Net zero construction in Yakima, WA — Climate Zone 5B
I have been a frequent reader of this website over the last couple years and didn’t see an article that addressed a few questions I have run into when attempting to design and build a net zero home in Yakima WA (climate zone 5B).
I do understand that net zero can be achieved in any house with a big enough solar array, but I’m looking to strike a balance and use as little energy as possible based on my design choices, building material decisions and construction techniques. The hard part seems to be finding the line on what is cost effective within the current market conditions. I’m by no means a rich man and can only afford to do what has an ROI that will realize within a decade or so.
Here’s it what our structure looks like so far from an energy conservation perspective:
– Property is a South facing hill slope with no obstructions perfect for passive solar gains.
– The structure is a single story 1000 sq ft apartment with an open floor plan (two bedrooms, single bath, laundry, kitchen and great room).
– The conditioned space is a 26 ft by 38 ft rectangle.
– Continuous insulation for all exterior walls (currently settled on a 2 x 6 wall filled with fiberglass batts and CI rigid foam on the exterior, probably 2″ of XPS).
– Concrete floors throughout with radiant heating via a Daikin Altherma System, which also includes an air handling unit for cooling in the summer (ducting housed with conditioned space of course).
– The air tightness of the conditioned space will achieve 1.5 ACH or better.
– The air quality and circulation will be done via an HRV.
– Atrium triple pane vinyl windows with Low-e glass, all but two are fixed, the remaining are casement.
– The majority of the windows are south facing with a pergola overhead on the exterior to provide necessary shade in the summer to avoid overheating the build envelope.
– The ceiling are 10 ft tall and the doors are all 8ft tall. I know this is not energy efficient but it is a necessary compromise with my wife for all my energy efficiency requirements lol… plus it makes the apartment feel much more spacious.
– Exterior doors are all single fiberglass doors, but they are 8 ft tall and 36 inches wide.
So now for the questions:
1) What is the sweet spot for R-values in the ceiling, walls and slab given this build envelope? Especially in terms of how much rigid foam is need in the slab, stem walls and exterior walls. Code in WA State is R-49 in the ceiling, R-21 in the walls and R-10 in the slab and stem walls. I have read the R10/R20/R40/R60 rule referenced here, but that seems overkill perhaps given all the other factors in play that I have noted above.
2) In my searches I found a great presentation on R-40 walls, comparing different construction strategies and methods (link here). Slide 31 does a great job at indicating what is effective for the money. Based on their results a 2×6 wall with exterior XPS CI anywhere from 1-3 inches is quite cost effective. It’s only beat by a 2×8 or double stud wall with exterior XPS CI. In everything I’ve read a rain gap is needed between the sheathing and foam, how much does that impact the insulating quality of the exterior XPS CI?
3) Given the exterior door height of 8ft I am considering a multi-point locking system, this upgrade is expensive from Therma Tru which is the door manufacturer we are looking at and it also limits lock/handle selection to Therma Tru’s options according to their literature. Is this kind of upgrade going to be worth it?
Any guidance on these questions would be much appreciated. I feel like I’ve been think about this stuff too much!