Expert: Ted Barnhill (Principal, Design45 LLC) - 1/23/2010

Question
Hi Ted,
We are going to build a new, single story (with basement) home in northern Indiana.  It will be out in the country, so it will get some winds.  We are going with 6" walls, and a 95% efficiency gas forced air furnace.  We will also have a heat pump.  A few questions:  What is better, house wrap or spray foam?  I'm not sure how much or where foam is sprayed.  Should I even bother with either since we have 6 inch walls?  Is a heat pump needed with a 95% furnace? I don't want to go overboard on these things, but if spending another $1000-$2000 will make a big difference, then I'm all for it.

Thank you

Answer
Mark,
I'm glad to see that you are taking the time to research appropriate technologies for your new home's heating and cooling system.  The right choices will repay themselves many-fold in lowered energy costs.

To take your questions one at a time:
1. House wrap and spray foam serve different, though somewhat overlapping purposes.  House wrap is a weather resistant barrier (WRB), an industry term for the required layer in the wall assembly that stops water from entering the wall if it gets past the siding (which it will at some point in the building's lifespan).  If the seams are properly taped, house wrap also serves as an air barrier, which is the code required layer that stops the wind from blowing through your wall.  These are two equally important but very different functions.

Spray foam, on the other hand, is the insulation that fills the cavities in your wall.  It is used in place of fiberglass batts or other insulation alternatives.  In your 2x6 walls, it goes between the studs and should fill the entire cavity.  Spray foam also acts as an air barrier, because it expands into all cracks, hardening to provide an excellent air barrier.

I think it is fair to say that spray foam is both the best air-barrier around, and the best insulation.  I recommend that you use it.  It will probably cost you thousands more, but will ensure a very tight, efficient house with less chance of future mold problems.  Depending on your heating usage, I would expect it to pay for itself (in energy savings) in 10-30 years.  If you do use spray foam, then your best bet for a WRB is probably felt paper instead of house wrap, as it allows interior wall moisture to dry out effectively.  Just be sure that you properly flash all windows and other openings.

2. Heat pumps are typically used in mild climates instead of gas-fired furnaces, not alongside them.  Heat pumps don't work as well in very cold temperatures, so here in Minnesota the gas furnace is the norm.  Another option that works well even in very cold climates is a geothermal-source heat pump, in which wells or trenches are placed in your yard as a heat sink for the heat pump.  This is an incredibly efficient system, but costly up front.  Your local HVAC installers will be able to provide data on likely annual energy costs in your area, so that you can compare the effectiveness of the various systems.

Ted Barnhill
Design 45, LLC