the next house will be very insulated. i hope to approach passive house levels. however, i'm still under tight budget constraints. i need lots of insulation, but i need it cheap.
insulation cost
mainstream insulation options are cellulose, fiberglass, and rigid foam. these are available at the big box stores. mineral wool is another option, but i haven't priced it out at any building suppliers. i assume if it's not in the big box stores, it's not cheap. mineral wool would probably be my first choice if it wasn't expensive. it has a good R-value, it's water and fire resistant, bug resistant, etc. natural product. but cost is the issue here.
cellulose and fiberglass are the two cheapest insulation options - cellulose performs better in the cold, and acts as a hygrothermal buffer. and i just don't like fiberglass - it makes your skin itch, and it can't be healthy breathing in loose fibers in the air. thus, cellulose insulation will be used as much as possible.
envelope insulation
it's straight-forward to integrate cellulose into most components of the envelope. to get cellulose into the floor, however, constrains the design a bit. a slab foundation couldn't be insulated with cellulose. you don't want cellulose in the ground. it sucks up water, and soaked cellulose doesn't insulate. sure, i suppose you could hypothetically waterproof it, but any small failure of the waterproofing would be a disaster.
using a raised floor, we can keep cellulose off the ground. say, a ventilated crawlspace with insulation in the floor system. this isn't perfect. there are thermal bridges where the floor rests on the foundation walls/piers. and if there were ever a plumbing leak or a window left open in a rain storm, i don't think that would bode well.
rigid foam, although expensive, is resistant to water damage. it can get soaked and dry out and you wouldn't know the difference. this is worth consideration. in the real world, water disasters happen. in this light, a concrete floor is also superior to a wooden floor.
plumbing leaks can occur at supply or drain lines. they will be at joints or at fixtures. a one-story building will have drains below the floor, so a wooden crawlspace floor needn't be concerned with drain lines that are below the floor system. a pex water-supply system has a minimal number of joints. they will be at the manifold/water heater location, and at the fixture locations.
fixture locations will be baths, kitchen, and laundry. we could install floor drains and tile/waterproofing in these select areas. this would seem, then, to negate any concerns about plumbing leaks and cellulose insulation in the floor. the only other scenarios to be concerned with would be an open window in a rain storm, and maybe a flood.
insurance
i suppose at this point its worth noting that home insurance should cover damage in the event of a flood. leaving a window open or a plumbing leak, however, i'm not so sure. an open window seems to be owner negligence. a plumbing leak may be covered though. if the cost of home insurance doesn't vary depending on the foundation and floor insulation type, then the impact of these design decisions doesn't seem so great. but it could save the owner alot of headache.
cost comparison
we need hard numbers to compare the benefits of concrete and rigid foam to their additional cost. back in 2009 i perused material prices from the big box stores and calculated a cost-per-R-value-per-square-foot figure. you multiply it by the desired R-value and the area of coverage (duh):
- dense-pack cellulose: $0.0246/R/sqft
- faced polyiso rigid foam: $0.0728/R/sqft
- unfaced XPS rigid foam: $0.0901/R/sqft
- cellulose: 0.0246 x 40 x 1000 = $984
- polyiso: 0.0728 x 40 x 1000 = $2,912 (+$1,928)
- XPS: 0.0901 x 40 x 1000 = $3,604 (+$2,620)
maintenance
if there is ever a problem with the drain piping in the home, a crawl space foundation will be much cheaper than a slab to repair the work. also, renovations and moving plumbing is much less costly with a crawlspace.
decisions, decisions..
