Insulation Basics: Heat, R-Value and the Building Envelope
Learn how heat moves through a home and the materials that can stop it, to make sure your insulation is as effective as you think
Mariana Pickering
2 June 2014
Houzz Contributor; Owner/CEO of Emu Building Science; LEED AP BD+C. After spending many years as an architectural designer in the high-efficiency residential Italian market, I now run a company that specializes in advanced construction science and Passive House design. We are located in Denver, Colorado, and Northern Italy, and we work internationally with builders, designers, and manufacturers of high-performance projects and products.
Houzz Contributor; Owner/CEO of Emu Building Science; LEED AP BD+C. After spending... More
Adding insulation to your home’s building envelope (the walls, slab and roof that protect your home from the outside) can be one of the most cost-efficient ways to reduce your heating and cooling bills. In new construction, investing in the insulation is a smart way to reduce future maintenance costs by reducing the home’s energy consumption.
However, because no two buildings are the same, and because there are so many ways to insulate, it can often be confusing to determine what’s best for a particular project.
Over the next few weeks, we’ll look at various types of thermal insulation and cover the pros and cons of each. Before we get started, it’s essential to understand the basics of heat flow and how insulation factors in.
However, because no two buildings are the same, and because there are so many ways to insulate, it can often be confusing to determine what’s best for a particular project.
Over the next few weeks, we’ll look at various types of thermal insulation and cover the pros and cons of each. Before we get started, it’s essential to understand the basics of heat flow and how insulation factors in.
Heat is always on the move. Heat will always find a way to flow toward colder areas until the two are neutralized. It moves in three different ways:
- Conduction. For example, when an uninsulated slab on the ground allows the heat in your home to transfer to the ground.
- Convection. Heat rises, making your attic hotter than your basement.
- Radiation. Underfloor radiant heating, such as in the home seen here
To keep heat from flowing through a building envelope, conductive resistance can be introduced in the form of insulation. In the winter this insulation helps to reduce heat loss by making it more difficult for heat to flow toward the cold outdoors. In the summer it helps keep the heat out of your cool interior environment.
There are other ways to help prevent heat flow, such as radiant barriers that reflect the heat away from the building envelope. Pictured here is a ventilated metal roof over a system that includes a radiant barrier. This ventilation strip along the ridge of the roof is important for releasing the reflected heat from the space underneath. Often a system like this will be used in combination with a well-insulated building envelope to help curb summer heat from invading the home and sending air conditioning bills through the roof.
The higher the R-value, the better the thermal resistance. You may have heard the term “R-value” when discussing thermal insulation. Here in Europe, we more often talk about the U-value (the inverse of the R-value) or the lambda value of a material. Lambda and U-values work the opposite way, with lower values representing higher performance.
All of these values are meant to help you understand the thermal resistance of a certain material or combination of materials to conductive heat flow. In the U.S., it’s actually legally required to provide the R-value of certain building materials at the point of sale (and sometimes in the advertisement).The intention of this law is to increase consumer awareness and prevent false claims about the potential of insulation components. However, the R-value itself can be tricky to understand.
All of these values are meant to help you understand the thermal resistance of a certain material or combination of materials to conductive heat flow. In the U.S., it’s actually legally required to provide the R-value of certain building materials at the point of sale (and sometimes in the advertisement).The intention of this law is to increase consumer awareness and prevent false claims about the potential of insulation components. However, the R-value itself can be tricky to understand.
R-values use different units of measurement in the U.S. versus the rest of the world. And they can easily be confusing, because the units are typically left off. Internationally, R-values are in watts per meters squared kelvin (m²K/W), whereas in the U.S. they are given in units of feet squared times degrees Fahrenheit times hours, divided by BTU (British thermal unit), or ft² °F hr/BTU. (Now do you see why they leave the units off?)
Sometimes Americans refer to the international system as RSI to differentiate. This can be important when you are researching products online, so keep it in mind. And be sure to know which system your product is referencing, especially when it comes to imported materials like expanded cork from the Mediterranean (seen here).
Sometimes Americans refer to the international system as RSI to differentiate. This can be important when you are researching products online, so keep it in mind. And be sure to know which system your product is referencing, especially when it comes to imported materials like expanded cork from the Mediterranean (seen here).
The R-value can change in a material due to varying thickness. You may have heard your architect say that the goal is to reach a certain R-value for the wall section, for example. This means the thermal resistance of the entire series of products that make up the wall section — all of them together — plus the internal and external layers of air surrounding the wall. When you purchase insulation, you should have access to a fact sheet telling you exactly what the R-value per inch is, with a table showing any change in the R-value over varying thicknesses of the material.
Thermal insulation is only one part of the puzzle. But it’s a pretty big one. It may help to try to imagine your home like a balloon full of hot water; the balloon is the building envelope. You can feel the heat trying to escape through the material (this is conductive heat loss). If you wrap the balloon in a towel, it may take longer for the water inside the balloon to lose all of its heat. But if there’s a hole in the balloon, all of that hot water will leak out at a much faster rate.
A home’s envelope is similar. You can add insulation to the roof or attic, the walls and the slabs, but if you have an air infiltration around your window frames (a very common problem in many homes), the effectiveness of that insulation that you added will be somewhat negated because all of the heat will just go out the window frame.
This is why the installation of your insulation is perhaps even more important than the material you select. In fact, ease of installation is one of the major reasons that spray foam insulation (seen here) is so popular in the United States.
