Insulation and peace of mind

[Rocksteady]

In all honesty, I'm still a ways away from breaking ground on this project so my enthusiasm is channeled into what some might call overzealous planning.  I plan on building a 14'x26' house in either a 5 or 6 climate zone (I can't live without legit winters).  The plans I have call for standard 2x4 framing.  I was planning on insulating with bonded logic's recycled denim R-13 batts in the walls, R-21 in the floor joists and R-30 between the attic floor joists.  This is essentially what the framing will allow.  I would like to heat ONLY with wood in a centrally located Jotul F 602 cb, which is rated at 28,000 btu/hr, and is capable of heating up to 800 sqft according to Jotul.  Seeing as how the house's footprint is only 356 square feet I am hoping I can get away with only mediocre insulation.  I am extremely interested in efficiency, but I think it's a two way street.  I'm only interested in adding more insulation/thickness to my walls if it's a must. Using more resources in my walls simply for the sake of a higher r-value seems silly to me and ironically an inefficient use of resources.  Anyone with experience heating small homes in cold climates please give me your two cents!

[NM_Shooter]

Actually, I think you would be better served with more insulation for small spaces, rather than less.  But I don't think what you are describing is mediocre.  It sounds pretty good.  Problem with the small cabins is that they have a higher ratio of window to wall space, and your windows are going to be leaking heat well before your R13 walls.

I stayed in a cabin that had no insulation other than OSB walls.  It had a wood stove for heat, and was 16X24.  The nights were cold...high teens or so.  We'd get that woodstove roaring before bed, warm the place up, and be freezing by 2 a.m. 

You get a heat soak in the stuff in the cabin, and the insulation keeps that from escaping.  You could run your wood stove all night long, but they are hard to modulate.  Trying to get it just right will be an art. 

Having more insulation in your cabin will allow it to stay more consistent, allow for longer heat cycles, and give you some additional sound insulation too. 

Insulation is pretty cheap. 

Having a flat, insulated ceiling helps a bunch too....

[MushCreek]

Pay very close attention to sealing everything well- some say it is as important as insulation. They seal everything these days- electrical outlets, plumbing penetrations, rim joists, the list goes on and on. Caulking and foam cans are cheap as long as you're doing the work yourself.

[Brian_G]

They seal everything these days- electrical outlets, plumbing penetrations, rim joists, the list goes on and on.

They do seal everything these days, but this has created some problems too, making a home to tight creates indoor air quality issues such as moisture build up (from showers, cooking and even breath) it can also seal in any chemicals that are used in the home (cooking spray/hairspray) While sealing is good like most things in life it should be done in moderation. The home needs to exchange the air inside a home a minimum of 4 times in an hour to maintain a healthy living space. This can be accomplished by either what most would consider air leaks in the construction, or through mechanical means (energy recovery ventilator) to me adding an E.R.V. is kind of silly as why would anyone want to add another mechanical device that could fail and requires maintenance and power when they can allow it to happen naturally. Just some food for thought.

Brian

[Rocksteady]

Thanks for the responses!  I'm not sure where I stand on the whole sealed house vs. breathable house issue.  I can totally see both sides of the argument.  As far as an Heat recovery ventilator is concerned, I'm not sure I could justify the expense in such a small space, and I too have a concern adding one more breakable appliance to a house.  I'll already have three mechanical air exchangers in the form of my kitchen exhaust fan, my bathroom fan, and the wood stove.  Between my kitchen and bathroom alone, I could move all of the air out of my house in a matter of minutes.  I'm trying to straddle the line between efficient building use, and efficient material use.  I don't want to eat up floor space (4 inches extra on both sides makes my living room/dining room a mere 12' 11") with a 2x6 wall unless it is essential to the comfort,usability, and longevity of the building.  Keep the responses coming! As far as I can tell in my research a 2x4 in a structure this small with a wood stove at 28,000 btu/hr should be just fine.

[Redoverfarm]

Depending on your budget you could spray the walls with foam and gain more R value in less depth of wall. Another alternative is to use foam insulation sheets which will give more R-value.  I would probably use more ceiling/attic insulation as that is where your major heat escapes other than windows.

