I've spent HOURS and HOURS on the Internet (actually days and days) investigating basement insulation techniques. If you don't agree with my method below, please take the following advice: DO NOT USE A VAPOR BARRIER on EITHER SIDE OF THE basement wall. I don't care what anyone advises. I've had builders recommend vapor barriers -----------------and I'm sorry to say, they may build a great house above the ground, they don't know what they are talking about when it comes to basements!!!!
I wouldn't insulate my NEXT basement this way, but this is what I did in 2006, one year after my NEW house was built. Seven years later my basement is dry and the walls are dry. (I use a dehumidifier in the warmer months as a precaution.)
- 1/2 inch blue extruded polystyrene styrofoam (R-5) glued top to bottom of basement wall vertically and directly to walls with FOAMBOARD adhesive. Why 1/2 inch? It was on sale for a great price-----$8 per sheet in 2006.
- Sealed joints with housewrap tape.
- Overlapped 1/2 polystyrene with 2 inch WHITE expanded styrofoam (R-8) HORIZONTALLY with FOAMBOARD adhesive on the TOP half of the wall only. Why 2"? It was on sale for a better "great" price--about $9 a sheet in 2006.
- Sealed joints with housewrap tape.
- Installed 2" by 4" stud walls against the white styrofoam.
- Installed 3 1/2 inches of Kraft paper backed fiberglass insulation (R-13) between studs and slashed the Kraft paper with a razorblade to allow any moisture trapped in wall to escape. (Do NOT use a vapor barrier on basement walls.)
- Installed housewrap over the now fiberglass-insulated wall. The house wrap allows moisture through--------------but prevents relatively warmer basement air from contacting relatively colder wall.
- Applied regular 1/2 inch drywall over the housewrapped wall.
- Painted wall with an eggshell paint. (not semigloss---------remember, you don't want to trap any moisture behind the wall.)
What would I do differently today? At my house-----------high on a hill and NO threat of standing water against the foundation--------------I'd still install the additional fiberglass insulation in a 2 by 4 basement wall. But, my first option would be to use spray foam between the studs------depending on the cost of the spray foam. If costs were prohibitive, I'd
- attach 2 inch blue styrofoam directly to wall top to bottom with FOAMBOARD adhesive
- seal joints with housewrap tape
- install 2" by 4" stud wall.
- install 3 1/2 inches of batt insulation MINUS the Kraft paper
- still install housewrap over the stud wall
- use MOISTURE RESISTANT drywall.
- still use paint with an EGGSHELL finish.
I live in the THUMB part of Michigan (Michigan is shaped like a glove?) and the winters are pretty active. I use propane to heat my house (2 by 6 inch exterior walls with R-21 fiberglass insulation and I have about 22" of blown-in insulation in the attic). For people who use propane in the northern states, you know that heating an 1800 square foot house would cost a pretty penny to insulate.
I have yet to spend more than $950 per year to heat my 1800 square foot house with propane. (I have a set-back thermostat, double pane windows, insulated window treatments, and most of my windows face south.) The $950 also includes heating my water with a propane hot water heater. Insulating the basement reduced my heating bill a shade over 10% or about $100 per year. Today, I'm saving even more because I paid $1.29/gallon for propane in 2006 and today --2013) I'm paying about $1.79/gallon.
EXPENSES FOR INSULATION
$360 on 1/2" foamboard $198 on 2" foamboard $20 on housewrap tape
$40 on housewrap $40 on FOAMBOARD adhesive
$658 not including 2 by 4's and drywall.
I've been saving about $100+ per year on my heating bills since insulating the basement. You can do the rest of the math.