Disclaimer: while I think all the information presented here is accurate and scientifically valid, you are advised to consult a *professional before changing your home. This article covers just one component of your home and your specific home may have conditions that override the comments contained herein.
Cathedral ceilings are very popular - they provide added ceiling height, giving rooms a feeling of openness and an added aesthetic dimension. At the same time, they are responsible for a variety of building catastrophes and homeowner heartbreak. What causes these problems and how do you avoid them?
The first thing to understand is that there are a variety of climate zones. The south-eastern United States is hot and humid, while the north east is cold. The mid-Atlantic states, where I live, is mixed - during the summer it is hot and humid, during the winter it is cold. The south west is mostly hot and dry and the northwest is moderate but very humid. Each of these climate zones has its own particular building details. However, all must follow the laws of physics.
Physics tells us that moisture moves from areas of high humidity to areas of low humidity. If it's more humid outside, moisture wants to come in but when it's more humid inside, the moisture will move toward the outside. Simple!
The trick is that the amount of moisture that air can hold depends upon the temperature of the air. Warm air can hold more moisture than cold. And, at some temperature, the air will reach a point where it can hold no more moisture. This temperature is called the dew point.
The next thing to know is that moisture in air is water vapor. Water vapor is much smaller than liquid water, so it moves much more easily. Water vapor is also lighter than air, so it rises up to the ceiling and through any cracks or holes. Warm air also rises, so there is a tendency for warm, humid air to exert lots of pressure on the ceiling. These concepts are also pretty simple. Just remember - humid air and warm air rise.
So what happens during the winter when you heat your house? The warm air in the house contains lots of water vapor. That warm air and water vapor rises up to the ceiling. If there are holes, like electrical boxes, recessed lights or ceiling fans, the vapor easily moves up into the space above the ceiling.
Let's look at a diagram of a typical cathedral ceiling during the winter....
The blue lines represent moisture trying to move from the inside of the house, where the air is warm, to the outside, where the air is cold and dry.
For most of the ceiling, the moisture encounters painted sheet rock first, which stops air flow and most of the water vapor. However, a small amount of the vapor is absorbed by the ceiling and travels through the sheet rock slowly (thin blue lines). Where there are any holes, such as at recessed lights, air moves through the hole, carrying water vapor right up into the insulation. In fact, a small hole can transmit tens or hundreds of times more water into the ceiling cavity than moves through the the painted sheet rock.
As the water vapor moves through the insulation, it encounters colder and colder temperatures the closer it gets to the roof. If the temperature reaches the dew-point, the water vapor condenses into liquid water and can drip back down through the insulation and back onto the ceiling, causing water damage.
In order to combat this effect, builders created the vented ceiling. This is a space between the insulation and the roof deck. At the bottom, there would be a soffit vent and at the top, a ridge vent. Such a construction allows convection currents to create airflow through the cavity. The ideas is that this airflow will carry away any moisture that builds up inside the cavity. It is also intended to keep the roof deck cold to avoid snow melt and ice dams
In moderate and sunny climates the roof may heat up during the day, warming it enough to carry away the water vapor and dry out the insulation. However, what if the roof doesn't get much sun or the moisture is moving through holes in the ceiling faster than it can get carried away?
As noted above, the moisture in the cavity can build and build until it leads to ceiling damage. This is especially prevalent when the ceiling has many recessed lights or, uses tongue and groove boards instead of sheet rock.
The thermal image shown here demonstrates just how leaky a tongue and groove ceiling can be. The dark areas show places in the ceiling that are colder than the surrounding areas. The lightest areas are where the insulation is intact.
If the moisture makes it through the insulation without condensing, it tries to move up and out of the cavity. However, this space is much like a refrigerator. As the air moves through, it gets colder. Eventually, it may form ice on the back of the cold roof deck. I've seen many roofs rotten and covered with mold because of this effect.
