Hello:

My single-story house (over a conditioned crawl-space) will have the following spec's:




According to my insulation/heating calculator, www.dropbox.com/s/cq9gu7xwotxygna/CockamameeCalc.xls, my house will require 63,000,000 BTU's to heat per year.

(an improved version of the spreadsheet called Insulcalc can be found here: https://www.dropbox.com/s/ei29pl3ac1331hk/InsulCalc.xls?n=8915286 )

If the house is heated by baseboard heaters only, my annual electric bill will be (in a perfect world based on the above) about $1,577.

We will have a small high-efficiency direct-vent, wood-burning fireplace insert on the main floor. There is plenty of maple & beech hardwood in my area at, say, $120 per cord.

I was thinking that it would be a good idea to put a high-efficiency wood burning stove in the crawl-space.

What I need help with is 'sizing' the 2 wood-burning units and to know if this is a good idea, in the first place.

I heard that, when heating with wood, you must burn your wood at 'full-bore' to achieve ultimate efficiency - this makes me worry about over-heating my small house.

Any thoughts on this would be appreciated.

Here is the stove I thought could be used in the crawl-space: http://www.homehardware.ca/en/rec/index.htm/Heating-Ventilation-Cooling-Home-Comfort/Heating/Stoves-Fireplaces/Wood-Burning/Rocket-EPA-Wood-Stove/_/N-ntmanZ1z140uy/Ne-ntc74/R-I5531412



Rob.

Why don't you put the house down on a heated slab, lowering your heat load further and buy a wood boiler for all? No to any wood burner unattended or out of sight in a crawl space.

Badger is right, putting this in crawl space sounds like a bad idea unless your crawl space is more like an easily accessed conditioned basement. Seems like you just need to size the woodstove for your expected area heat loss. If you oversize it, it will be both less efficient and cause more pollution.

Rob - if I understand your situation right, you may have a higher heat loss in the crawlspace than you are predicting. Using the soils values as you have, it is possible that the frost will bypass this through the bedrock. There will be a strong heat loss path that prevails once the ground is frozen.

You say your infiltration rate is .045 ACH and your air changes per hour (assuming you mean HRV) is .05?
Over10 times tighter than passivhaus?

Buy a small catalytic woodstove like the Woodstock Fireview. Burns very efficiently, even at low throttle.

Posted By whirnot on 08 Mar 2013 02:55 PM
You say your infiltration rate is .045 ACH and your air changes per hour (assuming you mean HRV) is .05?
Over10 times tighter than passivhaus?

The Passivhaus spec is 0.6 air changes per hour (ACH) at 50 Pa differential pressure.  To convert this to "natural" air changes per hour, assuming average wind speeds, one rule of thumb is to divide the infiltration rate measured at 50 Pa differential by a factor of 20, giving 0.03 ACH.  So Rob's planning on a tight house, but not as tight as Passivhaus.  See papers by Max Sherman for a more complete discussion of the conversion process, and for values more specific than the rule of thumb value of 20.     

Rob,

My house is similar in size and design to your house, but I am using 23 MillionBtu of natural gas to heat it (as well as hot water and cooking) rather than your estimated 63 MillionBtu for your house. (My value includes losses due to furnace ineffiiciency, but those are low, 2.5% and my hot water and cooking requirements, although the hot water is mostly solar thermal.) I am trying to rationalize the reason for the difference. My floor area is a little greater, 1602 ft^2 versus 1418 ft^2. Insulation levels are similar. Your heating degree days are higher, 9300 HDD(degf) versus my 6500 HDD(degF) for the last couple of warmer-than-average winters, and your indoor temperature setpoint is 73 F versus my 67 F. Your soil thermal resistance is lower than mine, 3.65 versus about 9. Your exposed crawl space wall height is greater than mine, 2' versus 0.5'. I meet about 30% of my heating requirement from passive solar.

The main difference between our houses must be in losses from the crawl space, which are difficult to calculate. If that is the case, I would be tempted to add some insulation to the crawl walls, or put some insulation in the floor between the house and the crawl space. Fiberglass batts in the floor should be relatively inexpensive and easy to install.

Like BadgerBoilerMN and Sailawayrb said, I do not think that a wood stove in the crawl space is a good idea. It would be unsafe and awkward to use.

Thanks people! :)

1. I can't build a slab foundation due to the sloping bedrock: http://youtu.be/Sal6hLuVlJQ
2. I will be building a trap door in the floor, with a "ship's ladder", to access the deepest part of the crawl-space - about 5 to 6 feet in height.
3. I will look into the Woodstock Fireview

Rob.

