Good move on insulating.

it probably is just as expensive to excavate around boulders than it is to drill

You might talk to some excavators directly to find out how true this is.

I understand that you want to get rid of your electric baseboard heaters, but keeping a few (perhaps the less visible ones) would serve as the aux heat that you need with geo. And I wouldn't rule out cold climate mini-splits + a few transfer ducts + some electric heat. $20K (or whatever it comes to) in upfront savings can pay for a lot of electric heat.

34KBTU with or without the wood stove? At what thermostat setting? Also the software does not always account for errors inherent with older houses.

34KBTU is based on manual J with indoor temp set to 68F using HVAC-calc. Since I have infiltration values from an energy audit, I'm guessing it is pretty accurate. Nothing was estimated when inputing into the software

Are you sure the R-values you put in were accurate?

Yes R13 for walls, R57 for attic (its actually R60 but that wasn't an option in HVAC calc). I used normal double pain for windows uncoated and an infiltration value of 0.3 (the house was measured at 2.5 ach50)

I'd use .45 for ACH-nat, but that's a more conservative wild guess based on average houses. I also wouldn't expect too much from the accuracy of a Manual J, consider actual measurements.

Jon did you really just suggest ignoring an actual measurement (in lieu of a wild guess) and then simultaneously suggest "consider actual measurements"? That's kinda funny.

Jon did you really just suggest ignoring an actual measurement (in lieu of a wild guess) and then simultaneously suggest "consider actual measurements"? That's kinda funny.

You might want to review the difference between ACH50 and ACH-nat. And why deriving one from the other is horribly inaccurate.

It's a 1-story house w/basement, thus a very modest stack effect. At 2.5ACH/50 it's reasonably tight but unless the band joist & foundation sill have already been sealed it can probably be brought under 2ACH/50 without a lot of expense. The greater value in blower door tests (more than the raw number) is finding the less-obvious leaks & thermal bypasses.

Even the best estimates from heat load tools using cfm/50 or ACH/50 numbers are little more than a WAG- the locations of the leaks matter. Substituting one WAG for another probably needs a better basis than jonr's house-unseen gut feel though.

If the Manual-J is coming in at 34K prior to insulating the basement wall and insulating/sealing the band joist the low to mid-20s numbers cited ring true for the condition post-treatment.

At <25KBTU/hr you're definitely in the zone discussed in this bit o' bloggery: http://www.greenbuildingadvisor.com/blogs/dept/musings/choosing-hvac-equipment-energy-efficient-home

At <25KBTU/hr you're definitely in the zone discussed in this bit o' bloggery

While the article has a good point about the prices of air to water heat pumps and geothermal (they both need to come down), its comments about them losing efficiency because they need to produce 120F made little sense. Even ignoring desuperheaters, heat pumps can run at 120F for DHW and then switch to 90F for radiant heating.

But going radiant is a significant cost-adder, with remarkably small (but still real) uptick in average comfort in homes with sub-20KBTU/hr heat loads. To get there as a retrofit on THIS house would be insane- you can knock yourself out taking it to the nth-degree on GSHP or hydronic-ASHP, but to what end? In a ~2000' home with a 20KBTU/hr peak load lke this one you'd definitely be feeling the radiant-floor cushies in yer socks at the design condition, but to a far lesser degree during the average heating season day, or even the average mid-winter day.

Even retrofitting radiant on this house with extruded aluminum plates under the subfloor would be expensive, and MIGHT be able to deliver design-day heat with 120F water (but questionable- depends on many particulars). But to get down to 100F would require a WarmBoard or gypcrete solution, which is even more expensive (but might be considered if you were planning to replace all the flooring.) Radiant floors are more rightly thought of as paying a premium for the comfort factor, not as a cost-effective method of improving system efficiency. In some slab-on-grade ranchers it may be cost effective for new construction, but never as retrofit.

