The Great Cabin Erection!

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Re: The Great Cabin Erection!

Post by halfwise on Sat Feb 10, 2018 5:07 pm

Foam makes sense. But I expect the main vapor barrier will be below the fiberglass insulation?

I know a friend who did a DIY addition to her house, put the vapor lock outside the insulation, ended up getting rained on when the vapor condensed against the cold plastic. I learned then that if I ever was to build something I'd keep that simple physics in mind and keep the vapor lock below all insulation.

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Re: The Great Cabin Erection!

Post by David H on Sat Feb 10, 2018 5:18 pm

halfwise wrote: But I expect the main vapor barrier will be below the fiberglass insulation?
that's what i'd do. i'm in a particularly wet climate but having a separate vapor barrier and air space below each layer of insulation makes a lot of sense to me. otherwise you can expect soggy insulation and rotten roof boards over time. what's the wood treatment you're using forest?

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Re: The Great Cabin Erection!

Post by Forest Shepherd on Sat Feb 10, 2018 8:20 pm

The wood treatment is Bora-care, a brand of borate solution that you dilute with water. I added blue dye for ease of knowing what I already hit. I don't know if it will actually work, but, I mean, it should do something. I freaked out after I found thousands of dry-wood termite wings all over my new construction last fall, so I ordered a bunch and tried to soak every square inch.

The silver is a radiant barrier on the underside of the roof sheathing. I believe it's perforated so that must mean that it's not a vapor barrier as well. I think it's necessary in the cabin because of all the direct sunlight it gets in the summer.

I had been talking to my boss about what to do for proper ventilation, and he had scoffed at the idea of a ventilation channel, saying that it wasn't needed. I decided to go for it anyway, and am using a cheap-o ventilation channel insert that basically allows air to flow into the roof from underneath the eaves, up through the roof between the roof sheathing and the fiberglass insulation, and then out through the ridge vents I've installed already. Basically, you can't see it in those pictures because I hadn't decided to use it yet. (I had to pull out the insulation and put it back in again.)

California has a pretty dry, warm climate. The coast is more humid, but overall it's quite dry most of the year. Perhaps that is why I'm not actually familiar with vapor barriers in my work. I've never had to put one in and haven't seen any when doing demo work. I don't think they're needed here in regular houses.

Except the roof is kind of different. This is rather confusing to me, even after reading some more about it. Basically, as I understand it, I had two main options for my cabin's cathedral ceiling: spray-foam insulation or fiberglass insulation. Spray-foam insulation is, you know, spray-foamed between the rafters on the underside of the roof sheathing. It forms a vapour, air, and thermal barrier. Not much heat travels through it, and no air or water vapour can get through it. Once it's in, you don't need anything else, you're good to go. Cover it up with your ceiling material of choice. This keeps the hot or warm air in your ceiling away from your roof sheathing and rafters.  

But fiberglass insulation is different. It's a thermal barrier, but vapour and air can flow through or around it. The sheet-rock I'm putting on my ceiling is an effective air barrier, but not a vapour barrier. So what you do then is to include an open channel between the roof sheathing and the insulation, and then drill holes or whatever in your soffit and put a continuous ridge vent in your roof's ridge. This allows the hot air in your ceiling to flow out the ridge and be replaced by cooler air from under the eaves in a self-regulating way. This drives the hot air up and away from your roof sheathing and rafters.

As I understand it, this second method keeps my roof cooler overall than the first method, which is a fine thing. But both methods keep condensation from forming on my roof sheathing or rafters when outside and inside temperatures are different from each other. The first method by keeping vapour away from the roof sheathing and rafters, and the second by allowing the roof to breathe out any warm humid air and breathe in cool air that matches exterior levels of humidity.

Does that make sense? I feel like I needed to type this out so I could reassure myself that I knew what was going on here.

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Re: The Great Cabin Erection!

Post by David H on Sun Feb 11, 2018 5:58 am

I think you're wise to go with the ventilated roof method Forest. It's a little more work to install but it breathes in a much healthier way for the whole house, plus it's a lot easier to track down and repair a leak or rot issue if you ever have one [a serious consideration here in coastal WA with 6-8 ft of rain a yeaar....]

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Re: The Great Cabin Erection!

Post by Forest Shepherd on Mon Feb 12, 2018 9:10 am

Send some of that rain down this-a-way whenever you feel like it. We haven't gotten rain for weeks and weeks now, right smack-dab in the middle of the "rainy season". It's crazy that you can get that much rain. I remember four years ago we had fewer than 10 inches for the whole year, counting total precipitation. I think with just rainfall we had so little rain that a few years later we had more rain in one weekend than we did throughout that entire winter.

I hate it. I really do. I know that frigid winters suck in their own way, but I sure do miss Minnesota rainstorms in the spring and summer.

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Boar is badger, named after wood,
Not after forest but trees.
Where did you play on a rainy day?
Where did I eat bread and cheese?
Search inside, stay indoors,
Look up and find the secret is yours.
Your castle your fort,
Or so you thought.
The way is in four trees.
The way is in Boar in Brockhall
Under ale, under bread, under cheese.

-Mossflower, by Brian Jacques.
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Re: The Great Cabin Erection!

Post by halfwise on Mon Feb 12, 2018 1:58 pm

I could talk about global circulation and rainfall patterns, but shall opt out.

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Re: The Great Cabin Erection!

Post by David H on Mon Feb 12, 2018 2:47 pm

Forest Shepherd wrote:Send some of that rain down this-a-way whenever you feel like it.

It's your for free* Forest! Very Happy

*Transportation not included.

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