Types of Insulation: Part 2 – Where does insulation go?

In part 1 of this series, I gave you an overview of the different insulation materials and the various forms they come in. This article covers where insulation goes and why. Knowing this helps you understand why you’d want to use a particular type of insulation for specific applications in your home.

Where do you use insulation?

  • On the attic floor
  • In attic cavities
  • On an attic knee wall
  • On the attic ceiling
  • In the walls
  • Around the windows and doors
  • Around pipes and other holes in the wall
  • In the basement and crawlspace ceilings
  • On the basement and crawlspace walls
  • Under the slab
  • Outside the foundations

Each of these areas really deserves an article of its own. In fact, if you look on the Building Science website, you’ll find highly detailed articles doing exactly that.  If you want to go straight to the source, consult these references.

Let’s look at some photos to get an idea of several of these cases…

Attic floor insulation

The attic floor, which is the ceiling of the room below, is perhaps the best known location for insulation – almost everybody starts by rolling our some fiberglass in their attic, hoping to reap the results of added insulation.

Indeed, it’s a great place to start because attics get very hot during the summer and are relatively cold in the winter. At the very least, you should take a look in your attic to see what state the insulation is in.

But before you dive into it and add lots of insulation, you’ll want to study up on air infiltration and sealing. You’ll find out that air sealing is crucial before you insulate or you might even cause problems by adding insulation. On the other hand, if you go into your attic and you see big areas missing insulation like shown in this picture, you should immediately fix it. Usually, you’ll find the missing insulation sitting in a pile next to the bare spot, right where the electrician or carpenter left it after doing work in the attic. Just replace it, making sure that the insulation rests firmly against the ceiling below. Any air gaps will seriously compromise the ability of the insulation to do its work.

Attic Cavities are also illustrated in the photo. These are just as important to insulate as is the attic floor. In fact, they can be more important because they often lead to areas with plumbing. If you’ve ever had frozen pipes, it’s very likely due to something like this. Here’s a great article on frozen pipes and how to deal with them.

Kneewall insulation

Knee walls are among the most forgotten important places to insulate. Depending upon the design of your home, you may or may not have one of these places. Cape style houses are almost guaranteed to have these, usually with an access door or hatch so you can enter and store stuff in there. Unfortunately, the door is often uninsulated, leading to a big energy-hole in the wall.

For some reason, building codes consider these spaces to be just “little walls” and only require modest insulation on them. But knee-walls divide the living space from a hot attic space (often 130°F to 150°F, so these walls should be insulated to the same level as the attic floor – around R40.

Quite often, when people complain of hot rooms during the summer, the cause it an improperly insulated knee-wall. All that heat coming from the roof turns the space into an oven, so those wall get very hot if not properly insulated. And in the winter, they’ll freeze you.

There are also other problems with these little attic spaces. I’ll cover these in detail in other articles. They can be quite tricky due to hidden air passages and moisture problems. So even a high quality insulation job on the back of a knee wall can leave the room uncomfortably hot or cold.

To get you started, try this link. There are numerous great resources talking about this problem.

Attic ceiling insulation was popular years ago and is regaining popularity today with the advent of spray foam insulation and its great insulation properties.

Attic ceiling insulated with spray foam

Why would you want to insulate right below the roof instead of on the attic floor?

Remember that I keep harping on providing a good air seal. If air from outside, (or from the attic or other spaces that should be outside the living space) isn’t perfectly sealed from the house, it will bypass the insulation, rendering the insulation useless.

With normal insulation techniques, you have to be very careful about air sealing. Even the little gaps between the walls and the wall framing allow attic air to circulate down through your walls. And big holes, like for plumbing, electrical wires and recessed lights really destroy the air seal. Add in attic pull down ladders and doors, light fixtures, and anything else that leaves an opening to the attic, and you’ll quickly see that you could spend forever trying to air seal all the holes.

Instead, it’s much simpler and more effective to do what was done in this photo – spray foaming the underside of the roof deck and rafters using “closed cell” foam. It’s actually amazing how many problems this type of construction alleviates when done properly! If I were building a new home or doing substantial renovations, I wouldn’t think twice – this would be the solution I would choose. While some use open cell foam, it is highly moisture permeable and some say this can lead to serious problems.

Read this research report if you want to learn more about “unvented roof systems”

This discussion asks the question: “Can Spray Foam Rot Your Roof?”

This is a hot topic that I may address in a future post. For now, read the links mentioned if you want to learn from the pros.

Applegate Cellulose Insulation

Insulation in the walls is just assumed…or is it?

Just like other insulations, a continuous, uniform coating of insulation is important for a home to perform well. Unfortunately, since it’s hidden inside walls, this is an area that is often done improperly.

Have you ever felt a draft coming through your electrical outlet even though you know your walls are well insulated? Well, that illustrates the problem. Without air sealing, the pressure differences in and around a house will drive air right through the insulation, unless the insulation provides an air seal. Loose fill fiberglass or loose cellulose do NOT work.