For those interested, here is the break down for the manual J calcs I did using HVAC-calc. Design conditions: 5F in winter, interior temp 68F, summer temp 86F, interior temp 75F, humidity 50%. Attic R-57 ventilated over dark roof, walls R-13, infiltration 0.35

This is the current exact conditions, note the large loss through the floors which are specified to be above an unconditioned basement:


This is finishing 800 sqft of basement, insulating basement walls to R-13, and insulating floor where the basement isn't conditioned tp R-19, it cuts off a whole ton!:

You can see I've taken the load of the first floor to 21K btuh and the basement adds another 3-4k btuh for a total of 24k btuh

I'm a bit skeptical that an "unconditioned basement" would eat up 15 kBTUH, because at that rate of heat addition, it wouldn't be unconditioned for long, unless it is vented to the outdoors like a crawl space.

By now it should be clear I advocate meeting Plan B with a two ton nominal geo heat pump, topped up with a 5 kW strip and, when the spirit moves, burning some wood. The two ton solution would be cheaper to install, quieter and more efficient while running and do a better job (comfort and efficiency) while cooling during summer.

I can't speak to the validity of the calculation, however my observation over the winter was that my basement was cold (easily 50s), and when I measured the foundation by IR temp in March it was 42F. The bulkhead is reasonably sealed and air sealing was done in the basement. There are only 5 small basement windows which have storms on them. As part of insulating my basement walls, I plan to also get the rim joist. Based on the way the house felt this winter, the calculation that most of the loss was in the basement agrees with how the house felt for whatever that is worth...

I should be up that-a-way in July, so have it insulated by then!

Posted By engineer on 30 Apr 2013 11:13 PM
I'm a bit skeptical that an "unconditioned basement" would eat up 15 kBTUH, because at that rate of heat addition, it wouldn't be unconditioned for long, unless it is vented to the outdoors like a crawl space.

By now it should be clear I advocate meeting Plan B with a two ton nominal geo heat pump, topped up with a 5 kW strip and, when the spirit moves, burning some wood. The two ton solution would be cheaper to install, quieter and more efficient while running and do a better job (comfort and efficiency) while cooling during summer.

Do the math:

Assuming the basement drops to 55F at the +5F outside design condition (common for uninsulated basements in this area) that's a 50F delta on a R1 wall for the above grade concrete. None of the heat is being sourced by the ~50F subsoil under the slab.

For a 40' x 50' footprint 2000' house that's 180' of perimeter, times 2' of exposed foundation is 360 square feet.

~360' x ~50F delta x ~U1 concrete= 18,000 BTU/hr

If it drops to 50, for a 45F delta that becomes:

360' x 45F x U1= 16,200 BTU/hr

The 15K figure reads like the right ballpark to me. Experience with retrofitting basement insulation and measuring the effect on the heating bills on other houses in the region tells a similar story.  I'm not sure how "conditioned" a 50-55F basement feels like to most people, but it's admittedly a lot more comfortable than 5F.

(edited to correct arithmetic keying error and add second example)

Fair point. Much depends on the square footage of U1 / R1 exposed foundation. I should have done a bit of the math before posting on 30 April.

At eastern MA design freeze depths & deep subsoil temps the below grade stuff matters, but the above grade portion is still going to be the bulk of it. Even with only 18" of exposure, you would still have the uninsulated band joist losses (figure U0.5-ish, and over 150 square feet) plus the sub-grade losses. Under 10KBTU/hr of loss would be rare, but over 25K would also be an outlier unless it were a walk-out basement. (And uninsulated walk-outs exist here too.)

It's surprising to me that local codes did not require foundation insulation until fairly recently. Even in the late-1990s some people thought I was nuts to go ICF for the foundation on an addition we put in, and most homes in MA (such a sbeausol's ) still have NO foundation insulation. For new construction MA is currently governed by IRC 2009 prescriptive R-values, which call out R10 c.i. for basement walls (or R13/cavity, though cavity-only fails the dew-point test, and would take on ground moisture if an interior side vapor barrier were used.) Slab on grade requires slab edge R10 c.i. to 2' below grade. or to the footing, whichever is less, though frost protection wing insulation would be required for footings as shallow as 2'. From a long term cost-effectiveness point of view, R10 c.i. PLUS R13 cavity (for an ~R20-ish whole-wall R) is still worth it at our higher-than-national-average energy pricing.