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What Type of Insulation Should You Use?

Part 1: The Basics

Click to Read Part 2: Identifying Your Priorities

Choosing the best insulation to maximize performance and minimize environmental impacts is one of the most complex decisions you can make for a building project.

Insulation is a critical component of any building--especially one designed and built to minimize environmental impacts.

Determining what type of insulation to install--and how much of it--can be complex. No other building product category offers such a diverse range of materials and impacts to consider: environmental, human health, performance, and building science.

Heat moves through our walls, roofs, and floors in three ways: our insulation and air sealing materials must contend with all three at once. Click for larger image.

Moreover, on top of the wide range of existing insulation products, new insulation materials and new variations of older insulation materials are appearing regularly. For example, in addition to standard fiberglass, cellulose, polystyrene, and polyisocyanurate insulation, we can now purchase insulation made from mineral wool, cementitious foam, radiant foil, cellular glass, vacuum panels, gas-filled panels, wool, recycled cotton, and polyester.

With such a diverse range of materials--and material properties--from which to choose, selecting the best insulation for a given application can be tremendously confusing.

GreenSpec is here with the information you need to make the correct decision for your project's needs and budget. Take GreenSpec's insulation quiz and learn how to make the right insulation choices.

How Insulation Works

To understand insulation materials, one needs to understand the basics of heat flow.

There are three primary mechanisms of heat flow: conduction, convection, and radiation.

Thermal conduction is the movement of heat from direct contact: one molecule is activated (excited) by heat and transfers that kinetic energy to an adjacent molecule.

We generally think of conduction occurring between solid materials--the handle of a hot skillet conducting its heat to your hand, for example--and that is the most efficient mode of conduction. Thermal conduction also occurs within liquids and gases, but more slowly.

Convection is the transfer of heat in liquids and gases by the physical movement of those molecules from one place to another. As air is warmed, it expands, becomes more buoyant, and rises--a process called natural convection. This occurs with liquids, too, as we experience with thermosiphon solar water heaters.

Finally, radiation is the transfer of heat from the surfaces of one body to another via the propagation of electromagnetic waves. When you sit in front of a fireplace and look into the fire, your face is warmed by the radiant transfer of energy from that heat source to your face. That radiant energy is not affected by air currents and occurs even across a vacuum--as we know from lying in the sun and experiencing radiant energy that has traveled 93 million miles through space.

Here on Earth, heat flow is almost always moving in all three modes simultaneously, and our insulation must reckon with that.

In Part II of this series, we'll discuss some of the thorniest questions building professionals confront when selecting insulation.

In preparation, be sure to test your basic knowledge of insulation materials by taking GreenSpec's insulation quiz.

Posted by Paula Melton on October 13, 2011


Your insulation quiz

One of the curious things about the web is the high frequency with which surveys and quizzes are erroneous and/or misleading. Even when they are created by well-informed people, it is frequent that the questions are ambiguous or meaningless. The answers are often incorrect, debatable, or downright confusing. Tunnel-vision and separation from the issues relevant to the real world are commonplace.

Finally, while these online quizzes are supposed to be fun and informative, they almost never provide useful information and discussion on the points that they raise.

I was disappointed to find that your insulation quiz suffers from most of these maladies.

Your insulation list should i

Your insulation list should include hemp, flax, mycelium (http://www.ecovativedesign.com/greensulate/), aerogel, and dynamic insulation (http://en.wikipedia.org/wiki/Dynamic_insulation), but radiant foil is not insulation in the conventional sense because it does not resist conductive heat flow – it has no R-value.

You acknowledge that insulation issues are "complex" and then claim that "GreenSpec is here with the information you need to make the correct decision", as if there is a single "correct" option for each project.

And there are some errors of fact in both this blog and the GreenSpec Insulation Quiz.

In the blog, you state that "thermal conduction also occurs within liquids and gases, but more slowly." Not necessarily. Water has a thermal conduction of about 4 Btu•in/(hr•ft²•°F), which is five times that of softwood and 30% greater than granite.

And, while it's true that "heat flow is almost always moving in all three modes simultaneously", it's not necessarily moving in the same direction through all three. On a sunny winter day with no snow on the roof, heat can be moving outwards by conduction while moving inwards by radiation and moving outward also by convection (air leakage).

Convection (leakage of moist air), carries both sensible heat (that is measured with a thermometer) and the latent heat of vaporization of the water – which is generally not substantial in winter but can be quite significant in summer.

The quiz claims that Perlite is a good choice for attic floor insulation when a low-density version is available regionally. But loose-fill Perlite offers no resistance to wind-washing, which is one of the answers, and offers among the lowest R-values per inch.