Hi Lee:

I am as perplexed as you are! I remember our discussions in a different thread and I am really confused by my 60,000,000 plus result!

Here is a link to my calculator, "InsulCalc": https://www.dropbox.com/s/ei29pl3ac1331hk/InsulCalc.xls?n=8915286

Maybe, if you have the time someday, you might put in your info and see if you can locate a flaw? (It has my info in it at the moment.)

Thanks,

Rob.

I don't know exactly how your spreadsheet calculates, but just looking at the comparitive values, what jumps out at me is that you are loosing 43% of your heat at your basement walls and slab. So if that is correct, I'd definitely increase the Rvalue substantially in those areas. I undetrstand you have a sloped ledge condition - why can't you level it out at perhaps 36" with sand. gravel or stone, then cover that with 12"-24" of EPS? Likewise the basement walls - they can be insulated easily with EPS or polyiso foam sheets.

You can use that spreadsheet to calculate what thicknesses make sense, and I'd strongly encourage you to play around with it without thinking of whether it makes sense or is affordable. If for instance, you find that you can get the results you're looking for by increasing some values, then concentrate on figuring out how to do just that. 60,000 btu is simply too big a heat loss to be satisfied with.

Posted By robert.thompson on 09 Mar 2013 11:30 AM
Hi Lee:

I am as perplexed as you are! I remember our discussions in a different thread and I am really confused by my 60,000,000 plus result!

Here is a link to my calculator, "InsulCalc": https://www.dropbox.com/s/ei29pl3ac1331hk/InsulCalc.xls?n=8915286

Maybe, if you have the time someday, you might put in your info and see if you can locate a flaw? (It has my info in it at the moment.)

Thanks,

Rob.

Rob,

I used your InsulCalc with the values for my house.  The measured degree days here for last year were 6347 HDD(degF), but I reduced that number by 15% to account for the fact that I operate my house at 67 F rather than 70F.  (HDD change here by 21.4% for each 5 degF reduction in base temp.)  I compute a heating-only heat loss of 34.8 MillionBtu.  If I claim that I meet 30% of my heating needs with solar, then the heating requirement by natural gas would be 24.4 MillionBtu.  Last year I used 22.8 MillionBtu of natural gas at 97.5% efficiency, giving 22.2 MillionBtu of heat.  However, some of that was for heating hot water (in addition to solar thermal), and some for cooking.  Let us assume 15% was for hot water and cooking, so about 19 MillionBtu for heating. 

Your model estimates 24.4 MillionBtu compared to an actual of 19 MillionBtu, or 28% high.  Your model estimates that I lose 13% of my overall heat losses through the "slab" so if I zero that out, then the model is closer to my measurements.  But then there is at least +/- 10% in my analysis. 

I think that you are assuming a thermal resistance for the soil for heat loss under the slab as just the thickness of the soil from the surface to the depth of the slab.  That is accurate for the edge of the slab, but not for the slab under the center of the house, where the heat energy has to travel from the center of the house through many feet of soil.  Just look at folks that have done 3-D finite element analyses of heat loss from a slab, and you will see that the losses are mostly from the area close to the outer walls.  See Figure 2 at www.newellinstruments.com/files/col_6.pdf.  You are overestimating the losses from the slab, but it is a hard fix.  Maybe just count the outer 3' around the periphery of the slab. 

Losses to the ground are significant in your case because of your relatively high soil thermal conductivity.  You are estimating 23.2% of your heat losses through the slab even with R-12 insulation.  I do not believe that to be accurate based on all the 3-D finite element work.  If I ignore heat losses from the slab with the exception of the outer 3' for your house, then the heat losses through the slab are reduced to 37% of what your computed, or about 8.5% of the total heat loss, a more reasonable sounding figure. 

If you decide to add further insulation in the crawl space area, it would probably be far cheaper to add fiberglass batts under the floor than more foam insulation under the slab, and you only need to add it one place rather than both the walls and the slab.  As I understand it, you don't have ducting in the crawl space anyway.  It would still be good to retain the foam where you have it to reduce the chances for condensation on the floor joists. 

Posted By robert.thompson on 09 Mar 2013 10:40 AM
Thanks people! :)

1. I can't build a slab foundation due to the sloping bedrock

Sure you can.  Pour a stepped footing using FastFoot.  The footing forms and braces will likely have to be drilled into the rock, but it is done all the time - just takes more time than driving stakes in dirt.  If you have significant slope on the rock, I would drill holes in the rock near the middle of the footing periodically and pin the footing to the rock with rebar.  After the footing is poured, have standard 8" concrete block (CMU) layed on the stepped footing to get your level foundation.  Install 2"-4" of foam insulation vertically against the inside of the foundation blocks (alternatively, install the vertical insulation outside of the blocks, but it is more difficult to protect the foam).  An alternative to the CMU block and vertical insulation would be ICF, but you have to provide some way to protect the exposed exterior foam.

