Posted by: Li Ling Young | September 4, 2012

Choosing Basement Insulation

For those of you not obsessed by the medieval fantasy novels and television show, Game of Thrones, it’s a dangerous world for those with power and those without: a sexually violent and unjust world for the cunning and the guileless alike; doom crowds in on all sides, and even if your enemies defeat each other before getting to you, there’s always Winter.  Now we can all appreciate the irony of the young, soft father pushing the tricked out stroller with the cup holder (for the pusher!) through our neighborhood to the bakery on a Sunday morning wearing a Game of Thrones shirt: Winter Is Coming… in black, of course.

Yet even in this suburban world of lawn mowing, baseball games, neighborhood bakeries and scooters left on the sidewalk, winter is actually coming.  It’s time to get serious about insulating this house.  The question today is, how do we insulate the foundation?  We’ve got two areas we can get to without disturbing any of the finish surfaces: the basement walls in the utility room (about half of the basement), and the crawlspace walls under the addition (pretty sizeable addition).  The walls are concrete block in the basement and poured concrete under the addition, so they are flat enough that foam board is the best choice.

Foam board is nice because it can be applied directly to the wall without any framing to hold it up; it’s easy to seal up; it’s got a high insulating value.  Downsides: expensive and slows drying.  And then there’s this one: as a plastic, foam board is made with some pretty nasty stuff.  One of them is the blowing agent that causes the bubbles to form so the plastic doesn’t conduct energy.  Some of the blowing agents are potent greenhouse gases.  If you want to do what you can to slow climate change, saving energy by using a greenhouse gas-intensive product is a bad proposition.

Following a hotly debated article by Alex Wilson of Building Green, energy efficiency consultant David Whitecreated a handy tool for comparing the global warming potential of insulations.  Here’s a little snip of the tool.

Greenhouse gases associated with insulation

As you add more insulation the amount of energy saved by each subsequent inch diminishes while the blowing agents stay the same.

Along the bottom it shows the level of insulation.  As you add more insulation, each additional bit saves less energy than the previous bit because there’s just less to save.  Therefore, the curves flatten out at the higher end of the scale because your benefit is falling off.

Along the left it shows greenhouse gas emissions: combined blowing agents released from the insulation plus emissions from burning fossil fuels and losing heat through the envelope of your house.  The curves start high on the y axis because the energy lost through low insulation means high greenhouse gas emissions.  Add a little insulation and the amount of greenhouse gas emissions drops rapidly because some insulation in a poorly insulated home makes a big difference.  Add more insulation and you save more energy, but you also have more blowing agents, so the line stops dropping quite so fast.  With some insulations you actually get to a point where more insulation contributes more greenhouse gases from blowing agents than it saves in fuel, so there’s a line that turns up and starts going the wrong way.

The one that’s going the wrong way, that’s the most common foam board insulation used in homes.  That insulation would make this house an energy freakshow, but in the wrong way: the hypocritical and incoherent way.  So, if we’re not going to use extruded polystyrene, the easy-to-buy default insulation, what do we use and where the heck do we get it?

The chart shows three other insulations.  One is there as a reference and isn’t a contender:  cellulose (red line), which is my favorite insulation, made entirely from mashed up newspaper and cardboard.  I thought it would be a good baseline because it is made from recycled materials.  Cellulose isn’t a foam board.  The curve in between extruded polystyrene and cellulose is two insulations that have almost the same greenhouse gas profile: expanded polystyrene and polyisocyanurate.  To keep them differentiated I use EPS for expanded polystyrene (good guy), XPS for extruded polystyrene (bad guy) and PIC for polyisocyanurate (other good guy).  Both EPS and PIC are good candidates for insulating our basement.

There are a few other things to consider.  There are small differences in the insulating value of the foams.  Two additional important considerations are moisture transport and fire.  Walls that have dirt on the other side are always a little bit wet.  In my opinion it’s nice to have a way for that moisture to evaporate away.  Unfortunately since the foam boards are plastic, they don’t allow for much vapor diffusion.  EPS is a little better on this score than XPS, and PIC is absolutely the worst because almost all the PIC products have some sort of foil facing which allows no drying whatsoever.

For fire safety foams are required by code to be covered by a thermal barrier.  During a fire this prevents the foam from emitting disabling gases for a few minutes so the occupants can get out of the building before frying their lungs.  The PIC comes in a version with thermal barrier attached.  Neither EPS nor XPS are manufactured with a thermal barrier, but drywall can be added over the foam to provide a thermal barrier.

Taking all that into consideration: greenhouse gas emissions, insulating value, drying potential, fire safety and cost, I decided on EPS foam with no thermal barrier.  Perhaps we’ll add drywall sometime later.

