Pretty Good House P1, How much insulation is enough?

Building Science is something I’ve been interested in for many years now. Below I’ll go over how that intersects with my current and largest project to date, a Pretty Good House which is a house almost as good as a passive house but trades off some of the ridiculous insulation and glazing for a reasonable operating cost (utilities)

My wife and I are adding a ~1000 sqft living space over a garage behind our business. The only source of power we’ll have is electricity, I don’t want gas/oil/propane inside the structure for indoor air quality reasons and general safety. So we heat with electricity, which can be expensive in a zone 5  borderline 6 climate where we live. To mitigate the costs of heating I’m building a super insulated and super airtight structure. The airtight portion is by design, there will only be a few openings in the entire structure where bulk air could leak through while the rest of it is sealed. Don’t worry about ventilation, I’ll have a heat exchanger which I’ll go over in a later post.

The superinsulated specification is a bit more complicated. Because I wanted to maximize the available floorspace in the second story above the existing structure I determined the insulation must be outside the 2×4 studs (2×4 again to maximize floorspace). To reduce my contractor’s construction quote it should be somewhat standard construction practices for the area. For this I decided on rigid exterior foam panels. My desire was for an R40 wall. Conveniently there is a guy somewhat local that sells reclaimed foam, the best $/R foam he had was 2.5″ Polyisocyanurate panels from an old commercial building. So I loaded up a borrowed trailer (Thanks Lynne!) and my truck and 7 hours of driving round trip I had my insulation. This was $16/sheet and I needed about 82 full sheets for 2 layers around the entire perimeter. The reclaimer threw in some additional sheets because many of the sheets were not perfect (either cracked, chewed, missing pieces). All in all I had 6*16 or 96 sheets total.

These were installed 2-layers at a time with staggered seams, furring strips and 7″ long screws.

Yielding a nice thick efficient wall

*Note the insulation is slightly compressed by the furring strips.

Below is a table showing all of the layers of the wall and their r-values along with a total of about R-40. This is a rough approximation and does not consider the effects of windows, doors, or the thermal bridging effects of the screws/nails/studs/headers and assumes one complete stud-bay. A thick build-up of polyiso has a higher effective R-value. New virgin polyiso has an R-value of 6 or 7 but as it off-gasses it becomes less effective. I used this data to compile my totals, other similar tables are all over the internet.

Wall Build-Up
Vented Vinyl Sheathing 0.61 0.61
Polyiso 5″ @ R5 27.5 27.5
Sheathing 3/8 osb 0.45 0.45
Wood 2×4 3.5
11.4595838
Dense Pack Cellulose 12.95
Drywall 0.45 0.45
Effective R Bare Wall 40.4695838

In future posts I’ll go over a bunch of other factors that make this a pretty good house like site orientation, floor and ceiling insulation, windows, doors,  and HVAC.

Waterwheel Microhydro P1, The Dam

This is the first part of my waterwheel series. In this video and subsequent videos I go over the construction of a dam, flume, waterwheel, and electrical system for my little microhydro scheme.

Stats:

  • Minimum Flow: 10 GPM
  • Maximum Flow: 1000+ GPM
  • Designed operating flow: ~50 GPM
  • Designed water drop (head): 32″
  • Desired wattage output at 12V: Peanuts, or somewhere between 5W and 20W continuous output.