Backfill and compact river sand or gravel to bring your area under the slab up to grade minus the thickness of the slab and any insulation under the slab.  Dry sand provides a decent amount of insulation if you have enough of it - install perforated drains from inside the foundation to the low point in the footing to insure the granular backfill stays dry.

Install rough in plumbing and anything else that needs to go under the slab (don't forget penetrations through your footing or foundation blocks). 

Cover sand/gravel backfill with vapor barrier. 

Then cover with 2"-4" EPS or XPS foam. 

Pour a 4" concrete slab over the foam and down into the perimeter foundation blocks.  Alternatively, fill the block cores with perlite insulation prior to pouring the slab.  If you go with the perlite block fill, I would leave at least one block core every 4' open to let the concrete from the slab pour flow down to the footing - use a vibrator to get good consolidation of the concrete in the block core. 

Some will argue that it is better to put the foam insulation down first then cover with the vapor barrier, or no vapor barrier is needed if you seal/tape the seams in your foam insulation.  If you keep the granular backfill dry with good drainage, it probably doesn't matter much either way.

Thanks for your input Bob & Lee. :)

I have corrected/changed InsulCalc to better handle the basement wall & slab calculations - I uploaded the corrected version.

The basement walls & slab still lose 37% of the heat (57,000,000 BTU's), assuming the calculations are getting closer to reality.

It looks like the basement walls are really the problem.

I am now off to consider Bob's 'leveling idea' and arkie6's tutorial on how to build my foundation - it looks really good!

The big news is that the bedrock IS NOT GRANITE, well, most of it isn't. It seems that the highest point of the bedrock is LIMESTONE which, apparently, can be 'easily' cut with a cement saw or broken with a jack-hammer.

If that is true, I might be able to do some leveling like Bob suggested and then build the foundation with my "mutinous crew", with a little help from someone who knows what they are doing, when it come to foundations.

Thanks again for your help.

Rob.

Rob,

While you are pushing dirt around, if you had only 6" of crawl space wall above ground instead of 24", you would further reduce your computed wall heat losses by 4300 Btu annually, or about 8% (from your earlier version model). You need some buildup of soil next to those walls for drainage anyway.

If you stick with the crawl space design with the same foam as in the current design, some R-19 fiberglass batts in the floor should cut the heat losses to the crawl space by a factor of two or a little more, and I assume that would be much cheaper than doubling the foam thickness. You would still need some foam on the walls and under the crawl space to avoid condensation problems on the exposed portions of the floor joists, and conditioning the crawl space would be more problematic. Building on a slab with lots of sand and some foam underneath would be another approach to reducing those heat losses.

Is the area you are located at have any rodent issues?

Out here in AZ crawl spaces are notorious for rodent infestations. It's a perfect spot for them to nest and breed. A friend of mine had a bunch of mice gnaw through the wood paneled crawl space floor boards and nest in the fiberglass insulation. They then made their way into his home and he had mice running around the home. When he went under the crawl space to fix the damage he had to be careful because there were Black Widows but even then he was stung by a scorpion while he was on his back. Not a good day for him.

Yikes!

We only have ants, mice, frogs, chip-monks, squirrels, skunks, lynx, raccoons, coyotes, wolves, deer, moose and, so I am told, the odd mountain lion. (plus birds)

My crawl-space will be completely sealed and treated as a 'conditioned' space. Hopefully the electrical entrance, pressure tank & hot-water tank will be the only things 'living' down there. :)

Rob.

A very cost effective, and relatively unknown,  insulation material suitable for earth contact is expanded Perlite, It costs about $1.25/cu ft.  and has an r value of about r3/"  for a cost of about $0.034/ r sq/ft.  right down with loose cellulose and about 1/3 of EPS.  It is placed in plastic bags of appropriate size to achieve the desired thickness.  The standard packaging is in 4 cu ft bags that naturally form an 8" layer so you'd have to repackage it if you wanted less thickness than r24.
I plan on using it under my basement slab. My logic is why not have r24 for the price of r8 EPS.

Is the perlite compressible?

"Is the perlite compressible?"
I think the answer is no but that needs some explanation.  Perlite, unlike plastic foams is "not elastic".  When perlite is stressed, below a critical level, it doesn't deform and spring back.  Stress above the critical level result in destructive collapse.  The critical level is about 300 PSI.  Also perlite is a loose granular material so in order to be stressed it must be constrained on the sides or it will flow sideways.