It suggests that the material with the strongest performance characteristics for sub-slab applications is cellular glass. This is true only if embodied energy (nearly double that of XPS per R) or cost are included as "performance characteristics".

And it claims that the insulation material requires the largest energy input is sheep's wool. Polyester is a petrochemical (pure energy) and polyester batts may or may not contain any recycled material, but even the recycled material requires heat inputs for production. Sheep's wool batts are typically made from the first shearing that would otherwise be composted or discarded and, unless bound with polyester fibers, has less embodied energy than any other commercial insulation product except blue-jean batts and cellulose.

Robert, thank you for pointin

Robert, thank you for pointing out the immense complexities of weighing performance against environmental impacts. That is exactly what the quiz is meant to get people thinking about.

All of the points you bring up are addressed in greater and more definitive detail in Alex Wilson's new insulation report. We're obviously not in agreement about all the specifics, but your comments are well taken--particularly your point about there being no hard and fast solution for every project, which was really the whole point of this post and our quiz. Many of the other issues you've mentioned will be addressed in more detail in Part 2 of our insulation series next week.

Thanks for getting the discussion started!

Paula, You say "we're obvious


You say "we're obviously not in agreement about all the specifics". But some statements are a matter of fact and not opinion. If you assert that sheep wool batts, made from material that would otherwise be discarded, are more energy intensive to produce than either recycled blue jeans or polyester (which may use either virgin or recycled material), then please share the raw data you're using to come to that conclusion.

The data I've seen contradicts such a conclusion. Polyester is a petrochemical, and even recycled plastic requires significant heat (energy) input to melt and re-configure it.

As for "more definitive detail in Alex Wilson's new insulation report", that's available only for a steep price. Information required to make responsible choices should be freely available in the public domain.

Wool carpet commonly has petr

Wool carpet commonly has petrochemical backing and binders, unlike wool batts which are minimally-processed natural fibers. The ICE report shows a range of embodied energy coefficients for wool (in the material profile section) from 106 to as low as 3. The New Zealand chart (www.victoria.ac.nz/cbpr/documents/pdfs/ee-coefficients.pdf) gives 106 for wool carpet but 14.6 for recycled wool insulation.

Even using the ICE recycled wool insulation coefficient of 20.9, with a density of 1.3 pcf and R-3.65 (an average) per inch, wool batts have about the same EE per R as mineral wool batts and about half to a third that of recycled polyester batts.

Any of these numbers are very dependent on site of manufacture practices, and life-cycle costing is a pretty arbitrary exercise. Do we include all the extreme environmental costs of paper-making in the impacts of recycled cellulose (otherwise waste) insulation, or the methane generation of sheep in the impact of diverting waste wool (1st shearing) into insulation? How do you factor in the landfill savings or reduction in waste incineration?

Depending on what variables are considered and with what weight, the results can be surprising, such as when Donella Meadows determined that throw-away plastic diapers were more environmentally sound than cotton diapers because of the exorbitant water contamination from washing and bleaching (as well as the intensive use of pesticides on the cotton).

The BEES data shows that the

The BEES data shows that the impacts of wool carpet are clearly agricultural in nature, and not as a result of the plastic backing.

That is not to say that such data is beyond reproach—to the contrary, as you allude to, setting boundaries for life-cycle assessment is notoriously difficult. It is argued that wool used in these settings is a waste product and should get a free ride. I would like to see more data supporting that.

There is little reliable data

There is little reliable data on the impact of wool insulation due to its small volume. However, one telling point is offered in the life-cycle assessment database used by the BEES tool (Building for Environmental and Economic Sustainability) from NIST. In this data, the global warming impact of wool carpet is off the charts compared to petrochemical-based carpet, primarily due to the agricultural impacts of wool production, especially eutrophication. Another data point, of debatable applicability, is provided by the Inventory of Carbon and Energy (ICE), which shows that "recycled wool" insulation (not sure how they define that, or whether this is a common product in the U.K., where this inventory is produced) has embodied energy per kg much higher than cellulose, higher than mineral wool, and not much less than fiberglass.

Do you have good data on the relative impact of wool?

Purchase of this report supports the many hours of research and production that it required, and in helping individuals make once-in-a-building insulation choices, represents a small cost. BuildingGreen will cheerfully refund the purchase price to anyone who is not satisfied with it.

Paula, I'm curious to know yo

Paula, I'm curious to know your opinion on reducing heat load (solar radiation) as a means to cool buildings. Cool roof coatings are getting some attention for commercial applications. Couldn't some be used in residential applications for exterior walls? Do you think the RE value is valid?


