Types of insulation: part 3-Insulations compared


Signs of moisture and air flow in fiberglass insulation

When you climb into an attic and see this, you know something is seriously wrong. In fact, if your home isn’t very old, your insulation should be clean like when it was installed. The reason you get black insulation like this is because air and moisture are moving through the insulation, and it’s acting like an air filter.

The reason I’m showing you this picture is because it’s an example of the wrong insulation being used for a job. I feel pretty strongly about this because I’ve seen numerous homes where fiberglass has been installed in open walls like this and in almost every case, the insulation was seriously compromised – it was buckling under its own weight or simply falling out of the wall cavity. Or, in cases like this, it was hiding a big hole in the wall that should have been air sealed.

Let’s walk through the different insulation types and compare their main attributes: R-value, ability to reduce air movement, suitability for retrofit applications, and other characteristics.

R-values are approximate. There’s a pretty wide range of published values. Keep in mind that there’s a lot more to insulation than R-value!

The exact values for vapor permeability are unimportant. What you need to know is: is it very permeable, somewhat permeable or very impermeable. A sponge is very permeable. Unpainted sheet rock is highly permeable. Painted sheetrock or foam board is slightly permeable. Plastic sheeting is basically impermeable.

Likewise for the air stopping ability. Is it good at stopping air movement or lousy? If it’s lousy, you better have a really good air barrier in place before installing it, or the insulation will be next to worthless.

Insulation Characteristics
Type R-value per inch Air blocking ability Permeability
Fiberglass batts 3.25 non-existent very high
Fiberglass, loose fill on attic floor 3.0 non-existent very high
Fiberglass, loose fill in enclosed walls 4.0 non-existent very high
Cellulose, loose fill on attic floor 3.5 non-existent very high
Cellulose, dense packed in enclosed walls 3.4 good very high
Spray foam, open cell 3.7 Excellent High. ~16
Spray foam, closed cell 6.25 Excellent Low. ~1
Board foam, expanded polystyrene 4.0 Excellent Moderate. ~3.5
Board foam, extruded polystyrene 5.0 Excellent Low. ~1.0
Board foam, foil faced polyisocyanurate 7.0 Excellent Very low. 0.02
8″ Cinder block 1.11 (total R-value) Lousy very high
4″ Poured concrete 0.32 (total R-value) Excellent moderate
4″ Lumber 1.1 Excellent moderate, ~3

There are certainly a lot of choices, and this is just a subset. However, what we’ll cover here should allow you to more intelligently choose an insulation material the next time you have to make the choice.

Questions to think about when choosing insulation:

  • What temperatures will it be used to insulate between?
  • Does it need to stop air?
  • Is it fireproof/resistant?
  • Can it be left exposed?
  • Does it matter if moisture moves through it? Should moisture move through it?
  • Does it matter if it traps moisture? Should it prevent moisture movement?
  • How difficult is it to install?
  • Will it maintain its R-value over time?
  • What is the installed cost for a given R-value?
  • Will it get wet or be exposed to water?
  • Are there ecological or health concerns about the product?

Thermal image showing poor attic insulation

Temperature extremes:

Attics are worst-case scenarios for insulation. During the summer, the temperature in a conventionally insulated attic (insulation on the floor) can reach 150°F. Without adequate insulation, that heat turns your ceiling into a big space heater. Not only is that uncomfortable, it’s really expensive to air condition.

What does this tell you about the insulation you need? It tells you that you want as high an R-value as you can afford separating your living space from those hot attic spaces. That’s why building codes in most of the U.S. call for about R-40 insulation.

Keep in mind, it doesn’t matter if the wall is horizontal, like your ceiling, or vertical, like in an upstairs room with a small attic space behind the knee-wall. The wall is subject to the same amount of heat so you better insulate it well!

What about other factors like moisture permeability and air stopping ability? For a typical ceiling without holes in it, you just want lots of insulation R-value. Whether it comes from 16″ of shredded fiberglass or cellulose or whatever – it really doesn’t matter. Just remember, air seal before you add all that insulation because it’s a real pain in the … to go back and air seal after you have an attic filled with two feet of insulation!

