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bukhntr
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# Posted: 29 Aug 2012 05:39pm
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I am starting construction on a 12x28 barn style half loft. The ceiling is off the trusses so no attic. It will be insulated and only heated and cooled when occupied. I am told I need to ventilate to keep condesation from forming when the building inside and outside change temps. Makes sense to me. Does anyone have experience with this happening or not. I am considering an 8x16 vent in the gable end to open and close leaving it open when not there for air to exchange. Would one work on the non loft end. I hate to leave a window cracked for rain to come in or critters and a second vent would be hard to squeeze in above the window at the loft. Thanks for reading
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MtnDon
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# Posted: 29 Aug 2012 06:32pm - Edited by: MtnDon
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Not sure I interpret the design correctly, but it sounds like the ceiling is to be fastened to the inside face of the rafter or truss material that the sloped roof sheathing is attached to on the exterior. ???
Ventilation is required in the rafter bays from soffit to ridge, above the insulation and below the roof sheathing. That is it is required with the usual fiberglass batt insulation. With batt insulation and no venting there will be condensation of water inside the insulated space up against the cold roof sheathing.
Ventilation can be unnecessary if the roof is either sheathed externally with several inches of overlapping rigid sheet foam. Thickness depends on the climate zone. Another non vented solution is to have the underside of the roof sheathing sprayed with foam. At least a couple inches usually works, but again varies with climate.
What size are the roof rafters going to be. You should probably consider oversizing them by a big factor as far load requirements go, in order to have sufficient depth of insulation. Sufficient depends on climate again, but roofs like that normally are built with an R-30 as a minimum.
What type of heating is being considered. Unvented or vent-free propane or kerosene heaters add lots of moisture to the interior air, making the condensation problem even more serious.
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bukhntr
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# Posted: 29 Aug 2012 11:08pm
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we are just having simple 2x4 rafters since it is only a 12' wide building and being barn style the lengths are not more than 5'. I am planning to put the ceiling against the inside of the truss with insulation between. Probably just r-13. It is a recreation cabin not a main living type cabin. With the small size it will heat ok with reduced insulation than you would need in a home. I am thinking against the ventless furnace leaning more toward a vented model for safety and reduced moisture. tell me more about filling the cavity with foam. are you talking about spray foam in a can or something a contractor would install. To recap I do not have an attic to vent, so my vents will be common to the living area. Thanks for replying
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Rob_O
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# Posted: 29 Aug 2012 11:24pm
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If you stuff R13 in 2*4 rafters without venting above the fiberglass you will have condensation and eventually mold. Your only choice for what you describe is spray foam - not the stuff at the home store
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bukhntr
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# Posted: 29 Aug 2012 11:28pm
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What type of spray foam do you suggest, is this something I do myself. Will the walls be ok with standard fiberglass insulation?
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Rob_O
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# Posted: 30 Aug 2012 12:18am
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You can do it yourself, the kits are available online. I'd recommend you build the roof with 2*6 rafters and put 1"of spray foam plus the R13 at a minimum
Fiberglass insulation should be fine in the walls, as long as you don't build a moisture trap
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TheCabinCalls
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# Posted: 30 Aug 2012 11:03am
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The other thing you can do if you haven't roof the place yet is to put rigid foam on the top. If you want 2" use two 1" boards so you can stagger the seams. Also polyiso will get you most r-value and vapor barrier.
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bobbotron
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# Posted: 30 Aug 2012 02:54pm - Edited by: bobbotron
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I know it's popular to suggest these days, but seems to me like using big rafters would be way less of a pain than trying to stick a ton of foam boards over a roof deck. If I did my cabin again, it'd be with 2x10 rafters (or more?), less of a roof pitch, and skip the foam on top unless it's a really shallow pitch, getting one layer of decking up there is enough of a headache. :P
Edit: I say this having never tried either way. Dragging 2x10 or 2x12 rafters up a roof would also be painful, I'm sure.
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MtnDon
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# Posted: 30 Aug 2012 03:13pm
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Quoting: bobbotron but seems to me like using big rafters would be way less of a pain than trying to stick a ton of foam boards over a roof deck.
I agree. Add ing foam on top works best when retrofitting insulation and needing a new roof at the same time. The issue is with lining up the long screws used to secure everything with the rafters. It can be done, but hauling rafters with a greater depth to the roof seems easier to me as after that everything remains the same.
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bobbotron
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# Posted: 30 Aug 2012 03:45pm
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Quoting: MtnDon I agree. Add ing foam on top works best when retrofitting insulation and needing a new roof at the same time. The issue is with lining up the long screws used to secure everything with the rafters. It can be done, but hauling rafters with a greater depth to the roof seems easier to me as after that everything remains the same.
And in the process you get one seriously strong roof!
Bringing the 2x6's up to the ridge board was seriously not my idea of fun, but I think if one make a clamping holder of them, bringing up larger wood wouldn't be too bad.
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PA_Bound
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# Posted: 31 Aug 2012 08:29am - Edited by: PA_Bound
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bukhntr... About 10 years ago a few fella's and I built a 16'x16', one room hunting cabin. It's typical- stick-built, 2x4 walls insulated to about R-13 (ceiling uninsulated (never got around to it)), painted drywall through-out. Standard roof- vented at the eaves, ridgevent and one gable vent. Built on piers (open underneath) and heated by a small woodstove. The interior living space was not vented, and all windows were closed tight. We use it mostly during hunting season, but do come-and-go a bit year-round.
We've never had any problem with condensation in the walls, ceiling or attic, but one summer 4 or 5 years ago the interior living space got damp by an unusually humid period of sumer days. Coupled with the summer temperatures, the place mildewed badly. The drywall and wood was fine, but any textiles (sleeping bags, blankets and pillows especially) had to be immediatly washed or, is some cases, replaced. And the smell was horrible. You almost couldn't stay there.
We have two standard double-hung windows on opposite walls, with the top protected by the eaves. Ever since we've taken to leaving each of them cracked about an inch. That's provided just enough ventilation to prevent this problem from occuring again.
In the recreation-use cabin I'm building now, I will definately make sure the interior living space is vented- with both a vent high enough to let the hot-humid air escape, and a vent low to let the cooler air in. Something that I can also close off if I'm there in the winter. I'm not sure exactly how I'm going to do this yet, but I have a couple ideas in mind.
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bukhntr
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# Posted: 3 Sep 2012 08:55pm
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ok changed my mind about the barn loft, I have changed the roof plan to a 5/12 with overhangs. i will have soffit vents and 2 gable end vents. Now I have a full attic that will breath. Not worth the worry. do i need ventilation for the inside when I am not there as mentioned by PA bound. I have windows I could leave cracked but would prefer to keep closed for weather and critters.
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OwenChristensen
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# Posted: 4 Sep 2012 07:37am
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You don't need it, but it will keep the stale smells down. It gets a bit funky in my cabin after weeks of being shut up.
Owen
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