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Envelope air tightness sanity check

Designing a "Contemorary Industrial Passiv Ranch Haus" for DIY build in SW Colorado (CZ6B). Working through data entry in PHPP now and I know the one level elongated ranch style is sub-optimal for PH design. It's 2250 ft^2 (OD) with conc. slab and shed roof. Floor/ceiling/wall envelope area is 6982 ft^2 and volume is about 25789 ft^3. An optimal PH two level cube would have envelope area of 4531 ft^2 and volume about 19125 ft^3. I'm over that by 154% and 135% respectively. Assuming a very good air sealing design and execution- am I going to have a reasonable chance of attaining 0.6ACH50?

Also want to say I've lurked for years here, it's a great forum, thankyou all for sharing your knowledge. I'll have quite a few questions forthcoming.

Asked by Chuck Jensen
Posted Sat, 02/15/2014 - 19:07


4 Answers

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That's a lot of numbers there and my head is already sore. But yes. 0.6 ACH50 no problem. After all, it's really only plugging holes. Think about connecting all of the assemblies together and you'll do fine. If you need help, call for help. It's out there.

You catch a break on your project since the larger the volume the easier it gets. Small volumes are the hardest.

It's pretty straight forward to join a slab to a wall to a shed lid. The parts to pay close attention and plan for are the transitions: Slab to wall, wall to lid, windows, etc. There are all sorts of products available and in Colorado you've got a couple of resources from PH communities and suppliers. A&E Building Supply or Main Stream corp can help. Or… your welcome to call The Small Planet Workshop.

0.6 ACH 50 is really a "sane" number.

Answered by albert rooks
Posted Sat, 02/15/2014 - 21:29

Helpful? 0

I agree with Albert.

The problem with attaining the Passivhaus standard with a stretched-out single-story ranch has nothing to do with whether or not you can hit the airtightness standard of 0.6 ach50. It has to do with the increased surface-to-volume ratio (which you also mention). Because of the high surface-to-volume ratio, you end up having to increase the thickness of the insulation -- in some cases to levels that aren't particularly cost-effective -- in order to hit the 15 kwh/m2*year goal.

Answered by Martin Holladay, GBA Advisor
Posted Sun, 02/16/2014 - 06:45

Helpful? 0

Ah yes, my sanity check was misplaced. While the envlope wall area is not too different between optimal cube shape and sprawling ranch shape, the floor and roof area are double so those heat losses are much larger given the same R. More concrete, more roof, and more inlulation to make up for it. I think I'll evaluate both shapes in PHPP just to see how much more material, labor, and cost is involved. Wanted a single level for old age convenience but the dollar cost might change that.

Answered by Chuck Jensen
Posted Sun, 02/16/2014 - 14:00

Helpful? 1

I would seriously consider abandoning Passive House certification before I would let it dictate whether I had to have a second story I didn't want. Your situation makes another case for adopting the Good Enough House standard instead.

Answered by Malcolm Taylor
Posted Sun, 02/16/2014 - 16:24

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