Air Stopping Ability:

Air infiltration, the unwanted movement of air through your home, has been shown to be responsible for 30% or more of the heat loss in most homes. Plus, as I’ve noted elsewhere, where air moves, it carries moisture. When air carries moisture to places it shouldn’t, you end up with mold and rot. So air stopping ability is far more than just a comfort issue. It is a health, safety and efficiency issue!

If you’ve ever seen a house being built, you know that most homes aren’t very air tight. Do you think a layer of Tyvek, on the outside of the house, filled with 50,000 staple holes, tears and seams, does anything to stop air from moving through the walls? Not a chance! Do you think that the wall sheathing, nailed to the studs is going to stop air? Nope!

So what happens when the wind blows? It goes right into the walls, carrying with it the 10°F arctic blast or the 95°F summer scorcher.

And what about other places, like in the basement around the top of the foundation at the band joist (the area of wood that sits on your foundation walls)? Do you think that is air tight? Not going to happen. So you have some serious choices to make.

  • Have your construction crew try to caulk every seam and hole in every piece of lumber installed, or
  • Use an insulation material that is intrinsically air sealing

Which do you think will be more successful?

For me, the choice here is obvious – pick an air sealing insulation that goes on easy without a lot of detail work. But remember, that’s just one characteristic. You have to think about the rest of the environment. If you’re sealing something in the basement, you have to consider that the basement is typically a relatively humid environment and if you use moisture permeable materials, you might have problems later due to condensation issues. Likewise in the walls, you have to think about what’s happening in walls. Builders are creating huge potential problems by “solving” the air sealing issue by spraying a thin layer of foam on the inside of the exterior wall sheathing then filling the wall cavity with fiberglass or other loose-fill material (so-called flash and batt). This is an entire article in itself, so I’m not going to tell you why it’s a problem here. If you’re interested, read these Google search results. It’s a hot topic!

Moisture Permeability:

This is directly related to the previous section. When selecting insulation products, you have to give serious consideration to the moisture permeability. In some cases, it really doesn’t matter while in others, it’s critical.

Take a look at the photo at the start of the article. There’s a bathroom behind this insulation. In fact, there’s an open wall right into the underside of a bathtub. That was one cold bathtub! And all the moisture from the bathroom moved right through the gaps in the wall and soaked the cold insulation with condensation during the winter. In this case, they should have had a perfect air and moisture barrier along with lots of R-value because it was exposed to the attic. Instead, they had no air or moisture barrier and a non-existent R-value because the insulation was wet and let air through.

Frankly, if I were building a home, even if I didn’t use foam everywhere, I would seal all bathroom walls and ceilings with closed cell spray foam so that the moisture couldn’t sneak out into the attic or walls and cause them to rot out over the years.

Basements are another place where moisture permeability is critical. We showed pictures in this article of the outside of the foundation walls that were spray foamed to lock out the moisture. Likewise, there are many situations where you want to spray foam the inside of the walls of basements or crawlspaces in order to minimize the chance of moisture problems.

Ease of Correct Installation:

The question of correct installation isn’t discussed much, but it weighs heavily in the actual cost and performance of insulation. For example, fiberglass batts are very inexpensive on a $ per R-value scale. Unfortunately, in order to get the performance, you have to install it very carefully – cut it to shape around any obstacles, take care not to compress it, ensure that it is in perfect contact with the surface of the wall/ceiling, air seal the surface etc. etc. Because of this, fiberglass batts are often installed sloppily and therefore it often doesn’t insulate very well.

Spray or blow in insulation product, whether fiberglass, cellulose or foam, are relatively easy to  install given the right equipment. In a small amount of time, an installer can blow your attic full of fiberglass or cellulose. Likewise, a trained foam installer will fill in every nook and cranny with foam, providing a near perfect air seal and high quality insulation.

