Hello,
I'm looking for a bit of sanity to try to save me from frustration in a project I was really, really excited about. My wife and I are building our first home. I have a tremendous interest in energy efficiency, both from a practical (i.e. cost savings) perspective and also because I believe it is a worthwhile goal even if not all added efficiencies will show immediate financial benefits.

The home is in McMurray, PA (just south of Pittsburgh - zip code 15317). This is custom construction, but we were not able to chose a green builder. Our contractor is willing to work with us on items that are of interest, and work with different subcontractors if I can demonstrate a good reason to do so. The house is two stories, 3600 square feet (plus basement, currently to be unfinished but plans are to finish it down the road). House faces northeast, there is a slight predominance of windows on the southwest side (back) of the home. Builder is pushing a "flash and batt" insulation package (this is their "upgrade" from code-standard) while I've been leaning toward blown-in cellulose, though an objective look at things shows that they may well be about equal. The house will be for the wife, me, and a baby that's expected in the fall. We broke ground yesterday, and in spite of working on this for a long time now, I can't say I've made much progress.

So to summarize, we have cold winters and pretty warm summers, something like 5400 square feet of house that we'd like to condition (3600 above grade). Traditional construction methods, with upgraded insulation and the involvement of a HERS rater to ensure air-sealing, etc is adequate. The big issue comes down to picking a geothermal system and contractor. I had proposals from a few contractors, and ultimately need to pick from one of these two. They are basically standardized as far as what is included including zoning, ventilation of bathrooms and kitchen, make-up air for the range hood on my commercial-style gas range, desuperheater, drilling, etc.

Contractor #1
This is the preferred HVAC sub of my general contractor. Almost all of their homes utilize these guys. Most of their experience is in more traditional HVAC.
5 ton Carrier High Efficiency, Two Stage Geothermal 50YD064JC311
$35,198

Contractor #2
This is my preferred sub, only because I felt he communicated the best and seems to make the most sense (and understand what I'm trying to accomplish). Also mostly does traditional HVAC.
4 ton Climatemaster Tranquility 22, two stage geothermal system TZ 048
$36,705

So, I like (personality-wise), Contractor #2. He'd be cheaper (by a lot) if we weren't asking him to do a bunch of the extras that the usual (Contractor #1) does for my builder. But those are a requirement for this project, and with them, he's actually more expensive, for a smaller system. The price difference isn't too awful much, but it really forces me to answer the question of what size system is best for this home.

So we get to load calculations. Contractor #1 hasn't supplied them, but upon questioning is insistent that he does not feel a 4 ton system would adequately heat the home in the winter (or more accurately, would have to rely too much on back-up heat). Both parties agree that the systems are more than enough to cool the home in the summer. When asked about concerns for oversizing the system, his response relies in the fact that a two-stage system can run a lesser capacity if that is all that is needed.

Contractor #2 is convinced that 5 tons is too much system, leading to short-cycling particularly in the summer, and worse efficiency overall. He calculates a heating load of 49,371 Btu/hr. Says that Heating Stage 1 will be 90% of heating time.

Now, they didn't work out for other reasons, but the other three contractors who looked at this all suggested 5 ton systems. I know that these things "always" tend to be oversized, but I have a lot of folks telling me I need a bigger system. (The heat loss estimates are interestingly all over the map. One threw 28,000 Btu/hr out there, and other came up with 77,782 Btu/hr and *both* of them said 5 tons.) Part of the issue with these heat loss estimates is that my insulation package is not 100% finalized, as noted above, but I gave everyone the same information to work with.

I can't ask Contractor #1 to put in a smaller system, because I feel that if anything is wrong, ever, I don't think he'll stand behind it because of that change. I can pay a little extra for Contractor #2, but if it really is an inadequately sized system, that's going to be a problem basically forever.

Can anyone provide some clarity? Even anecdotes may help me out. I would really appreciate it.

Heatloss suggests 4 ton unit, pretty much everything below 57-58 KBTU/h does when matched with your weather data. But is the heatloss correct? Why don't you have your HERS rater run the heatloss independent of the contractors. If your 1st contractor does not know the heatloss, chances are that he is just guessing. Then you know where you are.

