TYVM!!
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Old 08-19-2020, 04:58 AM   #2
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Re: Shed roof insulation options, on a budget . . .


Welcome from North Georgia. Insulation alone won't get the job done without compromising moisture damage. I'd go ahead and plan on cutting in a ridge vent and some soffit vents, installing foam baffles to allow air to move above your insulation. You may have to install 2x2's to the edge of your rafters. Then put in your insulation with the paper down and cover with wood or sheetrock. Flatlanders have to worry more about moisture and heat.
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Old 08-19-2020, 07:53 AM   #3
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Re: Shed roof insulation options, on a budget . . .


For baffles to work, you will need to install soffit vents.
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Old 08-19-2020, 10:59 AM   #4
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Re: Shed roof insulation options, on a budget . . .


Thanks Chandler and dj. Looks like I have some work cut out or me here. Reckon I'll face the rafters with 2x2's, install foam baffles and R-15, bore out some soffit vents, and cut in a ridge vent. Now I need to do some calculations.

The "eaves", if you want to call them eaves, are simply 2x4's installed flat and extending out from the sidewall top plates. They are beveled on the outer edge to match the slope of the lower section of the gambrel roof. The underside of the OSB roof sheathing is only 3-inches from the siding at the eave.

The only thing I can figure for soffit vents is to bore two 2-inch holes in each rafter bay on each side of the building. A 2-inch diameter hole provides 3.14 inches of vent. With 4 holes per rafter bay (2 on each eave), that would be a total soffit vent space per rafter bay of 12.56 square inches. So how wide will my ridge cut need to be in order to ensure there's plenty of "suck" at the top? My understanding is that you want equal soffit and ridge vent area, or slightly higher soffit vent area. I'm thinking the ridge opening at the peak will need to be about 1/2 inch wide. That would give me right at 12 square inches of opening per rafter bay at the peak under the ridge cap.

Does this sound right? Will this create proper/sufficient air flow and control moisture without causing other problems?
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Old 08-19-2020, 04:50 PM   #5
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Re: Shed roof insulation options, on a budget . . .


And use these to plug the holes to keep critters out. https://www.homedepot.com/p/Speedi-P...SV-2/300558927

I'd cut 2" on either side of the peak to allow for the ridge vent to draw well. Then place ridge venting over it and shingle over it all.
https://www.homedepot.com/p/GAF-Cobr...7000/206940210
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Old 08-19-2020, 05:55 PM   #6
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Re: Shed roof insulation options, on a budget . . .


Thanks again, Chandler. The 2" soffit vents are perfect. I'll have to adapt the ridge vent plan because the roof deck is metal over OSB (not shingles). Easy enough to figure that out, but it's gonna be a challenge cutting the metal and OSB. Is that a grinder job? Or is there a proper circular saw blade for that kind of cutting?
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Old 08-19-2020, 06:03 PM   #7
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Re: Shed roof insulation options, on a budget . . .


Ouch. Is there no gap at the ridge? Usually there is a small gap and the metal ridge vent is placed over it to allow for ventilation under the ribbing. If not, yeah, it may be grinder time, then circle saw for the OSB. Be sure to stop the gap a foot or so from the end to prevent water from blowing in the sides and getting things wet.
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Old 08-19-2020, 07:25 PM   #8
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Re: Shed roof insulation options, on a budget . . .


Well, I can't tell if there's a gap in the metal until I remove the ridge cap for a closer inspection under there. But the OSB, which I can see from the inside, has a gap at the peak ranging from 1/8 to 1/4-inch wide. Doubtful that was deliberate (ie: for ventilation purposes). Probably just how the sheets worked out. Lawdy I hate to cut metal. Reckon I can't avoid buying a grinder any more. Huge thanks for the reminder to leave a foot intact at each end to keep the rain out.
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Old 08-22-2020, 11:33 AM   #9
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Re: Shed roof insulation options, on a budget . . .


