Since I managed to draw the ire of at least 3 people (at least one of whom clearly didn't understand the technology) with my last post pimping ground-assist heat pumps, I thought I'd give it another go, but this time I'd like to focus on another really cool high-tech, low-cost and low-work way to boost home energy efficiency.  Before I go there, I'd just like to offer up this link to those of you who find geo-thermal or ground-assist heating and cooling systems intriguing.  It's a cool savings calculator that allows you to plug in your location, home size, existing heating & cooling system info, and then shows you the annual savings on energy bills you could realize with a geo-thermal heat pump system.  Keep in mind that this IS a manufacturer's website, and the calculator doesn't take into account your personal energy usage habits; in my case, the fact that for 2 - 3 months per year I open up the house and run a whole-house fan means that I wouldn't save as much as they indicate.  But still it's a cool toy that can help you see the savings these systems could offer in your part of the country. 

Now, on to today's geek topic:  radiant heat barrier.

Basically what we're talking about here is aluminum foil that you staple to the framing in your attic, floor, or exterior walls.  It operates on the principle that most heat loss/heat gain is due to radiant transfer - heat moving through an object or air.  The "R-value" you see cited on insulation stands for "resistance".  Basically, that's what insulation does - it "resists" heat transfer, mostly through trapped air pockets.  It doesn't stop heat transfer; it just slows it down by trapping the heat in the insulation's air pockets.  And when it has trapped all the heat it can hold, it then "radiates" that heat out, in all directions.  That's at least one of the reasons why, in summer, a house that's been shut all day with the A/C off will still be hotter than the outside air when you return home at night - there's insulation and air up in the attic that's now releasing the heat it stored during the day into your living space.  The same thing happens in reverse during the winter, only now the heat trapped in the insulation is heat that's rising from your living space.

What radiant barrier does is reflect heat.  In summer, the heat in your attic is mostly radiant heat being given off by the underside of the roof.  A radiant barrier under the roof joists will reflect up to 97% of that heat back out so the attic space never heats up in the first place (and the insulation on the attic floor never traps it to release it later back into your home).

There are a number of radiant barrier products on the market now.  One that I have used successfully in several installations is Prodex, which is a layer of closed-cell foam sandwiched between aluminum foil on each surface.  It has an Energy Star rating.  I first used this product in a 100-year-old home which had closets tucked under the roof slope - one of them on the south side of the house.  So much heat radiated through the roof in the summer that the homeowners complained they couldn't sleep in the bedroom during hot weather.  They wanted to demolish the back plaster wall and sloped ceiling of the closet and install insulation, then close it back up.  But since insulation needs an air space to work effectively, and there was only about 6" of space from the back of the wall surface to the bottom of the roof deck, I told them I didn't think they would gain much relief with fiberglass batts.  So I started researching and found this stuff.  Working with it was easy - it's flexible, only 1/4" thick, can be cut with scissors and installed with a staple gun, and it has an R-value of 15.  With the space we had to work with, we weren't going to get even that high of an R-value with fiberglass.  We left air gaps between the material and the roof deck, and between the material and the wallboard.  By the time we put the last piece in, the temperature in the bedroom had dropped - by about 20 degrees!

Well, after that I was sold, and since then, I've recommended to every customer that we install the barrier any time we open an exterior wall or someone wants to insulate an attic or crawl space.  For one thing, it's easier to handle than batts or blown-in insulation and it works better, too.  For another thing, the cost of the material is only 50 cents per square foot, less if you are buying a larger quantity - this compares favorably with the cost of both fiberglass batt and blown-in insulation.  For those customers doing new additions, I've recommended it as a house wrap in place of Tyvek - it provides the same vapor barrier, but adds insulation and soundproofing as well.  And thanks to the energy incentives included in the bailout, if you install this stuff any time in 2009, you'll get a $300 tax credit.

As for myself, I'm planning on installing it next year in my own home.  I'm going to be installing it over the ceiling joists so that I get the winter benefit of heat being reflected back down into my living area.  We're going to screed down the top couple inches of existing blown cellulose to allow an air gap and then staple this stuff to the tops of the ceiling joists.  Then, we'll run 2" sleepers over the top of it, perpendicular to the ceiling joists, and install decking above.  The decking will keep the foil surface free of dust so the top refelective layer can do its thing during the summer - reflecting the heat of the attic up and away from my living area.  One of the reasons I'm installing this way is due to my only concern with this product - I would hesitate to place it in direct contact with the bottom side of a roof deck, or on top of a roof deck under the tar paper, because I worry that the additional heat being reflected by the product would cause shingles to deteriorate more quickly.  I don't think this is a problem if the proper air gap is allowed for and the attic is well-ventilated with a ridge vent, where the heat can flow up the gap between the barrier and the roof deck and out through the vent, but I want the winter benefit as well.  I'll be jacketing the water heater and wrapping the water pipes under the house with it as well.

So, for those of you who advocate "envelope improvements", this one's for you - though you'd still get better efficiency and greater energy savings with a ground-assist heat pump.