We have pretty much what you are asking about: a highly insulated house (SIPS that are R-36 in the walls and R-45 in the roof), an ERV with radiant heat on the first floor and a Russian fireplace that heats the entire building on between 1 and 2 cords of wood a winter. (We live in Central Massachusetts).  Radiant heat is the most comfortable form of heat I've ever experienced and the ERV gives us all the fresh air we want during the heating season.  (We use the windows from late March through mid- Oct.)  We live in a rural, heavily wooded area and have never had the need for AC in the 18 years we've lived here.  However, we have the possiblity of using our geo-thermal heat pump to create cool air should summer temps rise in the future - climate crisis anyone?
  My understanding is that (in our neck of the woods) if you can get an R-50 envelope for the house, you don't need ANY heat source (outside of appliances, human heat, etc.)  Anybody know if this is accurate?

My understanding is that (in our neck of the woods) if you can get an R-50 envelope for the house, you don't need ANY heat source (outside of appliances, human heat, etc.) Anybody know if this is accurate?

You might not think that you will need heat, but I'll bet your banker will! I heard of a strawbail home in Tucson, AZ that had no heat or A/C for three years after it was built, and the temperature inside stayed between 65 and 75 degrees all of the time. They had to put in baseboard heaters when they wanted to get a loan.

Posted By rkillough-miller on 23 Apr 2010 01:26 PM
We have pretty much what you are asking about: a highly insulated house (SIPS that are R-36 in the walls and R-45 in the roof), an ERV with radiant heat on the first floor and a Russian fireplace that heats the entire building on between 1 and 2 cords of wood a winter. (We live in Central Massachusetts).  Radiant heat is the most comfortable form of heat I've ever experienced and the ERV gives us all the fresh air we want during the heating season.  (We use the windows from late March through mid- Oct.)  We live in a rural, heavily wooded area and have never had the need for AC in the 18 years we've lived here.  However, we have the possiblity of using our geo-thermal heat pump to create cool air should summer temps rise in the future - climate crisis anyone?
  My understanding is that (in our neck of the woods) if you can get an R-50 envelope for the house, you don't need ANY heat source (outside of appliances, human heat, etc.)  Anybody know if this is accurate?

If my kid had his way, that crossover would happen at ~R25, given the number of computers, lights, and game machines left on!

But R50 isn't some magic number.  With average central MA January temps of ~18-20F there is still a significant average heat load, and at 150-180watts per adult human body it would have to be a very small house with very few windows, and VERY tightly controlled ventilation for R50 to be the magic balance point. 

The amount you get from appliances will vary a LOT with the efficiency & duty cycle of what you have.  If you're the type that lights up the living areas with twenty 75 watt R30s for 7-8 hours/day, has a TIVO & 2 computers & a game machine on 24/365, and watch 4-5 hours of TV on the 52" plasma display, you probably don't need R50 walls to make a go of it.  If you have all high-efficiency lighting controlled by motion sensors (or diligence of user), and spend your evenings under an 18W fluoro or LED reading lamp, and have all Energy Star appliances, of the smallest size that works for you, R50 is probably your baseline starting point.

Somewhere north of R30 in this climate, with sufficient interior thermal mass, insulation against the ~50F central MA subsoil, low enough and appropriately placed glazing, the bulk of space heating function can be reduced to managing solar gain & ventilation rates, which sounds like the house in this thread.   R50 clear-wall R-values make this a lot easier to go with no heating system than the bare-minimum R30 required for space heating in high-solar-gain house.  With higher R you gain you need ergo less glazing for even lower nighttime loss, etc. - you needn't live in the dark, but you can't have huge panoramic view of the neighborhood and not suffer losses at night & low-solar days.

The PassiveHouse tools are allegedly pretty good at modeling designs based on site conditions & local weather history.  Anybody serious about getting the most-bang/buck with superinsulation will likely save far more on materials than the software costs, provide they have time to use the tool properly.