Now, how much foam?  One of the advantages of EPS is that it is manufactured in many different thicknesses.  Here’s a product made with 10” of EPS.

Structural Insulated Panels with 10" EPS core

EPS is manufactured in many different thicknesses.

Given the insulating value of EPS, about R-4 per inch, and that we will have the option of adding more layers of foam in the future I decided on a minimum of 4” of foam for our first effort.  If I were building new or had a more roomy budget I’d want to double that.  I feel pretty good about 4” given that we’re starting with completely uninsulated basement walls.  When I’ve recovered emotionally and financially from this first flush of energy improvements, I’ll tackle the question of how to insulate the walls of the finished basement.  Only then will it make sense to go back and add more insulation over the 4” of EPS.

Costing…  To start with, EPS is a bit of an oddball product.  It’s used in a lot of packaging (the molded foam blocks that cushion the toaster in the box).  But it’s not as common to find it used in buildings.  When I call around looking for EPS at the supply houses I’m going to have to be extra careful that they don’t think I want XPS, otherwise my whole exercise with the greenhouse gases will be lost.  Our local building supply, Curtis Lumber, wants to know how many sheets I’m buying.  In all likelihood I’ll have to buy this stuff by the palette, so no small orders.  After a week they still haven’t gotten back to me.  Nicholas suggests Sticks n’ Stuff, which is pretty far away, but will deliver for cheap.  Again, it’s a special order, so palette-fuls only.  They get back to us with a price for 4” XPS.  After some clarification they call us back with a price for 4” EPS, $42/sheet (4’X8’).  That’s less than half what the XPS cost.  We need 21 sheets for the basement and crawlspace: about $950 all told.


  1. Since this is your first winter there, what is your best guess for how long it will take you to make back your $950 in energy savings?

  2. Oh, Jessica, that is a sticky question. The short answer is, a long time. But the bigger picture is that part of what we’re gaining by insulating the basement well is 1) greater comfort and 2) the ability to control the environment in the house. Having uninsulated parts of a house is what first gave rise to the original energy freakshows: desperate, poorly-conceived attempts to be comfortable. The insulation and heating improvements to the house need to be considered as a package. The end game is system that manages energy so gracefully that it becomes elegantly simple. You can’t hope to get there with uncontrolled heat loss in any part of the house.
    But, if you really want a number, it’ll be something like 15-20 years. Of course, energy costs have a lot to do with that, and as we’re changing our heating fuel it becomes a bit muddled, and then there are all the geopolitical issues around fuel like fracking (makes fuel cheaper, payback times longer), peak oil (fuel more dear, payback times shorter), higher standard of living in hot countries like India (payback times shorter)… It’s a very complicated question you ask.

  3. That is a long time, LiLing! So, how long will it take you to make back the expenditure on the solar panels? To my mind, it doesn’t really matter, because what is important is that you’re doing the right thing for the environment. But still…I’m curious.

    • Well, the solar situation is a bit sunnier, if you’ll forgive the metaphor. We financed the solar system, so one way you can look at the “payback” question is monthly cash flow. Our loan payment is $149/month for the $20k we borrowed for the solar system. In February of next year we’ll get a $6500 tax refund on the solar system, and when we put that toward the loan and reamortize (which the bank will do for us since they know we borrowed the money for a solar system), our monthly loan payments will be around $109/month.
      We haven’t had the panels up for quite a month, but during that time the value of the electricity we’ve generated is $210. So, already we’re in positive cash flow.
      It’s been a sunny September, nonetheless over the course of a year the average monthly solar “income” should be around $158/month. So, we should be in good shape for immediate “payback”, if you want to think about it that way.
      Another way to look at it is the lifetime cost of electricity from the solar system. I’m not sure that Nicholas has done the numbers (you can expect a lot about the solar system on this blog in the future), but I think the lifetime cost of electricity out of our solar system is going to be something like $.11/kWh. Right now we’re paying over $.14/kWh, and of course that will go up over time, while the cost of the solar electricity won’t. The solar system has a life expectancy of 30 years, but there are many 30 year old solar systems out there that are still going strong.
      And finally, the most direct answer to your question is in 10 1/2 years we will have generated solar value equivalent to the cost of the solar system. That assumes that the utility will continue to pay us the same rate over that time, and ignores the time value of money. I don’t really have a head for all those subtleties, so if you want a more nuanced answer, find me an economist and we’ll put our heads together and get you an answer in a few weeks.

      • That is a sunnier situation! I’ll work on that economist…in the mean time, congratulations on your positive cash flow!

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