Hey onyx,
I'm in GA, and am getting ready to build a 12x24 shed. I'd consider buying one, though. Where did you get yours?
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Old 08-22-2020, 11:50 AM   #10
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Re: Shed roof insulation options, on a budget . . .


Researched buildings all over Georgia, visited every manufacturer in the Montezuma, GA area (there are quite a few). Premier Buildings cost a couple hundred dollars more than others, but their build quality sold me on their product. I didn't order through their website (https://www.premierbuildings.us/). I did all my business face-to-face with the actual builders (James, Grant) and the front office lady (Lucinda). They build for Premier, but their company is called Maco Builders. Very pleased with them. Good luck!
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Old 08-22-2020, 08:30 PM   #11
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Re: Shed roof insulation options, on a budget . . .


Quote:
Originally Posted by OnyxTheDawg View Post
Greetings Neighbors! Browsed a bit and learned a lot, but still need help with a decision on insulation. I recently bought a 12 x 24 wooden shed on 4x6 skids with a gambrel roof. Originally I planned to use the shed just for storage, so I gave no thought to things like ridge vents and soffit vents. But now it'll also be a man-cave and tinker-shop, so I'll be installing a/c and heat and will need to insulate the building. I live in the boonies of middle-Georgia, so I'm not concerned with code issues. I'll use R-13 kraft-faced fiberglass for the walls (2x4 on 16-inch centers), but the roof has me stumped because I want to avoid condensation and other blah-blah (and still be solvent). The roof deck is metal over OSB. The roof joists are also 2 x 4, but on 24-inch centers. There is an 8-inch by 8-inch vent near the top of both gambrel ends, but no ridge vent and no soffit vents. The building has a 12'x12' loft (2x6 on 24-inch centers with OSB decking) covering half of the structure.

Spray foam is cost-prohibitive. I see two options:

1. Just add R-15 (or R-13?) kraft-faced fiberglass in the roof bays without further venting and start saving money to replace the roof framing and OSB in a few years. I realize R-15/R-13 isn't enough, but it's all that will fit in the rafter bays. (Alternative might be some flavor of XPS).
2. Frame out/extend the loft over the 2nd half of the building (effectively creating a ceiling/attic situation), add R-19 kraft-faced fiberglass to the "ceiling" joists (loft floor), and figure out how to add soffit and ridge venting to the newly-formed "attic" area.

I'm leaning toward option 1. Grateful for any guidance you can give. TYVM!!

Why are you saving to replace the roof in a few years? It may never rot out. It is also just an outbuilding. I would not go overboard and start hacking away at my new building until I absolutely had to. It might take decades before you need to do any repair.
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Old 08-22-2020, 09:04 PM   #12
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Re: Shed roof insulation options, on a budget . . .


Thanks APA. You have some very good points. Actually was able to get a contractor to come by today and assess the building. After actually examining the roof, his opinion was that it will vent as it is. He recommended adding insulation and sheet rock. After all, it's a shed, not a house. All things considered, that is probably the way I'll go. I guess there is no risk-free option. Thanks to all for the input.
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Old 08-22-2020, 09:31 PM   #13
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Re: Shed roof insulation options, on a budget . . .


Quote:
Originally Posted by OnyxTheDawg View Post
Thanks APA. You have some very good points. Actually was able to get a contractor to come by today and assess the building. After actually examining the roof, his opinion was that it will vent as it is. He recommended adding insulation and sheet rock. After all, it's a shed, not a house. All things considered, that is probably the way I'll go. I guess there is no risk-free option. Thanks to all for the input.

I have a relative with a vaulted ceiling in their house covered by tongue and groove boards. When they built the house, they crammed insulation in the bays with no venting at all. No air gap. NOTHING. Just insulation touching the deck and no vapor barrier. That was 35 years ago. By all the literature I have read, it should have been raining in that living room by now and the roof should have collapsed from rot. Neither have happened. My thoughts is that it is a crapshoot.
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Last edited by APA; 08-22-2020 at 09:45 PM.
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