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PBC312
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# Posted: 17 Feb 2023 14:14
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I've been kicking around ideas for a small, simple bunkie on a piece of property I own. It will be 10x16 or 12x16. It will have electricity but no plumbing or water. I've seen videos and stories about a "Perfect Wall" house built in Austin which wraps walls and roof with a lot of foam board insulation on the exterior. There is no insulation at all on the inside, including in wall stud bays. This is a high end "building science" project in the south. My bunkie will be a DIY project just for kicks in Zone 6. But I love the look of the open stud bays and ceiling. My idea is to sheath the bunkie with pine or cedar boards, not OSB or plywood, then add some exterior foam board insulation. No insulation on the inside. The pine boards then become inside walls as I will leave the stud bays empty. No drywall. I would do something similar on the roof, using exterior foam board insulation over pine or cedar planks used for roof sheathing, so those planks become the visible ceiling on the inside. I would include an air barrier or self adhering membrane over the pine boards, the rigid foam, then a rain screen with metal siding / metal roofing on top. This bunkie will just get occasional use (maybe 20 days per year) so I just want enough insulation so that it will hold some heat and A/C when it is used. I'm not going for maximum efficiency here. If I put 1.5 inches of foam board on the exterior walls (the width of a 2x4) and 3 inches on the roof, would that be enough to (inefficiently) hold heat and A/C for a weekend visit? I understand you wouldn't want to build a full time house this way that was so under insulated but I'm willing to burn a little extra wood or run a space heater to keep it warm enough for short visits. Not only do I like the looks of the empty stud bays and ceiling it's practical to "finish" the interior. I would just paint the studs, rafters, and pine boards and it's done. Is this crazy? Any obvious or not obvious reasons this wouldn't work? Thanks for any feedback.
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PBC312
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# Posted: 17 Feb 2023 14:17
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Here's a picture of the inside of the house in Austin, showing the stud bays.
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gcrank1
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# Posted: 17 Feb 2023 15:49 - Edited by: gcrank1
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Yes, it works! I did it, well, even less than what you have described, on my 1st cabin build in 1983-84. We used it for over 30yrs heated only with an inefficient antique woodstove I 'rebarreled'. All off-grid and dry, rain barrel for utility water, hauled in potable.
Basically used post & beam main construction (tore down an old barn for salvage), if you havent looked at that do check it out.
Sheathed over, put 1" blue DOW styro outside and re-used the red barn boards on top. If doing today I would do much the same but use the 'rain-wall' technique.
When I built the floor deck (normal type joist) I tacked in 1x2 furring strips 1" down each side and cut/fit in the same styro, then the 3/4" decking. Now it looks like folks are putting the styro right on the joists and decking on top sandwiched in. That would be lots faster and make for a thermal break.
Had an exposed conventional rafters/peaked roof and always had intended to insulate inside and put up 'boxcar siding' but never got 'aroundtoit'. We did just fine even in southern Wisconsin winter visits. I did have to get up about every 4 hrs to feed the stove, still had coals so it wasnt a total cold re-start.
Were I doing it today I think Id go with the 'hot roof' construction technique on a small build, the next step up in efficiency and technique looks to be the 'cold roof' that allows air movement by convection between the insulation and metal roofing.
And rather than the old asphalt shinges Id go metal right off.
Too many cabins are kind of dark inside. Use lots of windows (and think passive solar) and light colors inside to bounce the light around.
LED lights were a game changer, but when you are off-grid energy conservation is important. With elec you have it made!
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PBC312
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# Posted: 17 Feb 2023 16:37
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Thanks for the validation! I'm attaching a photo from a different build that shows exposed ceiling planks, with insulation on the exterior of the roof. I love the look of the cedar planks (pine?) and the contrast with the white rafters. I might try to recreate this look by using 4x6 rafters. It would be more expensive but I think I could space them 32 inches on center in which case I would only need 6 of them on a 16 foot building. (It will have a shed / single slope roof.). Any concerns there? I'll double check the structural questions with someone knowledgeable before I finalize a plan.
Not having to do any work to finish the ceiling and interior walls seems almost too good to be true. Having the open stud bays will also make it super easy to install shelves, hooks, etc. For weekend bunkie type projects it makes me wonder why most people don't go this route. Maybe most people don't like the look. Or maybe it's more common than I realize. Or maybe it's a cost issue. This is such a small building that's not a factor for me.
One question I had is whether I'll have moisture / vapor problems. I haven't thought through where moisture would condensate on really cold days and really hot days on an under-insulated building. But there won't be a ton of moisture aside from what comes from 1-2 people. (No shower, no heavy cooking aside from a coffee pot and a hot plate.) I don't think it should be an issue in part because the exposed stud bays and ceiling should allow moisture to dry to the inside if moisture did build up with people inside for a couple of days. Anyone have thoughts about that?
To insulate the floor my plan was to drop rigid foam between the joists, resting on furring strips, with some kind of hardware cloth underneath for critter insurance. (It will be built on piers or maybe skids.) I haven't heard of people just laying rigid foam right over the joists then putting subfloor on top of the foam. The foam is strong enough to handle that?
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Brettny
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# Posted: 17 Feb 2023 20:16
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Have you thought about buying kiln dried rough cut lumber and running them through a planer for your structural lumber, I guess it really depends on the look your going for
Yes it may sound wonderfull not to have to do any finish work...but in the end your doing it when your still rough framing the building with no weather proofing. It's going to more tedious, more fragile and your one missed nail away from scraping things.
But yes your insulation plan will work find.
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PBC312
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# Posted: 18 Feb 2023 09:28
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I haven't decided on the type of wood for the structure. It will mostly be based on aesthetics and I will need my wife to weigh in. I'll look at the rough cut lumber suggestion. Were you suggesting that for the wall and roof planks and/or for the rafters and 2x4 studs? I was wondering about how the exposed studs will look. I'll paint them, probably white, but I don't know if there are different grades that make a difference. In other words will a stock 2x4 from a Home Depot look OK when painted or might I need to get some higher grade of lumber? I will investigate options.
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gcrank1
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# Posted: 18 Feb 2023 11:03
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My current prev. owner built 'sawmill lumber' cabin is 16x24, one room, peaked 'cath. ceiling'. The lumber is natural, rough cut, sorta the same sizes (lol). Id hate to paint it rough as the surfaces are. It is a good look to us, light/reflective enough too. Says RUSTIC for sure and certain.
In comparison is our Old Hickory 10x16 toolshed/workshop prebuilt installed late last summer. It is the typical 2x4 exposed studs, etc sheathed in the T-111 ply siding. It has that 'unfinished interior' look but its a toolshed. The smooth'ish surfaces would brush paint Ok and probably a roller could be used on the sheathing.
If you did sawmill lumber you could go with full'ish dimension sizing, a 2x4 would be 2x4 instead of the sticks we get now. Structurally for a bunkie, or a toolshed, the sticks are Ok but full size would look more substantial.
The 'impression I get from your description of your vision and the pics is you like the old-school seaside 'cottage' look. It is a good look.
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Brettny
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# Posted: 18 Feb 2023 16:35
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Quoting: PBC312 In other words will a stock 2x4 from a Home Depot look OK when painted or might I need to get some higher grade of lumber
If you like the look of standard studs. A 2x4 you buy at the store will always look like a 2x4 no matter what color you paint it. It's mostly the rounded edges that keep it looking like a 2x4. This is why I asked about buying rough cut and planning it your self. It wont cost any more.
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