ARTICLE 25
Thick Doesn’t Mean Insulated
There is a misconception I hear surprisingly often in construction and design:
If a wall is thick, heavy and solid, it must be well insulated.
Not necessarily.
Take concrete.
Concrete is strong.
It is durable.
It can be excellent for sound control.
It can absorb and store an enormous amount of heat.
But as insulation?
Concrete is actually pretty terrible.
You Might Need About 20 Feet of Concrete
Normal-weight concrete has a very low R-value compared with materials specifically designed to insulate a building.
An 8-inch concrete wall may not even give you an R-value of 1 from the concrete itself.
If you wanted to achieve somewhere around R-21 using normal-weight concrete alone, you could be looking at something approaching 20 feet of concrete.
Twenty feet.
Not inches.
Feet.
So yes, technically we could solve the insulation problem by making the concrete wall thicker.
We would just need a wall roughly the width of a small room.
There may be a better way.
First, Think About Heat Transfer
Heat naturally moves from warmer areas toward cooler areas.
In buildings, that heat can move in several different ways, including through materials, through air movement and through radiation.
For this discussion, we are mainly talking about heat moving through the building assembly itself.
That is where R-value and U-factor come in.
They are simply different ways of helping us understand how easily heat moves through part of a building.
Think of R-Value Like a Winter Coat
The easiest way I can explain R-value is to think about a winter coat.
Your body is warm.
The air outside is cold.
The coat helps slow the movement of heat away from your body.
A thicker or better-performing coat generally provides more resistance to that heat flow.
That is essentially what insulation is doing in a building.
R-value measures resistance to heat flow.
The higher the R-value, the more resistance there is.
Fiberglass, mineral wool, rigid insulation and similar products are specifically designed to slow that movement of heat.
Concrete is not.
And that is where people sometimes confuse mass with insulation.
Something can be heavy.
Something can be thick.
Something can feel incredibly substantial.
That does not necessarily mean it is good at resisting heat flow.
So What Is U-Factor?
Then we have U-factor.
If R-value tells us how much something resists heat flow, U-factor tells us how easily heat can pass through it.
They are essentially looking at the same problem from opposite directions.
In simplified terms:
U = 1 / R
So the rule of thumb is easy:
Higher R-value = better.
Lower U-factor = better.
And one of the easiest places to understand U-factor is something most homeowners have dealt with:
Windows.
Windows Are a Great Example
When you shop for a window, you may see a U-factor listed on the performance information.
The lower that number, the better the window is at reducing heat transfer.
A high-performing residential window might have a U-factor somewhere around 0.22 to 0.30, depending on the window and climate.
A U-factor of 0.22 works out to an equivalent thermal resistance of only about R-4.5.
Think about what goes into getting there.
We may have two or three panes of glass.
Low-emissivity coatings.
Insulated spacers.
Better-performing frames.
Argon or another insulating gas sealed between the panes.
A tremendous amount of engineering goes into making a modern window perform well.
And after all of that, even a very good window still does not come close to the thermal resistance we can achieve relatively easily in an insulated wall.
An R-21 layer of insulation, in simple reciprocal terms, corresponds to a U-value of roughly 0.048.
That does not mean every wall and window can be compared perfectly by simply flipping one number over.
Real wall assemblies include framing, sheathing, fasteners, structural elements and thermal bridges.
But the comparison makes one thing very clear:
Glass is difficult to insulate.
A good window can still be one of the weaker thermal portions of a building envelope.
That does not mean the window is bad.
It means walls and windows are doing very different things.
“Double Pane” Does Not Tell the Whole Story
This is also why saying, “It has double-pane windows,” does not necessarily tell us whether a window performs particularly well.
Two windows can both be double pane and perform very differently.
You have to ask:
What kind of low-e coating is being used?
What is between the panes?
How does the frame perform?
How is the edge of the glass constructed?
What is the actual whole-window U-factor?
There is also another measurement called Solar Heat Gain Coefficient, or SHGC.
That tells us how much solar heat is being allowed through the window.
Which means the best window is not always simply the window with the lowest possible numbers everywhere.
Climate matters.
Orientation matters.
Solar exposure matters.
Architecture matters.
Concrete Is More Like a Cast-Iron Skillet
So if concrete is not a great insulator, why can massive concrete and masonry buildings sometimes perform really well?
Because concrete does something insulation does not.
It has thermal mass.
The easiest comparison might be a cast-iron skillet.
A cast-iron skillet takes some time to heat up.
But once it gets hot, it holds that heat for a long time.
Concrete behaves somewhat similarly.
It can absorb a tremendous amount of heat energy, store it and release it later.
That is very different from insulation.
Think of it this way:
Insulation is the winter coat.
Concrete is the cast-iron skillet.
One slows heat movement.
The other stores heat.
Those are not the same thing.
And That Can Be Very Useful
Imagine a desert climate.
It may be extremely hot during the day and considerably cooler at night.
A massive wall or floor can absorb heat during the day and delay how quickly that heat affects the interior.
Later, when outdoor temperatures fall, that stored energy can begin to release.
That time delay can be extremely useful when the building and climate are working together.
This is one reason thermal mass has been used for thousands of years.
Adobe buildings are a great example.
Those buildings were not working because somebody discovered an ancient version of R-30 insulation.
They worked because mass, shade, orientation, ventilation, openings and climate were all working together.
But Thermal Mass Is Not Magic
Now imagine putting that same massive wall somewhere where it stays extremely hot day and night.
The wall absorbs heat all day.
Then nighttime comes.
But it never gets cool enough for the wall to release that heat effectively.
Eventually, that stored heat is going somewhere.
This is why you cannot simply say:
“Concrete has thermal mass, therefore concrete walls are energy efficient.”
Just like you cannot say:
“The wall is thick, therefore it is insulated.”
The climate and the entire wall assembly matter.
The Better Approach Is Usually to Let Materials Do What They Are Good At
A high-performing concrete wall can combine the advantages of both systems.
The concrete can provide:
Structure.
Durability.
Fire resistance.
Sound control.
Thermal mass.
And then we can add insulation to provide:
Thermal resistance.
For example, continuous insulation can be placed outside a concrete wall.
Now the insulation is slowing the movement of heat through the building envelope while the concrete remains closer to the conditioned interior temperature.
The two materials are working together.
We are not asking concrete to suddenly become insulation.
Buildings Are Systems
And this is where building-envelope design becomes interesting.
A wall is not just one material.
Neither is a roof.
Neither is a window.
Each layer is doing something.
Structure.
Insulation.
Air control.
Water control.
Vapor control.
Interior finish.
Exterior protection.
Sometimes one material can perform several of those jobs.
Sometimes it cannot.
Good design comes from understanding the difference.
Because adding more of the wrong material does not necessarily solve the problem.
Sometimes it just gives you more of the wrong material.
Thick, Heavy and Insulated Are Not the Same Thing
A massive concrete wall can be an excellent wall.
A high-performance window can be an excellent window.
A lightweight insulated wall can be an excellent wall.
But they all behave differently.
That is why I think the easiest way to remember all of this is:
R-value is the winter coat.
It resists heat movement.
Thermal mass is the cast-iron skillet.
It absorbs and stores heat.
U-factor tells us how easily heat gets through.
And if anyone still thinks concrete is insulation, we can absolutely get there.
We just might need about 20 feet of it.
Of course, none of this even gets into air leakage or moisture moving through a wall.
That is another conversation entirely.
And probably another article.
Michael Demetrios Frudakis, AIA, NCARB
Principal Architect