As far as the air exchange.  Yes you have fans to remove moisture/odors but that air has to be replaced when expended to the outside.  The woodstove will do nothing to exchange air.  It will draw air from within the room and put it out the flue.  What heat is derived is just radiant.  If there is a blower on the stove it will just circulate air not replinish it. 

One true test is if your stove will not draw properly and the flue heigth is not an issue then your house is too tight to allow air movement for the stove to draw air to combustion.

Just my $.02

[JRR]

Yes, a smaller house requires less insulation thickness for the same total heat retention.   Even though the heat loss is greater per square foot of exterior wall or roof... there are just fewer square feet total of exterior surface area for heat loss to occur through.  Simple physics and math.  I (also) would suggest you apply the insulation, and vapor control, on the exterior of the framing system.  Because you have a simple shape to work with, it would be very easy to apply felt paper and rigid foam onto the wall sheathing and roof sheathing ... then furring strips to support the exterior wall cover and the roofing.  This approach mimizes the chance of ever having wetness issues in the framing system.

Unless you have other restraints, I would suggest a 16' width instead of 14'.  Won't cost that much more. ... and the more closely you approach a "cube" shape, the better for insulation effectiveness. 

[MountainDon]

I don't want to eat up floor space (4 inches extra on both sides makes my living room/dining room a mere 12' 11") with a 2x6 wall

I went through the same thing while designing our cabin. We started with a 12 foot wide design. Moved to 14 quickly and then settled on 16 ft wide. I am glad we have the extra thickness in 2x6 walls both for winters snows and for summer heat.

One thing that proportionately eats up a lot of space in a small cabin is the wood stove clearances to combustibles. That was one reason we went wider and longer (30 feet).

R45 in our ceiling. We don't have to worry about keeping a fire going overnight. And once the interior is warm it does not take much wood to keep it comfortable, just a few pieces now and then during the day. Cooking also adds heat, especially if some bread is baked. We use an exterior fresh air inlet for the wood stove.

[Alan Gage]

I'm in climate zone 6. Started off planning to do 2x4 walls in a 500-600 sq. ft house. Why bother with more insulation since I'll be heating with wood and it will be free? Plus it's a small space anyway.  Well, as planning progressed things started to expand I ended up with a 1000 sq. ft. house with 10" thick walls. Maybe a bit of overkill but I have no regrets. Every room in the house is comfortable, even the farthest away from the stove. Nice not to be constantly feeding the stove.

Air sealing is extremely important as that's where much of your heat will be lost. I was pretty religious about it with air tight drywall, which also meant sealing around every outlet and light box. I went with an exhaust only air exchange system which uses a couple Aldes fresh air inlets in the bedroom closets. So when the kitchen and bath fans run that's where it should draw the fresh air in. The bath fan is set up on a timer that runs it a set amount of time every hour. The wood stove has dedicated make up air from outside the house.

My build in case you're interested:

http://countryplans.com/smf/index.php?topic=10497.0

Alan

[Woodswalker]

My cabin is 320 sqft, one room, vaulted ceiling, and sheetrocked.  Has insulation similar to what you propose, and heated by a woodstove same size as yours.  I have not stayed there in the dead of winter, but have seen spring temps in the 20's and summer near 100.  I think you will be fine with your proposed insulation/woodstove combo.  The sheetrock I used adds considerable mass that really helps moderate temp swings.  If you have electricity for a ceiling fan, that would help both the heating and cooling.

Steve

[Squirl]

I have a 12x8 with R13, walls, R19 roof and floor.  I don't heat with wood.  It heats up quickly with propane, but it will get cold by morning.  As NM_Shooter said, insulation is cheap.  The stove model you chose will throw off more than enough heat.  The purpose with wood heat, is to retain a comfortable environment overnight while you sleep.  Interior thermal mass will help with this a lot.  Heavy drywall, tile, some bricks/stone around the stove.  Just be sure your design can handle some of the extra weight.

