To Passive House or not Passive House – That is the Question

OK, so building a Passive House is not such a big deal, right?
- PHPP modelling? check
- lots of insulation? check (R70 walls, R90 roof)
- very good air tightness? check (We’re around 0.70 ACH50 before drywall)
- minimize thermal breaks? check (4″ rigid polyiso on outside)
- fancy windows? check (Paradigm triple pane)
- heat recovery ventilation (HRV or ERV) system? check (Lifebreath 155 ECM)
- passive solar? check. (well, kinda. active solar coming soon…)

OK, but in the US at the moment it’s a bit harder. As Mark Siddall writes over here, to be a true passivhaus — a certified passivehouse / passivhaus — one really has to go the extra extra mile on the items above. And well… we haven’t exactly. Part of it is that, from what I can tell, even if we do 50% or 100% worse than the Passive House standard, this is still really really really good. And in the process we have saved many thousands of dollars.

But let’s take a closer look. Let’s say our house ends up costing $700/year to heat vs a certified passive house which maybe would be $350 for a house of our size. So that’s $350 a year more. (One edit… I think the numbers are probably more like $250 vs $500, so I suppose that’s an even tougher sell. As John Straube put it… that’s less than a Starbucks coffee a day… LINK Valid point. But there are good reasons to do this stuff money aside!)

But this means the owner now just has to buy 25
MMBtu/year or about $300 at todays gas prices. Even for a motivated
homeowner, it is hard to spend too much time, money, give up
convenience and aesthetics to reduce the cost of home heating below the
“annual cost of a Starbucks Venti latte per week”.

First of all, the “net zero homes” and some superinsulation people say it’s cheaper to make up for that $350 using PVs rather than spending more on further improvements in the envelope. example discussion… The basic problem with some net-zero thinking I think is that PVs eventually break or wear-out. And insulation doesn’t. So comparing their cost is difficult. So we did go pretty crazy with insulation (R70 walls, R90 roof). But on the other hand, passivhaus-level windows and HRVs will eventually break too. So these are areas we didn’t go quite as far on. Also, we were selfish and wanted double hung windows. That’s a no-no with certified passive house windows as far as I know. But we got the best ones we could find on the east coast (Paradigm Premium Double Hung — triple pane, warm edge spacers, foam insulated frames, R5). And we’ll get cellular / honeycomb shades for night, since I want them anyway. Better than looking out black holes.

Second of all… just for curiosity’s sake, what is the simple payoff for (let’s say it would have been $10,000 additional spent on fancier windows and heat recovery, etc?) vs that extra $350 a year in heat. BTW, that $350 is assuming you/we have “GreenUp” energy from your utility — National Grid or NStar. (We don’t, since we have a municipal electric company, but we do pay into the Mass Wind program, so I guess you could say we do…) Anyway, so whether it’s PVs or green electricity from the grid, we’re supposedly on pretty equal footing.

So that’s $10,000 / $350 = 28.57 years

Really… simple payoff is misleading and one should figure out what it costs to borrow that $10k, since maybe it is most realistic that it’s in a mortgage. Either that, or if it’s $ spent in cash, then theoretically it would be earning some interest over 30 years. So you have to include opportunity cost. So maybe you could double the number of years for the payoff in either scenario.

In any case, you can see it’s probably a fairly close tradeoff, assuming my numbers are somewhere near right.

Third of all… what we are *really* planning to do is install a much fancier *active* solar heating (and hot water) system for the house which I think will reduce our “primary” or “source” energy use by more than the fancier windows or HRV would have. Thermosyphon air collector … based on the proven principles here at builditsolar.com.

And heck, we can always put in a fancier HRV. Apparently they make them in a different/better way in Europe. Ours is still pretty fancy. It’s a Lifebreath 155 ECM.

OK, and just a little more on windows… the “even better windows” we were considering are from Thermotech, in Canada. The increase in cost (for doors and windows) would have been substantial over the Paradigm. Almost double… maybe more like 70% more to be roughly precise from the numbers I crunched last night looking at old quotes. They would have been awesome, no doubt. Even Thermotech windows I don’t think(?) are technically “certified” Passivhaus windows, which are maybe only available from Europe at this point. But it seems like the company is at least obsessed enough that one could get the calculations and measurements one would need to fill in the blanks in the PHPP modelling software. Versus many other companies are not willing to do that for you, even if they have pretty darn good windows. That’s my understanding anyway. Here’s a discussion about the differences between European and North American window calculations.

Just for completeness I should mention 5 other ways (besides house insulation, etc.) to use substantially less energy. Specifically fossil fuels:
1. Don’t live in New England!
2. Live in a smaller house! Or in an apartment building with shared walls. (*)
3. Heat with wood. From locally, sustainably forested forests. (**)
4. Don’t drive. Or drive less. (*)
5. Don’t eat meat.

* Number 2 and 4: It’s green living in a city. Thank goodness for New York City, etc, etc!

** Number 3 is tricky with a passive house because it is SO tight, that even with direct-vent pellet stoves or masonry stoves like Tulikivi, etc. (where the make-up air comes from outside) one has to worry about the tightness of the firebox itself. You don’t want to be breathing smoke. So there is maybe some argument for building a little less tight and burning wood if you want to be fossil fuel free. But that’s not possible everywhere because of local laws or non-availability of local wood. And I’d have to find a recent study, but I’m not sure it’s sustainable for everyone in the northeast to burn wood. Let me know if you know the answer to this!
Here’s a recent discussion of heating a passivhaus with wood

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