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Thread: Cabin dimensions

  1. #1
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    Default Cabin dimensions

    As some of you know, I own some land in the Healy area.
    I would like to build a cabin onsite. Probably just for my wife and I. We would occupy the cabin between May and the end of September. Not continuously but from time to time.
    Thinking we'd probably use wood to heat when needed. Can drive within 100' of property.
    What might be the 'average' size of a cabin in Alaska?
    What 'R' factor of insulation might be appropriate?
    Are there Alaska specific building codes that I may not be aware of?
    What kind of snow loads do I need to plan for?
    Is it ok for me to build for myself?

    Thanks for the help!

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    What might be the 'average' size of a cabin in Alaska?
    16x20, 16x24, 20x20 are all good places to start. It depends on what you want in it and your budget and experience.


    What 'R' factor of insulation might be appropriate?
    I recommend r21 walls and r38 cieling/roof. It's false economy to underbuild as much as it is to overbuild.

    Are there Alaska specific building codes that I may not be aware of?
    Not yet. But the liberal busybodies are always trying to make life suck everywhere. I mean there are methodologies recommended for cold climates but I don't think any "codes" are being enforced outside of municipalities. Build it as you please. As long as it isn't utilized commercially I don't think anyone other than DEC has any concerns.

    What kind of snow loads do I need to plan for?
    Depends on the the roof type but generally 100# esp. if you are going to leave it unoccupied all winter.

    Is it ok for me to build for myself?
    Absolutely.

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    Member AKFishOn's Avatar
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    How does one achieve R38 for the ceiling?

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    How does one achieve R38 for the ceiling?

    With the corresponding thickness of insulation required to get an R38
    Approximately...

    Fiberglass batts about 12"

    Blown in cellulose blanket about 10"

    ridgid foam panels about R5.5/in

    Sprayed foam about R7/in

    your mileage may vary

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    R-38 insulation, lol. They want R-52 for a 5+-star house now.( R-38 is 12" thick) ( R-38 will give you a 5 star, but the banks are looking at the + sign now.)

    To get the R-38,

    Just make sure you have 16" clearance from where the outside wall connects with the top of the truss. It most times include having a heal as they call them built onto the truss. It is common pratice to have 16" from the wall to top of truss so that you can get 4" of air flow on top of the insulation to keep a cold roof. (This is important) This includes a vented soffit, for this to work.

    You can have the truss's built so as to where you have the same effect without the heal, and this method is a lot eaiser for you to install your soffit.

    Kinda depends on the pitch of your roof.

    Let me say this, if you are going to build this without code's involved, a R-19 ceiling in any place under 750 s.f. is more than enough.

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    I used R31 in my roof. No room for more.

    Narrower and longer saves money. I went 20 wide and would drop to 16' wide if I had it to do over.

    Wood heat is great.... until you go to sleep. Then it runs out. I plan to supplement my wood stove with a diesel heater for early and late season use. Check PM.

    Look at a Saltbox style roof. It would seem easier to do remotely.

    Mike

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    10 inches is considered an energy heel on trusses and minimum required for ventilation depending upon insulation type. A 1 inch space the length of the eaves between rafters over the bird blocks is adequate for ventilation at the heels.

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    I'm not following you Mics. ( the man was talking R-38) (R-38 is 12") and your talking 10"

    I'm talking from where it sits on the wall, to a squared up figure to the top of the truss.

    The 4" air flow is in no question, that is what they want, unless you install a top of the roof equlizer.

    1" is not going to do you.

    Go to home depot, or others and they have the 4" baffels to install so you can blow in more and still get the same airflow.

    Another thing is there is a calculation about how much air is supposed to flow, same as if you were to install fin tub for a hotwater system, there is supposed to be so much fin tube per square footage.

    I'm sure from some of your other posts that you have been doing this for some time, but I'm not geting it. Heck I've even learned a couple things from you, but this is not one of them.

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    10 inches of celulose in a cieling is R38

    a 10" heel on a truss is an energy heel.

    10 inches at the exterior of the wall from the plate gives you more than that 6 inches inboard at the interior of the wall. Again as I stated previously...this is the minimum heel requirement depending upon insulation type.

    The code is as follows...
    2006 IRC
    R806.1 Ventilation required. Enclosed attics and enclosed rafter spaces formed where
    ceilings are applied directly to the underside of roof rafters shall have cross ventilation
    for each separate space by ventilating openings protected against the entrance of rain or
    snow. Ventilating openings shall be provided with corrosion-resistant wire mesh, with
    1/8 inch minimum and V* inch maximum openings.
    R806.2 Minimum area. The total net free ventilating area shall not be less than 1 to 150
    of the space ventilated except that the total area is permitted to be reduced to 1 to 300,
    provided at least 50 percent and not more than 80 percent of the required ventilating area
    is provide by ventilators located in the upper portion of the space to be ventilated at least
    3 feet above eave or cornice vents with the balance of the required ventilation provided
    by eave or cornice vents. As an alternative, the net cross- ventilation area may be reduced
    to 1 to 300 when a vapor barrier having a transmission rate not exceeding 1 perm is
    installed on the warm-in-winter side of the ceiling.
    R806.3 Vent and insulation clearance. Where eave or cornice vents are installed,
    insulation shall not block the free flow of air. A minimum of 1-inch space shall be
    provided between the insulation and the roof sheathing at the location of the vent.

