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Author Topic: Small cabin insulation requirements  (Read 1971 times)

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Offline TimFromNB

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Small cabin insulation requirements
« on: November 03, 2016, 12:21:04 pm »
Hi folks,

I am trying to finalize my SketchUp plans for a 16'x20' cabin I want to build next summer. I still need to figure out the window sizes in order to place the girts in the right places.

My plan for enclosing it is a wrap and strap. I will have 4/4 vertical boards on walls and horizontal on the roof. I'll cover the whole thing in felt paper, then cover with a layer of rigid foam held down with house wrap and strapping. My question is how much insulation is really required for a cabin that will be at most used every weekend during winter? I live in a colder climate with the average low temperature in January/February around -14C/6F.

My plan was to use 2"x4'x8' XPS sheets (http://www.kent.ca/kbs/en/product.jsp?skuId=1620820&prdId=1620820&catalogId=45), however I'm not sure it is enough. But the stuff is expensive compared to the rest of the materials.

My plan is to heat with a small wood stove.

Looking for your input, thanks.



Offline timberwrestler

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Re: Small cabin insulation requirements
« Reply #1 on: November 03, 2016, 01:39:12 pm »
I did the exact same cabin with 2" XPS as an outbuilding, and it stays quite toasty with a small woodstove.  They also have a very small minisplit.  It can't hurt to put more insulation on.  Look around on Craigslist (or the NB version of it) for reclaimed rigid foam.  It's generally 1/4 to 1/3 the price of new. 

Offline ChugiakTinkerer

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Re: Small cabin insulation requirements
« Reply #2 on: November 03, 2016, 01:39:41 pm »
I think you are on the right track to consider having less insulation for the weekend cabin compared to what would go in a full-time residence.  My planned cabin build is a little bit bigger than yours but I'll be facing the exact same issue.  If you check out the cabin forum on Alaska Outdoors, you'll see some threads that talk about wood stoves and weekend cabins.  Typically a small stove will output way more heat than the small cabin needs, and you end up opening a window to maintain a comfortable temperature.

By cutting back on the insulation you can allow the heat to pass through the walls and roof rather than through an open window, allowing for a more even temperature.  This is especially so if you're looking at a high efficiency stove that doesn't allow you to completely shut off the oxygen to the burn chamber.  It will burn hot and you'll probably not want your cabin to be a sauna.  The problem I see is that it is counter-intuitive to have less insulation in order to be comfortable.  And there doesn't appear to be any useful rule of thumb for how to size your stove or how much insulation to use.  I'll probably just wing it by insulating at half the R value of what would otherwise be recommended.  The alternative would be to go through some painfully complex thermodynamics calculations that I currently know nothing about.
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Offline ballen

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Re: Small cabin insulation requirements
« Reply #3 on: November 03, 2016, 05:05:15 pm »
I agree that a wood stove will definately keep you warm. The only issue to consider is if you can keep it going all night long. I know I will be adding more insulation to my cabin for this reason.

Offline TimFromNB

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Re: Small cabin insulation requirements
« Reply #4 on: November 03, 2016, 05:24:55 pm »
Thanks for the replies. I checked on kijiji (Canadian Craiglist) and actually found this: http://www.kijiji.ca/v-renovation-other/moncton/3-type-4-sm-styrofoam-insulation/1211868535

It is 3" and cheaper than the new 2". There is enough to cover the roof and walls. I just need to make sure I can use it for exterior walls.

Tim

Offline ChugiakTinkerer

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Re: Small cabin insulation requirements
« Reply #5 on: November 04, 2016, 02:09:22 pm »
Looks like an excellent find.  The Dow XPS that I have worked with is all R5 per inch of material.  Do the marking pictured give any clues if this stuff is R5 as well?

For a weekend cabin it may not be much of a concern, but the felt wrap isn't a truly a vapor barrier.  As I understand, it is a moisture barrier much like Tyvek and is used to keep rain away from the wood.  You might give some thought to replacing the felt with a less permeable material.

Edit: Looks like I need to retract my comment about vapor barriers.  US Dept of Energy has a publication about insulation and moisture control: cold_climate_guide_40percent.pdf (39.57 MB)

Page 8.18 has this to say
Quote
Use of rigid foam exterior sheathing reduces the need for a Class I
or II interior vapor retarder. This insulated sheathing reduces the
probability of condensation inside the wall. In hot, humid weather,
insulating sheathing acts as a vapor retarder and helps prevent
moist outdoor air from entering the wall, reducing the potential for
condensation to form on the back side of the interior gypsum board
of air-conditioned homes. In cold weather, the exterior foam layer
minimizes thermal bridging from the studs to the outside and keeps
the wall cavity temperature above dew point, reducing the likelihood
of condensation forming on the interior surface of the sheeting. If
the foam is taped at the seams and caulked at the edges, it serves as
an air barrier and rain screen.



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Offline TimFromNB

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Re: Small cabin insulation requirements
« Reply #6 on: November 04, 2016, 07:11:42 pm »
Thanks. Worst case I can add a layer of poly over the felt. The felt is to give a black background between the boards after they dry and shrink.

