Posted By robert.thompson on 13 Dec 2012 09:57 AM
I can insulate the crawl space walls but probably not the bedrock floor, well, at least not easily nor inexpensively. I will cover the bedrock with heavy VB

What should the crawl space wall R-value be if it is heated?

And, most importantly, should I insulate the floor over the crawl space? Do I VB it?

Thanks.

P.S Is there someway to turn this pain-in-the-ass bedrock problem into an asset by using it to store solar heat?

Builders do not usually insulate under conditioned crawl spaces, although I did.  I would not sweat it about the exposed bedrock other than using the vapor barrier.  It would be possible to spray it with closed cell foam, but then you would have to add a fire barrier which would add to the expense.   

The insulation levels that we were discussing for basement walls would apply to the crawl space walls for a conditioned crawl space with the walls mostly below ground.  I chose R-19 for my conditioned crawl, which should be plenty.  (The heating degree days here are slightly higher than your 6500, assuming that value is based on deg. F and not deg. C?)  If you want to treat the crawl space as a conditioned crawl space, which is generally recommended these days, then you would not insulate the floor, just the walls.  You would also probably want vents between the main floor and the crawl space and some forced airflow between them, like an exhaust fan to the outside in the crawl space.   

Since the bedrock is thermally connected to the ground (and ground water), I do not think that it can be used to store solar heat. 

6500 heating degree days Celsius is a lot more seasonal heating than 6500HDD-F, and is a realistic number only for places well north of Montreal, or locations at much higher elevations. Whitehorse Yukon runs about 6500-6800 HDD-C, and Edmonton Alberta only ~5700-5800 HDD-C.

But the heating degree days in Montreal are well over 6500HDDF. Montreal proper over the past 25 years had averaged about 4400-4500HDD-C (or about 8000HDD-F) but that's down from ~4600HDD-C from the middle of the last century. (Burlington VT which is a bit south of Montreal runs about 7800HDD-F, or 4300HDD-C.)

Subsoil temps in southern Quebec are cool (low 40s F, maybe 5-6C), but nowhere near permafrost. Unless it's very light porous insulating type of bedrock (not likely) it's of no use for heat storage.

Insulating the crawlspace walls AND the bottom side of the floor is probably going to be necessary for performance in a high-R house. Leveling up the top of the bedrock with sand and insulating the floor of the crawlspace with at least 4-6" (R15-R25) of cheap low density Type-I EPS (roofing insulation for flat commercial roofs) protected by a non-structural 1-2" concrete "rat-slab" is also a good approach. If you're at PassiveHouse type R for the walls & attic, even going to ~10"/R40 under the rat-slab may be in order. (Using reclaimed roofing foam rather than virgin-stock is cheaper than batts or blown fiber between the joists.)

Posted By robert.thompson on 13 Dec 2012 09:57 AM
I can insulate the crawl space walls but probably not the bedrock floor, well, at least not easily nor inexpensively. I will cover the bedrock with heavy VB.

If you have enough room between the bedrock and floor joists, dry sand has an R value of ~0.6R per inch.  Putting ~18"-24" of sand on top of your vapor barrier would provide R10 to R14.

Posted By Dana1 on 13 Dec 2012 12:08 PM
...snip...
If you're at PassiveHouse type R for the walls & attic, even going to ~10"/R40 under the rat-slab may be in order.

Rob's stated goal is a breakeven at 17 years, not PassivHaus standards. Are you suggesting that your calculations show that your recommended approach will pay off in 17 years? What is the clearance height for the crawl space after adding 10" of EPS plus a rat slab. Sounds like 3', which sounds tight.

If I insulate the heated crawl space walls to R-20 but do not insulate the bedrock floor, am I correct in assuming that the bedrock will remain at about 45 degrees F?

If this is about right, do I calculate the heating requirements for this space as the amount of electricity to warm the volume of air from 45 degrees to say 70 degrees?

Thanks.

17 years might payback at R40 if it's reclaimed roofing EPS, definitely NOT if virgin stock. Reclaimed EPS @ R40 costs about the same as R10 code-min done with virgin stock.

The whole notion that ANYTHING "pays back" in 17 years is subject to a lot of speculative assumptions about energy cost inflation and the cost of money. Even code minimums in the US are rarely NPV+ on energy cost saving in a 17 year time frame even over an uninsulated but reasonably air-tight house. At IRC 2009 levels even 25 years is iffy.

The R30/R20/R10 attic/wall/foundation code min might not be paying off in 17 years compared to R19/R13/R5 if his 7.5cent electricity price remains stable, heating mostly with ductless mini-splits, but there's perceptible difference in comfort between the two.

A R15-R25 foam floor and an R20-25 sealed & insulated crawlspace walls "pays back" in comfort, and as an insurance package against mold on the joists, etc.