A home’s envelope is similar. You can add insulation to the roof or attic, the walls and the slabs, but if you have an air infiltration around your window frames (a very common problem in many homes), the effectiveness of that insulation that you added will be somewhat negated because all of the heat will just go out the window frame.
This is why the installation of your insulation is perhaps even more important than the material you select. In fact, ease of installation is one of the major reasons that spray foam insulation (seen here) is so popular in the United States.
The amount of insulation you will need and its R-value will be dependent on many factors, including:
A home’s building envelope is only as strong as its weakest link. Keep this in mind when investing in insulation. It may not make financial sense to improve the quality of your balloon if it has a hole in it anyway.
See more on mechanical ventilation
Finally, there are a few other factors to consider when shopping around for insulation. The heat capacity of a material can have a big impact on its performance in the summer or in hot climates. Also, the density and the transpiration of a product may be important factors to consider. I’ll tackle these in more detail in future ideabooks about insulation materials.
More common-sense ways to save energy at home
- Your climate
- Your type of heating and cooling system
- Your budget
- Whether you are installing the material yourself or having a professional do it
- What part of the house you’re insulating
- The importance of indoor air quality to you and your family
- Life-cycle cost considerations
- Recycled content and embodied energy of the material
A home’s building envelope is only as strong as its weakest link. Keep this in mind when investing in insulation. It may not make financial sense to improve the quality of your balloon if it has a hole in it anyway.
See more on mechanical ventilation
Finally, there are a few other factors to consider when shopping around for insulation. The heat capacity of a material can have a big impact on its performance in the summer or in hot climates. Also, the density and the transpiration of a product may be important factors to consider. I’ll tackle these in more detail in future ideabooks about insulation materials.
More common-sense ways to save energy at home
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I don't know what a Zen window is, apart from a brand upon googling the term. Maybe you can fill me in. But my original point was that sacrificing windows for the sake of energy savings - which was what you were advocating 3 years ago - may not be a worthwhile tradeoff. My second point was that simply "going outside" rather than enjoying the non-existent view from inside for lack of windows - which you also advocated - is not a practical solution due to the multi-tasking required given people's many responsibilities and the impracticality thereof. Even without TV.
As for my enlightened and humble diplomacy, I prefer to engage people without sarcasm so I've never been good at recognizing it. Maybe God just didn't put enough between my ears for that.
It appeared to me the pot was calling the kettle black. Your terms have bias - I am not sacrificing anything. I am judiciously using windows for a multitude of reasons rather than just one, which in doing that (expansive view) compromises the rest of my project program criteria one of which is always energy and another cost effectiveness.
Windows are expensive from a cost, energy (thermal performance) and environmental perspective (they do not recycle). And I could potentially add privacy and structurally if there is a lot of them in one place.
Zen view - first introduced to me in "A Pattern Language". Aptly described in this link.
http://www.dilloncarroll.com/blog/pattern-languages-and-zen-views-part-ii
I have incorporated single, clear view glass block as windows for the Zen effect. It is "not there all the time" to be taken for granted. You either make an effort to see that particular view or it presents its self to you unexpectedly as you walk past it.
I would venture the position that having an expansive view of glass more likely results in you not going outside as often, which in a skyscraper in Manhattan might be appropriate(?) but I have a garden and go for a hike at least once a week (and have been known to just wander around the yard to check things out). We don't have many natural areas here in Ag land so it is almost always a repeat but I have to get outside - it is my favorite classroom - if you dig past the surface you can find some interesting gems outside even here.
It makes me cringe when people ask a lot of windows arbitrarily and then say how important energy efficiency is - like saying both the economy's strong growth and the environment's health are important to you - one grows at the expense of the other - I just shake my head.
The solar addition performs many functions - I built it all my self - 1/4" plate glass - inexpensive and strong. When it is not gaining solar energy the doors are closed to it. Passive solar is not a fashion and needs to be well thought out to make sense - most places it does not without some specific strategy (Columbus Ohio - never). There is a hot tub and two story shower in it and when I use that I am very conscious of the view - that is why I am there. It is not coincidental.
Wish I hadn't put so many window in the master bedroom - makes it hard to sleep in on the weekends.
Those timelessly applicable sayings, "Less is more."
My apologies if I hit a nerve - you caught me at the end of a long day and hit one of mine.
Not many windows in this house. I think I would enjoy living there.
I remembered a conversation I had with someone about insulation - I had told them about the system I developed and how efficient it was. They said that the system would insulate me from the outside - given the context of the discussion I thought they were just being disagreeable or elevating themselves. But the comment stuck with me. Not to be indelicate but when nature calls and it is "nice" out I go outside - we live in the country. The stars go almost to the horizon - I am awestruck every time - how I go from my center-of-the-universe home environment to the Great Out Doors and see a tiny fraction of the immense universe and the perfection of it. The Milky Way looks like foot prints in the sky and I am aware of my place - I am humbled and yet do not feel small for I am a part of that not separate from it. My problems feel small and my spirits are lifted.
The Greeks gave us the constellations - they spent much time outside studying the heavens and were curious and in touch with nature. Our universe often shrinks to the tiny domain of man and we act like that. "Will this vision of perfection turn into our own reflection." A city and its lights wipes out the question of relative size for many - cities are a man made environment - it is easy to forget the complexities, beauty and perfection of the natural world - we are making it over in our image and not doing all that great a job compared to the master. A landfill does not give me the same feeling as a filed of prairie. We have much to learn.
Insulate your home and office well and get out of it often to see all that is not in there.