Tim, there is some great pers

Tim, there is some great perspective on cool roofing installations, and when they are and aren't effective, in our current feature article, on low-slope roofing:


They are definitely a valid way to keep buildings cool, particularly in hotter climates, and to some extent the same principles can be applied to residential settings, and walls rather than roofs. However, I would not consider reflective coatings a substitute for insulation. In itself such a coating has no R-value, and it is R-value that we need in many settings.

Insulation Types? Huh?


Generally a nice and basic article, and I agree with much of the previous response/commentary posted to the site. I know you are writing for diverse audiences of differing expertise, and this isn't the forum to get into the greatest detail or comprehensiveness. But I must say from a fairness standpoint that you appeared to leave out any mention of the role of air-sealing which is widely recognized as delivering double the level of energy savings over just insulation. And yes while it may seem self-serving, I think neglecting to even mention spray polyurethane foam (SPF) insulation, which has grown double digits consistently since 2006 based upon its obvious value propositions, is beyond an oversight.

I also began taking your insulation quiz and saw very limited options, and again not one reference to SPF. Even on the questions that dealt with vapor permeance and air-sealing, not one mention of the product that is best in class in both arenas.

To write a legitimate article about insulation and neglect to mention SPF, which is arguably one of the best performing all around products, is inexplicable. I believe as many of the folks that have responded, that there is no one particular insulation that is always the best choice. This is a diverse industry, with complex demands and differing value propositions. If you can't offer a fair and inclusive overview of these considerations, you are better off not doing anything.

There is always a chance I missed something. If so, please make me stand-corrected.

Kurt Riesenberg Executive Director Spray Polyurethane Foam Alliance (SPFA)

We can reduce solar radiation

We can reduce solar radiation at the earth's surface by increasing particulate emissions as a method of geo-engineering - but that's adding insult to injury and probably not what you meant.

Using reflective and high-emittance roof surfaces is not a method for cooling buildings but is a useful technique for reducing air-conditioning loads and also urban heat island effects.

Because of high summer sun angles, most radiant gain occurs at the roof, not the walls. EPA's Energy Star program rates roofing materials according to their ability to reduce summer heat loads for residential applications. Florida Solar Energy Center studies have demonstrated that light-colored metal and high-mass (Spanish tile) roofs are the most effective methods for reducing summer heat gain.

Re: Insulation Types? Huh?

Kurt, thanks for your comment! Publishing a comprehensive list of insulation types was not the aim of this post or the quiz, but we'll be unpacking some of the issues having to do a few specific insulation types throughout this series.

By a strange coincidence, I'll be posting one that addresses performance characteristics as well as health and environmental impacts of SPF later today. Rest assured that Alex's insulation report is quite comprehensive and does cover both closed- and open-cell SPF.

I read a paper by LBL (Potent

I read a paper by LBL (Potential benefits of cool roofs on commercial buildings) that said cool roofs were beneficial in all states, out weighing the heating penalty. Obviously a solar radiation deterrent needs to work in conjunction with retardants of convective and conductive heat transfer to be most effective.

It is true that the majority of radiant heat gain occurs on the roof. But what would be the effect of lowering solar heat gain on the walls in combination with a weather resistive barrier the slows air and moisture in/exfiltration to semi-permeable? It seems most cool roof coatings have these properties in common. Many are modified elastomeric coatings. If it is safe to do so, what would be the harm? Would it not provide a level of benefit for the occupants?


Immense complexities

There are none in heating/cooling performance vs. environmental considerations if you do it right. However 99.9% of all new construction gets it wrong and LEED isn't helping.

Making dog house warm? Insulation info needed

Hello, I have feral cats that were TNR. Trapped Neuter and Return. I bought a dog house for them to use in the winter. The problem is the dog house is not insulated. I put a self thermal mat on the floor for them, and a cat bed inside. But the house has 2 openings. The front door and a side window (escape route) I am trying to keep it as warm as possible for ny winters. Is there something I can use on the walls that is not harmful to animals. They only sleep there, no playing. The house is made of wood. I am reading alot of information and now could use some help. I thank you for your time.

re: doghouse insulation

Renee, are you planning to cover the insulation with anything after installing? If not, I would probably avoid any kind of batt, since the cats might scratch at it and get dust, pest-control chemicals, or binders in their lungs.

Maybe rigid foam on the outside could be an option? We cover our beehives with a box made of foil-faced rigid foam for the winter. You can paint it a nice color if the "look" doesn't appeal to you or your neighbors!

I'm sure there are other good options, though. What ideas have you had? Have you explored what materials are available in your area in small quantities for this project? It's entirely possible that feral cats don't even really need the extra warmth.

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