Because this is a full time house and not a cabin, I would veer toward an additional 1" of high R value rigid foam on the exterior.  It gives 50% more insulation and it covers the normally 16" O/C 2x4 studs. By cutting down on stud heat loss (which is highest with 2x4 construction) it will give even more than a 50% gain and you don't lose any interior space. 

Along the same principle, coving ceiling joists helps too.  If doing a non-cathedral rafter tied ceiling and you are blowing in insulation, having a complete cover of all the rafter ties (ceiling joists) creates a solid blanket of insulation.  Ceiling insulation recommendations for climate zone 5 has been R38 for a while now, and has recently been raised to R49 in many areas.  A few extra inches in the space is really cheap as long as you are up there.

[Brian_G]

Another thing to consider when thinking of retaining heat in the home is what you put into it. For example a wood or Formica counter top will not aid in the retaining/maintaining interior temperature like a concrete counter top will, Stone floor tiles will help to maintain an even temperature longer than a linoleum floor coating will. Simple little things but they add up across a home and can make a lot of difference. 

[MushCreek]

Actually, I'm firmly in the 'fresh air' camp. We have our windows open between 55 and 85 degrees, as a rule. I worry about stale air and chemical off-gassing, esp. in a very small house. I'd still rather have a 'controlled' leak, such as an ERV, than myriad little leaks all over the place. And in severely hot or cold weather, when the climate control is struggling, you can seal up tight for a while. I'm looking into an ERV that incorporates the kitchen vent fan- another new trend.

[NM_Shooter]

With regards to getting air in your cabin, I have a scary story.

I had my cabin buttoned up tight, and had cooked some dinner.  Had a coleman lantern going... I only have a 14x20 cabin. 

I am at 10,000+ feet, and the air is thin anyway.  At no time did my CO alarm go off.

I got up to put the stove back on to heat water for dishes, and was unable to get my lighter to light.  Tried two other lighters, no joy.  Had a thought about oxygen being low, and went outside.  All three lighters lit right up. 

Left the door open and windows for a minute, and was able to light the lighters inside.  Subsequently, I now always keep a window cracked when cooking, or during the night for that matter. 

I suspect that the oxygen in the cabin dropped significantly, but not enough to be immediately noticeable.  I would like to have an O2 meter to see what the range of density is during a night.

[Rob_O]

I'm looking into an ERV that incorporates the kitchen vent fan- another new trend.

Details, please. I've seen the inside of a kitchen vent stack and really wouldn't want that stuff building up in my ERV core, I'd like to know how they're keeping the greasy stuff out

[Rocksteady]

Thanks for all of the awesome responses! Let me explain where and why I'm not willing to budge on a few details.  I will NOT budge on the 14' width because I am in love with the aesthetics of a 14' wide house.  I realize adding 2 feet of width would be pocket change as far as extra cost is concerned, but I like the philosophical and aesthetic qualities of the 14' wide version proposed in my plans.  When it comes to installing the foam on the outside, if i go that route, how do I detail around windows and doors?  I would probably go with 2 inches (same work as 1 inch, just more insulation in my opinion).  I think i'd have to do window bucks in this case.  What foam thickness would make window bucks surplus to requirements?  I'm also assuming I'd have to sheath the wall in foam up past the attic insulation on the gable walls even though I'm not really insulating the attic gable walls.  I'd considered doing the foam on the outside route.  Building Science (an awesome resource by the way) highly recommends this after considering cost, ease of installation, and return on investment.  I like this idea, but I have to admit some vanity.  The designer of my house is very conscious of aesthetics and I'm reluctant to tack on an extra 2 inches on either side for fear of throwing off what he refers to as "sacred geometry". I could eat up the two inches on the inside and keep the exterior dimensions the same but I'd prefer not to as mentioned earlier.  Lastly, while I'm not a hardcore environmentalist using an expensive petroleum product does not thrill me, so if i could forego that I'd be happier.

[Alan Gage]

If you haven't before start reading Green Building Advisor. All the answers to your foam questions should be there, as well as discussions on foam vs. no foam for environmental reasons and other alternatives. I think a lot of people there go a bit over the edge (maybe I did too) but there's some good stuff.

Alan