We own a concrete casting and septic business about 11 miles from your town in Washington, PA.  Private message me your builders information to see if we work with him currently if you need some assistance or email me at [email protected]

Why do you have subcontractors "guessing" about heatloss and system sizing when you are committed to energy efficiency and have a HERS rater? If you are interested in energy efficiency, the "big issue" is not choosing a furnace, the big issues are envelope and air sealing.

I sust looked at 5 tons for a 53 kbtu zone in an area colder than yours (Chicagoish) the 5 ton required virtually no auxilliary heat (open loop) and stayed in 1st stage 98% of the time. I commented that we had reinvented a one stage heat pump!
Nededless to say it cost more to run than the 4 ton (as it was to big for the job until outside temp was down around 8*) but it also cost more to run than the 3 ton. I don't know your electric rates, but I would look for ways to improve the insulation (building envelope first) and finance it with the savings you should get for purchasing a 3 ton instead of a 5 ton.
.......n I gotta tell ya, I'm really getting aggravated with our colleagues who don't do load calcs and know so little about op cost that they keep grossly oversizing stuff. The reason they want you to buy a 5 ton is to cover their butts 'cause they don't know $#!+ about geo or design.
My vote is for the 4 ton guy, but get him to work with you on bringing the load down to 3 and that'll take care of his higher price (BTW based on what you've told us he is already worth more than the others you have spoken with).

What joe.ami said- seriously!

It's quite likely that the load can be reduced to under 3 tons for about the same money as the cost delta between a 3 ton and 4 ton system. Even if it costs substantially more than that cost delta to get there, those envelope improvements have ZERO operating cost, and provide a lot more greature comfort due to higher radiation temps from the exterior wall & window surfaces. Insulation, air-sealing, and better-than-code windows.

I'd be curious to see the calculations from the guy that came up with a 28K heat load for a 3600' house (5400', with basement), but it may in fact be there if you've gone with exterior foam to reduce thermal bridging, an insulated foundation, and an R50 roof, better-than-code windows, and air leakage < 2ACH/50. (I can't imagine why that contractor would recommend 5 tons though.)

BTW: Flash & batt vs. cellulose is pretty much a wash from a whole-wall R point of view (as long as the batt-job is truly first-class- most aren't), and dense-pack cellulose is only very modestly more air-leaky than a foam-job. But putting the foam exterior to the sheathing does FAR more for both air sealing and whole-wall R by doubling the R at the ~25% of the wall area thermally bridged by the framing. A 2x6 16" o.c. framed wall with either flash & batt or cellulose comes in at about R13 for a 25% framing fraction, R14 if it's a 20% framing fraction. Those will meet code minimum for western PA, but aren't exactly high performance walls. The flash & batt is only about a 10% bump over code-min even for center-cavity R value, but with the thermal bridging of the framing that "improvement" is in the statistical noise- a reduction from a 25% framing fraction to a 24% framing fraction makes a comparable difference in performance. Designers who care about thermal performance can usually get it down to 20% framing fraction if they're very careful about window & door placements and wall-lengths, minimizing the number of extra studs, etc.

Adding an inch of closed-cell foam or rigid sheet polyiso on the exterior brings that R13-R14 2x6 studwall up to about R20. Adding 2" would put it in the mid-20s, or ~ 2x that of the no-exterior foam assembly. When looking at the cost/ton of geo vs. the cost/ton-reduction of adding insulating sheathing, the first inch of exterior foam is usually clearly cost effective, and often the second would be too. Most contractors should be able to handle the 1" rigid foam proposition- 2x4 framing R13 cavity + R5 sheathing is a standard code-min construction standard for your area too. For exterior foam thicker than 1.5" it helps to have a contractor with some experience implementing it though.

Also note: The whole-wall R of a 2x4 framing w/cellulose fill + 2" of exterior iso runs about R22-R23, and is dimensionally the same wall thickness as standard 2x6 construction, which means standard doors & windows set up for 2x6 framing can be used with very little customization. Also, with more than half the R value exterior to the sheathing you don't need to be concerned at all about interior side vapor retarders, since the sheathing never drops below the dew point of the conditioned space air. I suspect this would be cost effective approach in your case, if your contractor can handle the exterior foam. Using insulating concrete forms (ICF) for the foundation probably would be too. Building it with the sheathing foam co-planar with the exterior foam of the ICF makes for a good thermal break at the sill plate too.