This image shows an example of this problem, looking up into the ceiling cavity. In this case, the white substance on the insulation is mold. Further into the cavity, the roof deck has turned black and is rotting.
Almost every roof put on these days includes a ridge vent, even if one is not necessary. This is done because roofing manufacturers have stated that they will not honor the warranty if the roof isn't properly vented. The problem is, putting a ridge vent on an old house actually creates these types of problems!
Older homes used gable vents, not ridge vents. As such, most older home do not have soffit vents. What do you suppose happens if a ridge vent is installed on roof that has no place for air to come from?
The ridge vent still pulls air out of the cavity. That air has to come from somewhere, and that somewhere is the house. Many of the problems I have seen are caused by the ridge vent.
I have no problem with ridge vents IF they're implemented along with a proper amount of soffit vents. I have huge problems with ridge vents when installed without soffit vents.
For my own projects, I use Cor-A-Vent. They make excellent products backed up by proper engineering and supported by excellent technical documentation. If you plan on doing any roof work, I strongly recommend visiting their site and reviewing their technical literature. If you only read one document, make it this one. I've also copied this document so that it is available below.
Now that you understand how and why things go wrong with cathedral ceilings (and roofs in general), let's look at how to do them right. But first, we need to know exactly what problem we're trying to solve (building rule #2).
Refer back to the first diagram to the left - what problems exist?
This is obvious, so why do people insist on poking so many holes in a perfectly good ceiling?
The biggest offender is the recessed light fixture. Most of these fixtures, even those rated for insulation contact, are worthless in cathedral ceilings. You may as well just drill holes in your ceiling to let the heat out.
Recessed lights are made of metal. Metal is a very good conductor of heat, which means that, in addition to sucking the warm air and moisture out of your house, the recessed light fixture conducts away your heat. Or, on a hot day, transmits heat from the roof into your house.
If you absolutely must install recessed lights, buy boxed fixtures rated ICAT - Insulation Contact Air Tight. Be warned that all ICAT fixture's are not created equal. For example, The fixture to the right is rated ICAT, but I don't recommend this style. Much better are the fully sealed box fixtures, like the next photo.
Next, be sure to seal the fixture to the sheet rock so that it is air tight. A continuous bead of high temperature caulk greatly reduces the air lost by mounting recessed lights.
Recessed light fixtures may be the most common hole in your ceiling, but there are others that cause similar problems.
Be careful of electrical boxes mounted in the ceiling. These are usually very leaky, and can lead to even greater problems than recessed lights. These can be found above ceiling fans, smoke detectors or other ceiling mounted lights.
Most of these are sealed with caulk or foam. Note that you need to do this carefully so as not to interfere with the electrical wires and/or contacts in the box. Best, have a licensed weatherization contractor deal with it.
Fiberglass is a cheap insulation product, but it is not a good one. It works by reducing conductive heat transfer, but it does almost nothing to slow air movement. It doesn't matter whether you put three inches or three feet of fiberglass insulation in the cavity, it's not going to stop air movement and the associated water movement.
Unfortunately, most contractors love fiberglass. Anyone can install it and it is cheap. Granted, almost everybody installs it incorrectly - they compress it (reducing the R-value) and they install it in areas with air movement, rendering it almost useless.
If you absolutely, positively must use fiberglass, then do it right. Install it flush to the sheet rock so no air can come between the fiberglass and the ceiling. Do not compress it - if wires must be routed, split the fiberglass so the wire runs through the middle. If it has to go around obstructions, don't compress it - cut it to the exact size and shape needed. And finally, make sure everything is sealed air tight so no air is tempted to flow through the fiberglass.
A much, much better solution is to use dense packed cellulose insulation. This is cellulose insulation that is installed to approximately 3.5 lbs./cubic foot density. At this density, cellulose does not allow air movement under normal conditions. The way it is blown in also forces it into all the nooks and crannies - around wires and pipes and fixtures. It also can be used in a "hot roof" design. With this installation, soffit and ridge vents are not used. The entire cavity is filled with cellulose. For details, see this link. A PDF of the Applegate Insulation technical bulletin is also available below.