There are some insulations that provide effective insulation and air sealing. Densely sprayed cellulose, like shown in this photo, have been proven to greatly reduce drafts. There’s also a single fiberglass product (Spider) that claims to do this. Spray foam, both open cell and closed cell, will also do the trick.

For retrofit applications, it can be hard to find a qualified contractor. With retrofits, you drill a hole in the wall and inject insulation at a high pressure. Unlike in this picture, you can’t see if it’s going in properly, which makes inspection nearly impossible (though you can do it with thermal imaging equipment). Because of this, I’ve seen many improperly done jobs, including, unfortunately, in my own home! The contractors will claim to be doing it properly, but most do it wrong.

Injected insulation requires the use of a small diameter tube inserted into the wall cavity and careful monitoring of pressures. This is called “dense packed” installation. The tube is inserted all the way into the wall cavity and the insulation is blown in starting at the bottom as the tube is slowly removed. If the installer isn’t using this tube-fill method, they’re doing it wrong and your insulation will not do it’s job.

Improper cellulose insulation job = cold feet

To give you an idea of how hard it is to find good contractors, I have found only one installer in my area, which ranges from Philadelphia up to Princeton and west about 50 miles. Every other contractor uses a large hose that they just put up to a hole in the wall and spray. This is guaranteed not to work properly. Trust me. Google dense packed cellulose for many articles on the subject.

That one installer? Brian Snedeker of SPS insulation. The man is an artist with cellulose!

Here’s at thermal image of an overhand section of my living room floor. The dark patches are colder, much colder, than the rest of the floor. The insulation contractor blew insulation into the cavity improperly so barely any insulation went in, leaving big gaps between the floor and the insulation. Unfortunately, I didn’t discover this until I bought my thermal camera and had finished residing my house, so there is no simple fix. Argh!!!

What a window looks like without the trim

Around windows and doors, insulation is often forgotten.

When you look at a house before the sheet rock goes up or if you pull the trim off your windows or doors, you’ll see something like shown here. The window “box” will be about 1″-2″ smaller than the framed out opening. There will be some shims installed to make sure it’s level and square, and then it will be nailed or screwed to the house framing.

Where insulation comes in is in the opening between the wall and the window box. This gap is open to the outside. Sometimes, there will be a “nailing flange” on the outside that is attached to the outer sheathing of the house. This is there to act as flashing and to provide structural support. But it does not air seal the window.

The result is often a big air leak around the windows. This will frequently be confused with a leaky window because you’ll feel air coming in around the window. But the window itself may be air tight.

The most commonly used insulation around windows and doors is fiberglass. Unfortunately, you’ll recall that fiberglass does not stop air or moisture. So the water and air can move right through it and into your house. This is the cause of many rotten windows, since the fiberglass acts like a sponge, holding the moisture against the unprotected wood of the window or your house.

A much better solution is to use low-expansion spray foam to fill in this gap. The spray foam is mostly moisture impermeable and forms a complete air seal. I talk about this in detail in this article. It comes down to this – if you use fiberglass to seal around your windows and doors, you’re going to have drafts. If you use foam, you’re much less likely to have drafts or moisture problems. Just follow the directions and go to town.

Insulation around pipes and holes in the wall is essential for several reasons. First, and most importantly, you want to stop drafts. Since the pipes go right through the wall, they’re a clear path for air and moisture to enter from the outside. Seal them air and water tight, with foam, and you’ll eliminate these drafts.

You’ll also eliminate the openings for rodents and bugs. Look around your basement and find these types of holes and most of the time you’ll find traces of these pests that sneak in through holes around pipes and wires.

Remember – no fiberglass! You want to be use foam because it will fill in all the nooks and crannies. Just be careful to follow the directions. I won’t be responsible if you over-do it or get the sticky, gooey substance on your new hardwood floors!

Damaged crawlspace insulation


Fiberglass is often improperly used in basement and crawlspace ceilings.

If you’ve got fiberglass in the ceilings of your crawlspaces, there’s a very good chance it’s moldy and falling apart. Fiberglass simply isn’t the right material for this job.

I’m going to start you with a Building Science reference on insulating crawlspaces so you can go right to detailed information on the topic. Much better than a few paragraphs here!

There are a number of reasons by this type of insulation is no good in crawlspaces, but most of them come back to moisture. Crawlspaces are notorious for being humid. And humidity and paper faced fiberglass do not mix. Every single installation I’ve seen of this sort is worse than useless and many of them result in rotten wood and potential structural damage. Don’t do it!

Spray foam on crawlspace walls

Much better is to insulate the walls and treat it just like the rest of your house. Why would you want to let the weather into one part of the house and use an inferior material to try to keep it out? The reason people keep doing it is because it’s cheap. That’s it.