In general, if it’s difficult to install, it’s going to be installed improperly and you’ll lose much of your R-value. This is why I’m very partial to blown in products, particularly spray foams since they air seal and insulate in one pass. So, even though foam appears expensive, the performance is typically much higher even if the R-values are the same. Plus, if you add in the cost of properly air sealing before you add fiberglass or cellulose to your attic, you’ll may end up with those solutions costing more than the foam.

Thermal image of a perfect insulation job

I’ve shown you lots of pictures of bad insulation jobs, so I had to show you a good job. The thermal image above shows a perfect cavity fill giving a uniform thermal image. The light areas show warmth, which is good because this was taken in winter when you want the walls to be warm. There are no gaps whatsoever. However, note the darkness of the studs. This “thermal bridging” through the wood can reduce the overall R-value of the wall by 30% or more

Will it Maintain its R-value Over Time?

Have you ever gone into an attic filled with blown in insulation and found bare spots where the electrician or cable-guy pushed the insulation aside so they could run a wire? Or found piles of fiberglass batts set aside? Or perhaps boxes piled on top of the insulation, squishing it down to a couple inches thick?

Things like this can easily double the amount of energy lost to the attic because they compromise the overall R-value of the attic. Likewise, in wall-fill applications where you can’t see the insulation, the insulation may sag, leaving big gaps.

Sagging and missing insulation in walls

This is another reason why I dislike batts and loose fill applications in attics. You know that you will have to work in the attic at some point, and when you do, you’re going to compress or display the insulation. Does your cable guy care if he ruins your insulation? Not usually – he’s getting paid to run a wire to a room! Can you see into the walls to ensure that the insulation was installed correctly and isn’t sagging? I can, with a thermal camera, but few people have access to this equipment.

What about basement and crawlspace applications? That crawlspace ceiling insulation is almost always soggy and falling out.

I know this article sounds like an advertisement for spray foam, but it is nearly a perfect product. Because it is rigid and tough, it’s much less likely to be removed or displace when worked around. And when it is, it’s easy to see as it has to be cut out with a saw. As such, once it’s installed, it usually never moves or loses its R-value.

There have been instances where the temperature or chemical balance was off during a spray foam installation, causing severe shrinkage, rendering it useless. So it isn’t perfect. However, this will occur shortly after installation. The key is to allow it to cure so it can be inspected before something permanent, like a ceiling, is installed over it.

Ecological or Health Concerns

I’d be remiss if I didn’t mention ecological or health concerns associated with different products. Each product has its advantages and disadvantages, and you have to be comfortable with whatever product you’re choosing. This would fill volumes if I covered all these considerations for every product, so I leave it to you to use Google and compare the products in depth. However, here are some issues associated with various products:

  • Fiberglass – reputation for itchy skin when handled. No long term adverse effects documented. Severe respiratory irritation from inhaling the product. Typically not an issue unless you work around it frequently or are going to do a DIY job in the attic.
  • Cellulose – loose fill is very dusty unless sprayed in with the damp glue binder. Usually treated with borate, which is a fire and insect treatment. Generally considered safe but can be a severe irritant if you work around it. Dust can be very annoying. Considered by most “green builders” to be a very good product since it is made from recycled newspaper.
  • Spray foams – each has a different chemical composition, but typically closed cell polyurethane is mostly a petroleum product. There are offgassing/smell issues soon after installation. Typically you want to let it “air out” for a couple weeks after installation. Not known to be generally harmful but some people may have sensitivities.
  • Denim/cotton – Generally considered safe. Made mostly from recycled blue jeans and waste cotton.

This has been a brief overview of the various properties of common insulation materials. When you’re planning your next insulation product, try to keep these things in mind. Make sure the product is appropriate for the desired use and ensure that it will last the life of your home.

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2 thoughts on “Types of insulation: part 3-Insulations compared

    • It’s true – it’s overwhelming. Every time I sit down to write a “short” article, it brings up more and more considerations.
      Maybe by the end of the year, I’ll have done a good enough brain-dump that someone could create a simplified “road map” for renovations.

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