What about moisture? Cellulose insulation can hold a tremendous amount of water, dispersing it throughout the material. In this way, like a sponge, any small amount of moisture that gets into the cellulose spreads out rather than puddling up. In a properly made roof assembly, this moisture then moves towards the shingles and out. Note - there is some controversy about using dense packed cellulose in ceiling cavities. Read this building science article by Joe Lstiburek. The problem arises when you have very porous ceilings, like the tongue and groove ceilings. Because so much moisture passes into the ceiling cavity, it can overcome the moisture flushing capacity of the insulation and roof. This is especially problematic if you have a completely vapor impermeable roof, like a metal roof or a roof covered with a rubber membrane (like all flat roofs). To avoid problems, listen to Joe and don't dense pack your cathedral ceiling if you have any doubts.
Even better than cellulose insulation is high density sprayed polyurethane foam. This foam creates an air-impervious barrier and is also very effective at slowing vapor movement when applied at adequate thicknesses (greater than about 2 inches). Because of these properties, foam is usually applied using the hot roof (no venting) method.
Polyurethane foam also has an excellent R-value, about twice that of fiberglass or cellulose.
Usually, when I explain the unvented roof to people, they ask "where does all the moisture that gets in there go?" To this I reply - where does all the moisture in your house go? Are your walls rotting out? Has your floor collapsed recently? Is there mold growing anywhere? Mostly, they say "no - but that's not the same.
In fact, it is exactly the same! The reason that your house doesn't rot or have mold growing everywhere is because the humidity of the air isn't high enough to cause condensation on normal surfaces. In the same way, when you apply foam, the air cannot come in contact with a surface cold enough for condensation to occur. The humidity within the ceiling cavity is the same as it is inside the house so you have no problems.
Except for one....
There is an unfortunate technique called "flash and batt" that has become popular among builders because it allows them to air seal using spray foam but keep the cost down by providing the majority of the R-value with cheap fiberglass. Unfortunately, many of these insulation contractors do not understand physics. In the cathedral ceiling example, they spray a thin layer of foam to the bottom of the roof deck, air sealing the cavity from the top. Then, they fill the cavity with fiberglass.
What's wrong with this picture? The fiberglass allows the warm air from the house to move into the cavity, through the fiberglass. But, the close the air moves towards the thin layer of foam on the roof deck, the colder it gets. If the foam is not thick enough (usually the case), then it will be very cold. Now, the moisture in the air can condense on the inner surface of the spray foam. Even worse, the air and moisture sealing properties of the foam then locks this liquid water into the ceiling cavity. Before you know it - rotten ceiling!
Now, you can do flash-and-batt in a way that it works, more or less. Remember the physics - we want to stop the air and moisture movement and prevent the water vapor from coming in contact with a cold surface. To do this, you would build your ceiling, install wires and fixtures and then spray the back side of the ceiling with foam. This seals everything and keeps the moisture in the house. The problem is, this means putting the roof on last! This is never done because you need the roof on as soon as possible to keep the weather out while you're building the house.
So, if anybody says you can save a lot of money by doing flash-and-batt for your ceiling, send them packing - they don't understand the physics of insulation.
Think about walls. They don't require ventilation. They're supposed to be sealed tight, and they don't rot out (except when they leak, but that's another story.) why do we build ceilings differently than walls? Well, there is a slight difference - remember that warm, moist air rises. So ceilings are more likely to have warm, high humidity conditions than walls. However, the same physics applies.
I will be writing a detailed document explaining whole-wall (or ceiling) R-value. At this point, I'll leave you with a teaser: you might be paying for an R-40 ceiling but only getting one that's actually R-25!
If you're interested in more insulation discussions, see my notes on insulation and heat transfer.
For more information, please see my blog. This is updated daily and provides a Q&A section so you can ask your questions there.