Cutting Edge Insights
with Dr. Lee

What is GWP?

GWP stands for global warming potential.

I’m not an environmental activist. I do like progress. I also like trees.  Progress means we can build more comfortable and more efficient structures because we know more than we used to know.  Progress means using trees for lumber production rather than living among the trees.  Progress means building and real estate development.

If a tree is in the way of progress, cut it down.  But consider planting 5 more trees somewhere else.

Q:  OK, what is the problem?  

A:  1. We cut a tree down, but we don’t plant 5 trees somewhere else.  We leave it to someone else and it doesn’t happen.

We all know the best time to have planted a tree – a couple decades ago.  And the second best time – today.

2. We use products and materials that have a beneficial effect on energy consumption in the short term yet contribute to bigger problems in the long term.

Fact – the ocean levels are rising.  Climate change is occurring.  Debate persists and some feel like global warming isn’t a legitimate “thing”; that effects are cyclical over a long period of time.

Q:  What is the real story?

A:  Regardless of whether global warming is a “thing” or whether climate change is a cyclical event that occurs with regularity over time horizons of thousands of years, it will affect us and our children in our lifetimes.  Understanding how our actions impact the big picture is essential.

The definition of global warming potential is, according to the EPA:

The Global Warming Potential (GWP) was developed to allow comparisons of the global warming impacts of different gases. Specifically, it is a measure of how much energy the emissions of 1 ton of a gas will absorb over a given period of time, relative to the emissions of 1 ton of carbon dioxide (CO2). The larger the GWP, the more that a given gas warms the Earth compared to CO2 over that time period. The time period usually used for GWPs is 100 years. GWPs provide a common unit of measure, which allows analysts to add up emissions estimates of different gases (e.g., to compile a national GHG inventory), and allows policymakers to compare emissions reduction opportunities across sectors and gases.

Q:  What are the global warming potentials of different building materials?

A:  That’s a tough question to answer. A better question would be, “What is the level of embodied carbon within different building materials?”  You see, GWP was developed to evaluate the “green-ness” or “lack of green-ness” of gases, which are used in either refrigeration (think air conditioning) or as blowing agents for the production of insulation (sheets of foam or spray foam).

The answer is, concrete is the least “green”, consuming tons of energy in its production, followed by steel, followed by wood.

With respect to foam insulation, the blowing agents used in closed cell spray foam (historically) have had an extremely high GWP, similar to those used in the production of extruded polystyrene sheet foam (in excess of 1,400…remember, that’s a ratio of comparison to carbon dioxide – so in essence what is being said is that XPS foam has 1,400 times the impact on the environment over 100 years compared to an equal mass of carbon dioxide).

EPS foam, polyisocyanurate foam, and mineral wool all have a tremendously lower GWP than XPS foam.  Of course, there are other material differences in properties – R-value, compressive strength, and the need to derate exterior polyiso in cold climates, but as long as that is considered ahead of time, you can design a functional and efficient wall/roof/slab assembly that has a low environmental impact.

Q:  Didn’t you say in a previous blog post that your local lumberyard didn’t know the difference?

A:  Yes – I wanted a sheet of polyisocyanurate and all they knew was the “blue board” that they had in stock. I’ll be sure to give them a copy of this article.

Q:  What about spray foam?

A:  Spray foam sucks.  (Sorry, have to be blunt).  Open cell spray foam has a lower environmental impact than closed cell, but a poorer insulating value.  It’s not even as good an air barrier as dense packed cellulose, which is recycled material (think lower impact).  Closed cell spray foam has a terrible environmental impact, unless you use one of the newer, lower GWP foams (such as Honeywell Solstice) that are quite expensive.  Spray foams with a water-based blowing agent have a low GWP, but you really take a hit in R-value and they’re still expensive.  In my opinion, spray foam (as a primary component of a wall or roof assembly) is a lazy person’s way to design.

Q:  Why are you suddenly on this soap box when the blog was started with real estate investing in mind?

A:  Several reasons.  It irritates me when someone thinks that their home is energy-efficient and therefore earth-friendly from the perspective of utility usage, but at the same time has more embodied carbon in its component materials than another home with less insulation and incrementally higher utility bills.  In other words, they brag about low “operational energy”, but are oblivious to the fact that they have an extremely high “embodied energy.”   Fact – all insulation is not created equal.

It also irritates me when builders insinuate that their finished product is “green” yet uses a significant amount of XPS foam. Some prey on the fact that the general public doesn’t understand building science principles. Some may not understand building science themselves.

Finally, all this may seem extreme, but the utility companies seem to get it. I’m looking at a cash rebate of $8,000 or more per home in an upcoming development for avoiding fossil fuel infrastructure.

Q:  Is it possible to build a home or structure that has a negative carbon footprint?

A:  Yes: use concrete minimally, don’t use XPS foam at all, and then go plant a bunch of trees.  Now you’ve contributed more to mother earth than you have consumed from her!

As you think about that, be cognizant of the “all insulation is good” mantra.  Whether you’re designing a new build or contemplating a deep energy retrofit for an existing structure, the right knowledge and the patience to plan ahead are what will help your utility bills, your construction budget, and your carbon footprint.

Until next time,
Dr. Lee Newton

What real estate investing, building science, or development questions do you have?  I’ll be happy to answer them here.  Send them to


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