There's no guarantee that the crawlspace will stay above 45F with R20 foundation walls and no bedrock-floor insulation even if you don't insulate between the joists. A lot depends on the thermal conductivity of the rock. If it were dry soil and the crawlspace walls were insulated down to the frost line depth it probably would. With R10 bedrock insulation and R20 crawlspace wall it would definitely stay warmer than 45F with a ~70F house above, but if you insulated between the joists, maybe not.

It's not the volume of enclosed air that determines the additional heating load of a conditioned crawlspace- it's the amount of wall surface area & R value against the outdoor design temp, and the amount of floor area & R value against some other temperature (a temperature somewhat lower than the deep subsoil temps.) Determining the heat loss out through the bedrock depends a lot the type of rock, and it's not simple to get to an accurate number. But it's going to be a much bigger number for bedrock than it is for dry soils. In nearby parts of New England R10 under basement slabs is now required by code for new construction, even on soil, not bedrock, even if with R19-R30 insulation between the joists, with R10 foam (or R13 between framing) on crawlspace walls. See:

http://energycode.pnl.gov/EnergyCodeReqs/?state=Vermont

Your climate is colder, and your bedrock far more thermally conductive than soil. Putting R10 under a rat-slab in the crawlspace and R15-20 on the crawlspace walls would make it more like a Vermont code-minimum basement.

Rob-

Calculating heat loss through the ground is the hard part. The literature that I have seen gives widely varying results, partly based on variations in moisture content and soil type, with increased moisture resulting in increased heat transfer. This paper http://www.nrel.gov/docs/fy01osti/29694.pdf looked like they did a nice job, but I think that when I used there numbers on my house, the computed heat losses though the ground were higher than my measured heat losses for the whole house. But clearly the heat losses near the edge of the foundation are higher than further in from the edge. I made a minimal effort to contact the authors, but without success.

I think that I judged this paper http://www.ornl.gov/sci/buildings/2010/Session%20PDFs/92_New.pdf to have more reasonable results.

I like the overall philosophy that Ty Newell used in constructing his net-zero energy house. He went through some very detailed calculations for heat loss to the ground for his location in central Illinois that led him to insulate the basement foundation walls but not under the floor (http://www.newellinstruments.com/files/col_6.pdf). One of his arguments was why spend more money on insulation than what he could spend solar PV to generate the same amount of energy.

In your case, I might be tempted to put insulation on the crawl walls and between the crawl space and the house, so it is no longer a conditioned crawl space. However, if you had ducting in the crawl space, you would have increased heat losses from the ducts to the crawl space. You could also get cost estimates on the approach suggested by Dana1, as well as evaluating the resulting clearance height.

Two issues: when developing the Passive House analysis, the developers decided that, for whatever reason, under slab insulation was extremely important, so the result is that PHs have a LOT of insulation. (I think the Illinois house had 16"+ of EPS) Say what you will about PH, they work.

To leave out all the under slab insulation in Illinois is to welcome a cold floor and unneeded moisture into the space. Its not simply a heating issue. A cold slab in the northern climate will always be losing heat to the ground below and increasing your discomfort.

Posted By Bob I on 13 Dec 2012 02:00 PM
Two issues: when developing the Passive House analysis, the developers decided that, for whatever reason, under slab insulation was extremely important, so the result is that PHs have a LOT of insulation.

To some of us, that sounds like kind of an arbitrary way to make decision about a house design. My personal approach lines up with that expressed by the developers of BEopt at NREL and Ty Newell referenced above, who say that you calculate the heat losses and gains as best you can, and make decisions based on those hard numbers and economic tradeoffs. These decisions are not based on some arbitrary criteria developed on another continent that has very different solar radiation and climate than the particular place that we might be building a house.

One clear difference between PassiveHouse folks and Ty Newell, the BEopt developers, and me is that we are not averse to using active solar systems in place of increased insulation when it looks economically more favorable to upsize the PV system to generate energy more economically efficiently than by trying to reduce energy consumption by adding the last R-1 of insulation to a ceiling that is already at R-99. The goal for my house was to try to achieve net-zero source energy use, not PassiveHouse standards, since I considered net-zero more noble. But I accept that other people might have a goal of PassiveHouse or whatever that they consider more noble.

When the Passive House folks start to lose the economic argument, they often bring out the comfort argument. Comfort is harder to quantify, but I want to see a blind test to see somebody discern the difference between two houses, one with an attic with R-60 versus a second at R-40 based on comfort. As jonr pointed out, that is sensing the radiation difference of 0.5 deg.

Now I made the decision to put R-5 under the crawl space at my house, and more like R-29 around the edges. I think that we all agree that getting some insulation between the ground and the living space in northern climates is an excellent goal.