Thank you all for your in-depth responses. Believe me, I'm not ignoring the question of improving the envelope at all, but trying to solve one problem at a time (though they are obviously related).

You've helped me be more confident in pushing for a 4 ton system (and the HVAC contractor who is willing to stand behind it). Maybe if I can really focus on the envelope, I can get that down to 3 tons.

Straying a bit off-top, but related. Forgive my relative ignorance, but from inside to outside, my walls would be-

Drywall
Vapor barrier (is this placed correctly? Or not needed at all?)
5.5" Cellulose (2x6 construction is basically set in stone at this point)
Sheathing
1" Foam Board (And this is the pink or blue stuff they sell at Home Depot, right? Nothing more exotic? Would this all need to be taped?)
Brick

Dana1, if I read you correctly, you're saying that's significantly better performing than construction without the foam board, due to reducing the effects of thermal bridging from the framing. That makes a lot of sense to me. We also have additional reduction of air infiltration, correct? Based on what you said, I doubt I'm going to be able to get into the thicker layers due to builder resistance/cost, but may really get an improved result if we can add just that.

I'd love to be able to get in an upgrade like that. Again, you are all an incredible resource. If I'm going to come back at my builder (and subcontractors) asking for something that seems radical to them, I've gotta be armed with knowledge.

I would do some reading on proper air sealing and also have someone come in and do a blower door test to find missed areas.

If I'm going to come back at my builder (and subcontractors) asking for something that seems radical to them, I've gotta be armed with knowledge.

That attitude will serve you well. Keep in mind that asking (and the contractors agreeing) is one thing. It''s another to make sure it is actually done. Once you settle on an envelope plan, read all you can, particularly on the details involving windows, doors and roof to wall connections.

What Dana1 said, and
Consider: Structural Insulated Sheathing and
Advanced Framing and
your wall assembly is not complete as described. Perhaps consider this resource:
http://www.apawood.org/pdfs/download_pdf.cfm?PDFFilename=managed/A530.pdf

With 1" XPS (the pink/blue/green stuff= R5/inch) outside the sheathing a 2x6 cellulose (or flash'n'batt) studwall runs a whole-wall R of ~ R18-R19, which is definitely better than code. At only R5 of insulating sheathing it will still need an interior vapor retarder to protect the sheating from wintertime moisture accumulation.  If you can bump it to 1.5" (R7.5) or use 1-1/4" polyiso (also R7.5) or thicker instead (1.5"/R9-R10 iso recommended) you can skip the vapor retarder entirely and just use standard latex paint as the interior vapor retarder.  A 2x6 cellulose-fill studwall with 1.5" of iso yields a whole-wall R of ~R23.  Foil-faced iso is relatively easy to air-seal too using FSK tape (compared to housewrap tape on XPS that will fail within a couple decades as the XPS shrinks slightly), and if you have at least a 3/4" ventilated gap (using 1x furring) between the iso and the siding, it will bump that up to about R25 average performance due to the radiant-barrier effect of the foil facer.

If the builder is planning on just nailing/screwing 10,000 fasteners through the foam for mounting the siding, the net thermal performance of the foam just won't be there, since steel is 3 orders of magnitude more thermally conductive than the foam.  For mounting the siding it's better to use exterior furring over the foam, through-screwed to the studs 24" o.c., to minimize the cross sectional area of steel that penetrating the foam.  Depending on the rigidity & type of siding, it's sometimes better to use 2x furring rather than 1x to achieve a flat wall without going to tighter fastener spacing on the furring screws.


If the builder insists on 1" max on the sheathing you might with polyiso rather than XPS, and use a vapor-retardent latex primer on the interior rather than poly sheeting. (If that doesn't fly with the builder or inspector use Certainteed MemBrain rather than poly, which should be accepted. It's low-permeance when the humidity is low, as it is in winter, but becomes vapor-open should the air in the wall cavity rise in humidity. It limits wintertime moisture accumulation, yet drys very well when needed.)

See:

 http://www.buildingscience.com/docu...uirements/ 
(you are in US climate zone 5)

See also:

http://www.greenbuildingadvisor...-sheathing

Getting a blower door test done BEFORE drywall goes up allows leaks to be found and fixed much more easily.