Instead, do it once and do it right. If you’re adding insulation to an existing crawlspace, forget everything you know about venting crawlspaces and read the paper referenced above. Instead, spray the walls with ~2″ of closed cell polyurethane spray foam, right up and over the band joists (the wood that sits on the foundation walls). The foam will block almost all the moisture that would otherwise come in through the walls and insulate the space, helping to keep the living space above much warmer. If you’ve got insulation like shown in the first picture, remove it and throw it out.

This is an over-simplification since you really have to adapt insulation techniques to your specific home. However most homes of semi-modern construction (i.e. 1950’s till now) that don’t have dirt floors, really should have their crawlspaces treated as short basement. That means seal it up and insulate the walls.

Slab insulation is one of the oft-neglected areas of the home due to some unfortunate myths and misconceptions.

Myth one: heat travels up. WRONG! Hot air rises, true. But heat goes any direction. If you have a cold slab, it’s going to suck the heat out of the living space. This is basic physics. So that argument against slab insulation is out the window.

Myth two: the slab will reach equilibrium once it warms up the ground below the house so no more heat will be lose. WRONG! WRONG! When I hear builders say this, it makes my head explode. It’s absolutely no different than opening the window. You turn up the heat in the house so it is more comfortable, but the heat keeps going out the window as long as it is open. DUH! Why would anybody think that the earth below the house will stop sucking heat from the house?

Myth three: related to myth two is the blief that if you do need insulation, you only insulation under the first couple feet of the slab. While it is true that you lose the most energy from the edge of the slab, it is not true that you don’t lose energy from other areas of the slab. Think about it. A geothermal system works because the ground temperature is relatively stable. About 50°F in much of the central to northern United States. That may not sound like much, but most homes have a slab that is 1000-2000 square feet. That’s a lot of square footage trying to be at 50°F when your house is 70°F. This is why basements are so comfortable during the summer – they’re naturally air conditioned!

Outside the foundation is a great place for the right type of insulation. What’s the right type? It’s durable, waterproof, and of course, insulating. Why is it such a good place for insulation? First, it keeps the moisture out of the basement or crawl space and out of the foundation. People don’t realize it but cinder blocks are like sponges. Moisture passes through very easily. So coating the outside of the foundation with a water barrier does a huge amount to reduce potentially damaging interior moisture.

Second, moisture is a very good carrier of heat, so damp foundation walls conduct heat out of your basement and into the ground much more readily than dry ones. Insulating and keeping the foundation dry goes a long way to keeping the underground parts of your home comfortable and dry.


That does it. I’m sure I’ve forgotten some important places, so feel free to drop me a note if I’ve forgotten your favorite places to insulate.

Till next time…..


6 thoughts on “Types of Insulation: Part 2 – Where does insulation go?

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  2. Thank you for your reply. To clarify: Yes, all interior walls have been insulated, sides and ceilings. As to the crawl space with it’s own roof vent and gable vent _one wall of crawl space has the 4 ft insulated knee wall. Do I insulate this crawl space area which is over the laundry room below?

  3. Hi Ted,
    I have a 1948 Cape Cod style home. The south facing attic with 5 ft knee wall has a ridge vent and gable vent with thermostatically controlled fan. Due to the upstairs cathedral ceilings there is no other gable vent on this side, and no soffits on a Cape Cod. Is this ventilation adequate as is, especially given the no fan explanation on previous pages? I would like to insulate this space as it does reach 120-125 degrees in summer. Would insulation help with the heat? The knee wall has fiberglass but no vapor barrier. The interior of house and attic have been sealed for air leakage. Would closed cell polyurethane in rafters help appreciatively? Would there be another choice? How does insulation help an area that has ventilation ie roof vent?

    • For insulation, the general rule is to apply it in a continuous manner to all the surfaces adjacent to the living space. So with attic spaces like this, you’d go up the knee wall then continue up the slope then across the flat ceiling portion. In this construction, the flat part can be hard to access, due to small size, but one can easily cut an access hole for the insulation work, do the insulation, then seal up the hole. Having proper insulation up there would certainly make a big difference in summer heating. Closed cell would be great, especially if they can carefully get down into the sloped section.
      That said, you definitely want to insulate the sloped section in some manner. Just behind the sheetrock that is the ceiling of the living space and being careful to use baffles or something else to insure that the insulation leaves 1″-2″ of gap between itself and the underside of the roof.
      One gable vent is better than none, and if you’ve been careful to minimize the amount of air that can move from the living space into the space behind the knee wall, ventilation may be sufficient. Impossible to tell whether it’s enough ventilation without a physical inspection during the winter when condensation prospects are high. I’ve seen well ventilated spaces of this construction that have terrible mold problems due to moisture from showers making its way into the space. The only way to know for sure is to monitor it during winter.

  4. I like that you talked about how heat does rise but is also goes anywhere it wants. It seems like it would be a good idea to have good insulation everywhere in your home and not just in the ceiling and roof. It might be smart to have a professional come help you figure out where you need more insulation.

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