I am about to start design work after mulling over several remodeling alternatives regarding my house in Silicon Valley near San Jose, CA. (Zone 4, 2500 HDD and 400 CDD.) It is a two level 3000 sq ft house built in 1962. The house has been built using standard 2x4 wall construction over an 18” tall vented and uninsulated crawlspace. It has some insulation in the roof and the attic, but pretty much nothing else. It has a wood burning fireplace and single pane windows and french doors. I am sure it is leaky as hell since it feels cold all the time in the first floor.

I will be doing an approx. 500 sq ft addition to the house while doing extensive reconfiguration of the layout and the rooms. I expect most rooms to be opened down to the studs. This is an opportunity to do energy efficiency upgrades, but I am now a bit paralyzed on how to proceed.

I have done some research on Passive House and Net Zero energy homes. While I generally like the ideas overall, I am not particularly keen on getting certifications etc. I have to work within my budget and am happy if I could get to 80% reduction in my energy consumption. I am likely to be the GC and will coordinate work between the various tradesmen who won’t really know much about these concepts. Do I need a consultant to advise me or advise the various trades?

The house has wood clapboard siding. Will I need to remove it in order to do a good job of air sealing? If so, then I can do external rigid foam insulation. Or should I give up on that idea and build a second stud wall inside and super insulate internally? I have a bunch of other questions, but I don’t want to spew them all in one go.

As you know, the primary concern is to get the house tight, so remove the siding and air seal the exterior wall. Ultimately this is better than adding a second interior wall.

I'd use software like BEopt to evaluate the options (on a $ ROI basis, not % reduction). Consider filling the walls with cellulose, adding XPS foam on the interior side and then taping it and covering with drywall. Air barriers on both sides is even better, but perhaps not worth the cost. Also look at PV solar panels.

You don't need to strip the clapboards to tighten the place up considerably. Dense-packing the wall cavities with 1.8lb fiberglass or 3lb cellulose can make a HUGE difference in a 1960s vintage house's air tightness. It's usually possible to dense-pack over aging R11 batts in-place, assuming the cavities have any insulation at all, but if you're opening it all up, gutting the interior walls you can air seal the studs to the sheathing with caulk (assuming it's plywood, not plank), and carefully fit R15 rock wool or fiberglass in there for less than the cost of dense packing.

You'd have to strip the clapboards to be able to install exterior rigid foam, but that may not be the highest priority. Installing 2" of continuous polyiso on the interior side would roughly double the wall's performance, at the cost of that much interior floor space. That's not super-insulated, only R22-23-ish whole-wall, but that's sufficient wall peformance to hit Net Zero Ready in zone 4B. Download a copy of BA-1005, and look at Table 2, the zone 4 row values:

https://articles.extension.org/sites/default/files/7.%20Outdoor_Design_Conditions_508.pdf

Note, that was drafted in 2009, when commodity panel PV was efficiency was 12-13% efficeincy and cost $8-9/watt installed. At today's PV solar efficiency it takes 10% less roof real-estate, and PV prices have dropped by more than half. If it's cheap or easy to hit those whole-assembly R numbers, do it, but otherwise, close is good enough.

Converting the crawlspace into sealed-insulated space with R12-R15 rigid fire rated polyiso on the interior side of the crawlspace walls is going to be worthwhile. It reduces stack effect infiltration drives and first-floor chill by quite a bit. When it's cold outside warm air is leaving via all the leaks to the attic, but primarily entering via the crawlspace.

Getting rid of the wood-burning fireplace is a priority, especially if its on an exterior wall. If you insist on keeping it it's unlikely that you'd make anywhere near an 80% reduction in energy use. But if you keep it, an air tight wood burning insert will help (a lot), as would insulating the outside of an exterior chimney with at least R12 continuous rigid rock wool. (You can't use rigid foam on chimneys and still meet fire codes.)

Thanks for the link to BEOpt and the BA-1005 documents. Unfortunately, I have a Mac and will have to figure out a way to run a VM for Windows or scrounge a Windows machine. I will then try to get some ROI figures.

The BA-1005 table lists these out for Zone 4 - 25 (Wall) 60 (Vented Attic) 45 (Compact Roof) 15 (Basement wall) 30 (Exposed Floor). I do have a section of the house that has a compact roof and some portion that has a vented attic because the house is a saltbox style with dormer. The dormer portion has the compact roof and the steeper pitch has a vented attic. I don't have a basement, but I assume the crawlspace/foundation wall needs to be at R-15 and the underside of the floor needs to be at R-30.

The wood burning fireplace is indeed on an external wall, and I don't intend to keep it. I am planning to turn that family room into a bedroom and seal the fireplace up. If I do end up stripping the clapboards, what should I use for the vapor/air barrier? The documents mention a liquid applied water control layer, but I don't know what that is. I am thinking that pulling out the clapboards will perforate the sheathing which will need to be smoothed out and a liquid based barrier could help with sealing that more effectively. What are my choices here?

For the addition, should I plan to have 2x6 walls or should I go with regular 2x4 walls?

I definitely want to go with Solar PV and perhaps also DIY (from BuildItSolar) solar water heating, but that will have to come in later on during the construction process. Now that we have the 5 yr extension, I don’t want to rush it. If I do solar DHW, should I consider radiant heating and cooling or is that overkill if I get the house tight and well insulated?

Self adhered building wraps make sense to me (as a vapor permeable air barrier + WRB). Vycor enV-S, Blueskin VP100, Delta-Vent SA, etc. Or taped XPS plus Tyvek.

On the crawlspace insulation, if you insulate the walls to R15 (continuous insulation), you can skip the R30 between the joists.

IRC 2015 code min for the walls in the addition would be 2x6/R20, which has a whole-wall R of about R15. If you installed R23 rock wool in 2x6 framing and added 1.5" of exterior polyiso you'd be in the ~ R25 whole-wall range recommended for zone 4 in Table 2 of BA-1005.

In your climate you don't need or want vapor barriers, but you DO want to build it air-tight. Detailing the structural sheathing as an air barrier by taping the seams with Wigluv or similiar, and taping the seams of the exterior foam, caulking the bottom plate of the studwall to the subfloor, caulking the seams of doubled up top plates, and caulking every stud bay to the sheathing prior to installing the cavity insulation makes for a very rugged & reliable air barrier. (Huber ZIP sheathing is dead-easy to air seal the seams using their tape, but other OSB or plywood can be air sealed without huge effort too.)

Solar thermal makes less financial sense and is higher maintenance than PV + heat pump water heaters.

Rather than just sealing the fireplace up, take it out completely. Masonry isn't nearly as air tight as it looks, and it's a huge R2 hole in your R10+ 2x4 wall and R50+ attic.

Thanks!

I got the BEOpt software and installed it on a windows machine. However, I am unable to create the roof into the style that is on the house. Is that not possible or is there some facility to create a custom roof using the software?

I had forgotten to mention that the front of the house current has red brick (veneer?) under the first floor windows and vertical siding from the bottom of the window on up. Will removing the brick require resheathing on the front?

Most of the time with partial brick veneers with other types of siding above the structural sheathing runs full height in a single plane behind both the brick and siding. It shouldn't need to be re-sheathed to replace the brick with other siding unless the sheathing has rotted (as sometimes happens behind brick veneers without venting the air space.)