The scientists who developed PH were, if anything, the opposite of arbitrary in developing the PH program. When doing the calculations (in the 30 page spreadsheet), you can change the one number (R value under the slab or any of hundreds of other components) and see the exact effect on the overall heat load in the house. I agree there is an argument to be made about the affordability of PH construction, even within the PH community, but he seems to have all the components of a good house nailed.

Here is a picture of my Bedrock
The picture is taken from the Northeast corner looking South. The hole is 30' x 40'. The bedrock is 1' below the earth at the NE corner, 1.5' at the SE corner, 3' at the SW corner & 4' at the NW corner. The bedrock is very uneven, with 2' & 3' ledges in the middle-left.
Can I get away with just VB over the entire bedrock, insulating the foundation walls (such as they will be), and insulate (VB?) the floor from underneath? I could minimally heat the space below the floor insulation.
If this is a reasonable solution, what R-value should the underfloor insulation be? What type of insulation? Where should the VB go?
An important factor would be that the floor be warm or my wife will throw me off the cliff! (the cliff is not in the picture.)
If you have any suggestions, i would really like to hear them.
Thanks.

nice lot! I'd recommend:
1. pour the footings which will be pinned to the rock,
2. pour the wall
3. backfill with 3/4" washed stone and level the stone
4. Cover with4" of XPS or EPS; be sure to place the foam around the edges so no part of the concrete floor is in contact with the ground or ledge
5. cover with 6 mil poly, seams taped
6. Pour a 2" concrete slab
7. Insulate the walls with at least 4" of XPS or EPS or polyiso. top layer should be code compliant Thermax polyiso, or painted with fire retardant paint.
your floor will not be cold.

the stone backfill is inside the foundation, if that isn't clear. Washed sand is the exterior backfill.

What Bob Irving said.  Leaving that cold stone uninsulated and facing the joist edges will make the joist retain more moisture and make the first-floor surface colder with or without joist insulation.  With even 4" of EPS or XPS (~R15, if low-density Type-I EPS,~ R20 if XPS) it'll be fine.

There are multiple vendors for reclaimed roofing foam in my area. Re-using goods is as green as it gets, and saves substantial cash on projects like that. InsulationDepot.com will ship within the US if you have a sufficiently large order, but I'm not sure if they have any facilities in Quebec or can easily ship to Canada , or how much the shipping charges would add.  (I'm close enough to be able to bring a truck to cart it away.)

Virgin-stock EPS from the orange box-store in Beabien Quest  4" of EPS would run ~$1.50CDN per square foot (about 10 cents per square foot per R) but if you can find a local source for factory seconds or used-goods it runs between 25-50 cents/foot for 4" (3 cents per square foot per R or less).

Here is a drawing of the Foundation
The top of the foundation will be 1 foot above grade at the NorthEast corner (the high point of the land) and will be about 3 feet above grade at the NortWest corner, the low point.
Will the foundation walls support all that 3/4 stone? I didn't plan on back filling to 5 feet on the west wall, do I need to?
And I guess that I should install the basement wall foam insulation before I start filling with the 3/4 stone, right?
Thanks.

The crawlspace/basement wall foam doesn't need to extend below the floor foam in your case.

If the walls rested on a footing dug into soil at local 100-year frost-line depth it would be worth insulating the wall down to the footing, but there is little advantage to that there. As long as the wall foam fully covers the edges of the foam under your 2" rat-slab there is no significant performance difference.

If I insulate as per "Building Science Corp in Boston recommend R values of 5/10/20/40/60 (windows/under slab/basement walls/walls/roof)" the total insulation cost would be $6,800 for under slab/basement walls/walls/roof, using cellulose above the foundation & foam on the foundation.

Using non-whole wall numbers and not allowing for air leakage, the annual cost of heating, with electricity, would be $775 or $65 per month, going 'full Boston'

If I insulated to one half the Boston values, the insultion would cost $3400 and the annual heating would cost $1,400 or $117 per month.

Therefore, the payback to go 'full Boston' would be ($6800-3400) / ($1400-775) = about 5.44 years.

By the way, to be paid out in 17 years, (using my cockamamee formula) the insulation values would need to be: 20/39/78/118 for under slab/basement walls/walls/roof and the annual heating would cost $396 or $33 per month.

I'm leaning toward the 'full Boston' approach.

Rob-

These costs must be insulation material costs only, and exclude any changes to the structure required to fit in this amount of insulation? For example, an R-78 wall might require some material and labor costs other than insulation when upgrading from an R-20 ("half Boston") wall. You might need to get some costs from your builder for the other labor and material costs to improve your trade-off analysis.

interesting that your 17 yr payout approaches Passive House values.

Also note that window losses and infiltration are staying constant, so the savings isn't as high as calculated. Once you add interest, 17 years is typically "never".

Open loop geothermal with an existing well and no ducts should be about 1/3 of the quotes you have - you can do it yourself if the vendors don't agree.