Offline AlaskaLes

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Re: Small cabin insulation requirements
« Reply #7 on: November 06, 2016, 02:31:52 pm »
We've made the transition recently to Living in our cabin.
The first 8 years were a weekend thing.
We live in a 12x32 cabin/dbl loft right now and will be for another year, till our house is dried in.
Cabin has good vinyl windows and a 50K btu Jotul woodstove.
Things that I've learned:
-Foam is way better than batting will ever be and the ratings that were started by the fiberglass manufacturers really don't cross over to the foam sheet accurately.
-In real Northern climates it is FAR better to have too much insulation and too much stove for the application.
You can always turn the stove down(get a good one) and you can always open a window.  It is a fun killer to have cold trying to seep in everywhere and making your cabin an icebox.
-Foam acts as an excellent vapor barrier and will slowly allow any moisture that may be in the used foam to dissipate out.
Good find on the foam. We picked up about 160 sheets of 2'x8'x4" thick blue foam for $8 a sheet a few years ago.
Serious score and we're still burning through that.
Matter of fact I was in town yesterday making a deal for a huge pile of 2" & 2.5" metal T&G/urethane foam siding that a contractor is trying to clear away.  Should be enough to side the Wife's 12'x20' craft shop and a new 30'x70' woodshop.
The 16'x32' metal/fab shop is built with the 4" foam and it is incredibly comfy with a 40K btu woodstove.
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Offline bic

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Re: Small cabin insulation requirements
« Reply #8 on: November 09, 2016, 06:54:54 am »
I'm with Alaskales, I researched this insulating issue to death, and ended up going with spray foam for the ceiling (cathedral). When it was all said and done it was a lot less labour and very little diffrence in cost to batts. Something to keep in mind that you will get to "know" your wood stove. Make sure it has a good adjusting damper system, and well cured wood, my opinion is I'm not real concerned about using a few extra sticks of wood so long as when I open and close off the damper I can hear the fire react to same.
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Offline TimFromNB

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Re: Small cabin insulation requirements
« Reply #9 on: November 09, 2016, 08:31:26 pm »
Thanks for the replies!

Offline TimFromNB

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Re: Small cabin insulation requirements
« Reply #10 on: November 26, 2016, 07:56:57 pm »
I've been able to find the minimum effective insulation value in my area which is R 16.9 for walls and R 28.5 for cathedral ceilings. For camps/cabins, insulation doesn't need to meet code.

I will be using EPS over the enclosure. I was thinking of 3" on the walls (R 12.78) and 5" on the roof (R 21.3). Do you guys think this would be sufficient?

Thanks,
Tim

Offline CJennings

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Re: Small cabin insulation requirements
« Reply #11 on: January 02, 2017, 07:46:59 am »
Do you have carpenter ant issues there? Foam insulation is a magnet for carpenter ants where my cabin is in VT so I went with fiberglass. Or rather am as it's not finished yet. The 15x15 cabin I built was un-heatable without insulation. All I'd do is throw wood in the woodstove and fireplace and it would maybe be 10 to 20 degrees warmer than outside. When the stove went out at night you'd know it quick. Just insulating the roof made a massive difference and I can't wait to get the walls done. So put me in the more insulation is better crowd.

Offline ScottCC

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Re: Small cabin insulation requirements
« Reply #12 on: January 05, 2017, 11:53:57 am »
I would like to advise given the small size, you insulate to code.  There may come a day the value of the camp will financially benifit you more if it could be considered a home.  Bank valuation and all.  The labor would most likely be the same to do so, just not materials.
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Offline DDW_OR

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Re: Small cabin insulation requirements
« Reply #13 on: January 05, 2017, 01:28:12 pm »
Looking for your input, thanks.

(Image hidden from quote, click to view.)

METAL roof

move the door to the left wall between the two windows, that way the snow/rain from the roof does not fall in front of the door and onto you.
have the roof overhang a minimum of two feet on all FOUR sides.
i assume the cabin has road access and grid power?
how are the bears in the area?
how are the bugs in the spring, summer, and fall. maybe a screened deck?

remember to remove all fluids, water, sodas, drinks, etc when you leave.

add one to two of these thermo fans on top of the stove
http://www.kent.ca/kbs/en/product.jsp?skuId=7165005&navAction=jump&navId=811&prdId=7165005&catalogId=811

choose a stove with a flat top so you can cook on it
and has an ash tray.

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Offline TimFromNB

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Re: Small cabin insulation requirements
« Reply #14 on: January 09, 2017, 12:02:21 pm »
Hi DDW_OR,

The plan is to have a metal roof and at least a porch to cover the door entry. Bugs are bad in summer, but OK the rest of the year. We do have bear and moose in the area in and around the camp. There is currently an old skidder trail leading to the camp location, it is about 1.5 km / 1 mile from the road and no power. I have to work on the trail so I can get in with a truck or tractor when the ground is thawed. There is no shale or any base to the road, only the natural top-soil so it turns to mush pretty quickly.

Thanks,
Tim

Offline RPF2509

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Re: Small cabin insulation requirements
« Reply #15 on: January 18, 2017, 05:25:20 pm »
I second the move the door comment.  Nothing sucks like having to shovel your way out the door after a heavy snowfall esp if your door opens outward.  You could put a small dormer over the door but its easier to exit onto a porch on the building ends - plus that's where you stack your wood for the weekend so you don't have to slog through the snow to get a piece.   A good 2' roof overhang is a blessing too, more on the ends- helps keep snow and rain splash off the walls.  I lived in rain country and now snow country and an overhang is just the ticket.

Offline AlaskaLes

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Re: Small cabin insulation requirements
« Reply #16 on: January 20, 2017, 04:13:12 pm »
Funny that I woke up to see this thread again.
We just had a few days where it got down to -37F and the 55,000btu stove was working to keep up.
You just can't have too much heat or too much insulation.

I'm voting for the door under the extended gable overhang.

You can see Mt McKinley from our backyard...Up Close!!

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Offline DDW_OR

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