With R36 SIPs you're already way ahead of the game, and if you have any solar exposure in your woodsy area, you may be able to optimize that envelope to take more advantage. But solarized or not, thermal bridging at structural elements is still an issue with SIP houses.  Other  oft overlooked details include air-sealing at the top of the foundation & insulating foundation walls (or slab-edges down at least to the frost line of slab-on-grade.) Foundation air-leakage & conducted heat losses can add up to literally half of the heat load of an R36 SIP house with a full basement.  (A guy in my office has an R25 SIP house built in the 1980s with an uninsulated crawlspace foundation and a lot of oak post & beam structural stuff penetrating the envelope.  His house is somewhat smaller than my not-so-insulated 1920s stick-built antique, yet uses more heating fuel than mine.  Even before insulating the basement walls & sealing the sill I was using slightly less fuel, now it's something like 20% less.  He even has better solar exposure than I do.  With better air-sealing & foundation insulation he would likely be using 20% less than ME rather than the other way around.)

"R50" doesn't necessarily mean a lot on it's own in my opinion. You could get 14" of pink fiberglass & be at R50 but air will move right through that. Without having thermal breaks & good air sealing, you don't have much.

Also, if you plan to not use heat at night, you need a way to capture that heat during the day - high solar gain windows, thermal mass, etc. I think, "if I have R50 I don't need heat" is a massive over-generalization.

Posted By jerkylips on 23 Apr 2010 05:48 PM
"R50" doesn't necessarily mean a lot on it's own in my opinion. You could get 14" of pink fiberglass & be at R50 but air will move right through that. Without having thermal breaks & good air sealing, you don't have much.

Also, if you plan to not use heat at night, you need a way to capture that heat during the day - high solar gain windows, thermal mass, etc. I think, "if I have R50 I don't need heat" is a massive over-generalization.

Yep,  I've repeated many times over the past few decades, a well insulated wind tunnel is still expensive to heat. For  making buildings energy efficient,  air sealing is job 1.

But R50+ fiberglass with both exterior & interior air-tightness in place isn't bad- not nearly as bad as the foam vendors would like you to believe, despite some issues with shifting R-values at larger delta-Ts (a factor foam is not immune to, albeit for different physical reasons.)  The very succesful  Urbana IL Passivehouse used ~R45  blown-fiberglass between 12" engineered I-beam studs, with another R16 of exterior EPS as a thermal break to achieve an ~R60 clear wall value.  (Note, they put R56 of EPS under the slab, despite not-so-permafrost-like subsoil temps in the 50s Fahrenheit.   They also used blown fiberglass in the attic.  Can't say if it was pink yellow green or white, but it demonstrably works.  The heating "system" is a resistive electrical element in the ventilation air stream, and this house has far less than the national average power consumption despite being heated with such a low-efficiency system.

Designing a building to work at that level of efficiency has many factors beyond mere R-values, but R50 clear wall R (which includes thermal breaks, not the center-cavity-R) is a good place to start.

Dana 1;  Sorry about your energy-enthused offspring.   I still get on my kids for not turning off lights when they leave a room for heaven's sakes.  Anyhow, I wanted to tell you about how I got around the thermal bridging with my SIPS.  This is not an original idea, but one I got from one of the most creative, talented timber framers I've ever met - Kenny Williams.   (1) insulate and air seal with caulking between foundation and sill; (2) extend sill past foundation by enough to "catch" the bottom of the SIPS;  (3) insulate between sill and band joist; (4) build deck with all point loads of timber frame engineered to take frame loads, but with insulation between each layer; (5) raise frame on deck; (6) Attach double-chip SIPS to timber frame with bolts on the INSIDE chip; (7) Foam all holes for bolts to outside chip; (7) Use rigid foam 5/8 inch spacers between timber frame and SIPS (this allows for sheet rock between frame and SIPS; (8) Foam all breaks between SIPS and bottom sills; (9) all electrical runs are put in chase on inside of SIPS; (10) all plumbing is put in one internal wet wall; (10) all external runs to septic, underground electric, etc are through the foundation; (11) SIPS have NOT ONE intrusion except for windows and doors. 
  There are a few other construction techniqes with a double roof, etc. that I don't need to go into here.  Steps #6 and #7 are Kenny's creativity at work.  I think you'll see that this COMPLETELY does away with any thermal bridging between the outside of the SIP and the inside.  This should be standard construction procedure for all timber frames with SIPS.