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    Misc, that is one I'm going to have to look up, thanks for that info tho.

    As far as the cabins that are being built in the woods I dought that they are going to be built with blown in insulation.

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    fiberglass works fine as well with 10 inch heels.

    Attic space.jpg

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    If you're going with piers then I suggest limiting the width to 20' max so you can support the structure with two beams. A third beam is a pain when it moves and jacks. Two is way easier to manage. With 20' width you can set the beams at 16' spacing and cantilever out 2' to the exterior walls. That makes venting the floor very simple. A 16' span between the beams is easy to manage with 2x12 lumber or BCI engineered joists. I stayed with 16' width because 20' was considerably more expensive. Longer lumber gets premium pricing. If you want a high-R ceiling the easiest way is to build some parallel chord trusses with enough depth to contain your fiberglass and provide vent space above it. Build a jig on the floor and you'll make your trusses in no time. You can use smaller lumber stock and save money, too. Eave vents and a ridge vent are necessary. Staple cardboard baffles into place and run the fiberglass over the exterior wall line.

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    I have a cabin 16x24 two story, and only 2x6s with open ceiling. It is not done yet and I have to insulate, it will mostly be used in the summer. Wondering if I could get by with R-21? Or should I blow in insulation to get a higher R factor? I have not looked at the costs of blow in, nor have I fully looked into proper venting yet, but the second story gables are not done yet either so I have a little time to think about it. Any thoughts on how to insulate the roof would be appreciated.
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    IMHO 4" ridgid (2x2") foil backed foam panels cut tight to fit leaving a space (1 1/2") to vent to ridge vents would be easiest fastest most cost effective.

    "blown in celulose" will tend to settle and not be my preference in your case.

    fiberglass and celulose isn't as effective as foam.

    If sealed around the edges the r-teck foil backed foam will meet perm rating for vapor barrier and eliminate the need for poly. You can tape it or use urethane canned foam to seal it...or put up a poly vapor barrier.

    The problem with celulose of fiberglass in this type of hot roof is IF moisture does occur to soaks the insulation and decreases the insulative value tremendously which causes more heat loss...more condensation..more r-value decrease...more condensation...yoou get the picture.

    Ridgid foam products tend to repell water and keep it's r-value.

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    I am a little concerned with your ridge support over a window on the gable wall with only a 2x6 header. I suppose it will be ok once the gable is framed and fully sheathed.

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    Heck, a rafter roof doesn't need a center beam or posts. I wouldn't worry about the header at all. For insulating, are you thinking of framing a flat ceiling? That would make insulating and venting way easier. If staying with a cathedral? I'd drop a false ceiling down several inches so you can use a thicker fiberglass batt and still leave an air gap between fiberglass and roof sheathing. It'll cost you some lumber and a few hours of framing but it'll be a good investment down the road.

    Lots of cabins out in the woods have 2x4 roof framing packed full with R-11 fiberglass and no venting. They seem to work, too. Do a good vapor barrier and keep the rodents out and yours would last a good long time, too.

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    Good point on the current ridge support, the gable will soon (I hope) be finished and sheeted. I will have to price out the 4" of foam panels, that might be my best option. I still have to do some research on the venting. I do plan on keeping it an open cathedral ceiling.

  18. #18

    Default Floor Venting Question...

    Quote Originally Posted by Mr. Pid View Post
    With 20' width you can set the beams at 16' spacing and cantilever out 2' to the exterior walls. That makes venting the floor very simple.
    Mr. Pid-

    I've heard others talk about "Venting the floor" -- where exactly would the floor need to be vented? I am assuming this would apply if there is a layer of plywood under the floor joists and then you're talking about leaving an air gap between this bottom layer of plywood and the insulation?

    I'm just trying to figure out where and why a floor would need to be vented.

    Thanks,
    Corn.

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    Bottom line for cabins. If you have a 16x20 and are heating it with wood your biggest battle is finding a stove small enough not to cook you out.. Anything over R-38 at that point is just wasting money in the roof. Lots of cabins around here with 2x12 rafters and R-30 in the roof, 1 1/2" airspace, R21 in the walls. Can heat the dang things with a candle if you get your vapor barrier tight.
    Bunny Boots and Bearcats: Utility Sled Mayhem

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    Heck, a rafter roof doesn't need a center beam or posts. I wouldn't worry about the header at all.
    That's a bit of an exageration.

    Structures with no interior walls for lateral support for midspan load bearing walls need a ridge that will carry the roof load. A roof that relies on the walls not spreading apart will sag if the walls are pushed outward, or if the rafter ends aren't fastened properly at the walls.

    Even on continuously sheathed structures, the roof structure, wall structure and floor structure all work together to stay ridgid. A system is only as strong as it's weakest element.

    How much sag would have to occur over a door of window in a gable wall from floor sag, ridge sag, lumber shrinkage, poor nailing of sheath/framing, wind or seismic movement to cause a door of window to stick?

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