In the first post, we looked at how adding insulation could lead to frozen pipes if the insulation was put in the wrong location. But, just like a sweater, if you put everything you want to be warm inside the insulation, you can keep your house and pipes happy and energy efficient.
This time, we’re looking at how to install insulation properly so that you don’t rot out your roof. Unfortunately, the photo above shows how not to insulate under your roof!
Excess moisture is a home’s worst enemy. It can lead to mold, wood rot, health problems and even destroy the house. The good news is, the physics of moisture is well understood, and if you understand a few things about moisture, you can avoid a lot of frustration and expense.
The most important thing to know is that warm air can hold a lot more water than cold air. If you remember this, then you can figure out just about everything else!
This is clearly demonstrated when you take a shower. Inside the shower, the air is hot and saturated with water vapor. When you pull the shower-curtain aside and let that air into the bathroom, you see a cloud move out of the shower. After a few moments, the mirror and especially cold windows are covered with fog or even dripping water.
When the warm air from the shower comes in contact with the cooler glass, the air cools and can no longer hold that moisture. The result is condensation – water vapor condenses into liquid water. What you don’t see is that the water vapor is condensing on every cool surface – wood, drywall, etc. You just see it on windows and mirrors because it’s more visible. Also, other surfaces tend to absorb water, so you typically don’t notice the moisture. But in your cold attic, it’s a different story.
During the winter, the roof at night is the coldest surface around the house, so if moisture problems are going to show, they’ll show there first.
If you’ve ever been in a cold attic during the winter, you’ve probably seen something like this. Rusty nails poking through the roof deck, dripping with water or even with little snow caps.
If the moisture is really high, you’ll even see frost all over the wood.
This is exactly the same as the bathroom window – water from warmer, higher moisture air condensing on the cold surfaces like the nails or roof deck.
Where does the moisture come from?
The important questions are:
- where does attic moisture come from?
- how do you avoid moisture problems in attics?
Follow this closely. I can’t tell you how many times I’ve tried to explain this to people who absolutely insist on things that violate the laws of physics. Don’t be one of those people!
The moisture is carried by warm air from the living space leaking into the attic. It’s as simple as that.
Remember the bathroom window? Warm, humid air coming in contact with a cold surface condenses because cold air can’t hold as much moisture. So, even normally humid air in the house at 70 degrees, when it comes in contact with those nails or the roof, condenses into liquid water. If this happens enough, the wood gets saturated with water and you’ll see frost.
How about outside air?
During the winter, the outside air is cold, so it holds much less moisture than the warm air from your house. So if that cold, dry air enters your attic, usually, nothing happens. In fact, the attic is almost always warmer than the outside, so it warms the air, allowing it to hold more moisture. If anything, outside air passing through the attic tends to dry it out because that cold, dry air sucks the excess moisture from the attic. This is why attics are ventilated with outside air.
Bad attic fan, bad!
I hate attic fans. They waste energy and often cause more harm than good. Why? Because they pull so much air from the house.
Usually attic fans are installed to eliminate some issue, either hot attics in the summer or moisture problems in the winter. But if you blow out lots of air, where does the fresh air in the attic come from? If the floor of the attic was perfectly air-tight, then all the air would have to come from the outside. But attics are almost never built this way. You have attic ladders and hatches, leaky ducts, sometimes bath fans venting right into the attic, leaky recessed lights and a hundred other holes and air leaks from the house up to the attic. So when you turn on an attic fan, it’s going to suck a hundred times as much warm, humid air into the attic. In the process, it’s likely to make matters even worse!
Adding a ridge vent can be bad too
For the last 20 years, ridge vents have gotten popular. People thought that they’d be the cure-all for attic moisture problems. After all, warm air rises, so shouldn’t a ridge vent take all that nasty, humid air and send it outside where it belongs?
It turns out, in many situations, ridge vents can make matters worse. Why?
Remember those holes between the house and the attic? Well, they’re still there. As the ridge vent sucks air from the attic, it’s acting just like the fan. Air is going to take the path of least resistance. So year round, that ridge vent is sucking that air from your house – air that you’re paying so much to heat and cool.
Ideally, when you install a ridge vent, you’re supposed to install matching, continuous soffit vents. Unfortunately, this usually doesn’t happen. If you’re lucky, somebody will drill a few holes and install a few round soffit vents. But have you ever looked at those? Do you think lots of air is going to flow through those little slits? How about after you paint your house and cover the vents with a couple coats of paint? Frankly, most soffit vent retrofits are worthless. Except for the Cor-A-Vent system. That system is awesome. I used it on my house and it works great. But you’re not so lucky. You probably have either no soffit vents or one so inadequate that adding a ridge vent just makes attic moisture problems worse.
What happens when you do it wrong?
In the worst case, you rip your roof off because it’s so rotten. Look at the photo at the top of this page. That’s insulation in the space between an attic bedroom ceiling and the roof. Remember our first lesson – cold air can hold less moisture than warm air. So when you insulate that space, you’re making the bedroom ceiling warmer (good) and the roof deck colder (not necessarily bad). In this particular case, it’s really bad.
In this case, the space above the bedroom has a little attic with a ridge vent. Since the attic isn’t otherwise ventilated, the ridge vent sucks air from the space where I took this photo. So imagine a constant airflow from the house, into the first attic space, then up through this fiberglass insulation. The warm, humid air from the house passes right by the very cold roof deck and condenses. To make matters worse, the fiberglass traps the water like a sponge, so now you have wet fiberglass pressed against wood. All winter long, this process continues and before you know it, your roof is rotted out.
If you’re lucky, this roof faces the sun during the day. Plywood is pretty tolerant of getting a little wet if it dries out quickly. During the day, the sun shining on the roof can bake out the water, letting it flow up and out. This process can continue for decades without damage. But a shaded roof or one facing away from the sun doesn’t stand a chance. That water just sits in there all winter long.
Vent chutes might not fix the problem
Because of problems like this, builders created another band-aid – the vent chute. The idea is that if you keep the insulation away from the roof deck, it will allow air to flow better and flush out the moisture.
Would this have helped in the situation above? Probably not. Why? Physics!
Remember, we have warm, humid air, moving from the house, into the attic then up past that cold roof deck. The water will still condense as the air cools. I’ve seen proof of this plenty of times – nice, moldy roof decks exactly where the air channels through the vent chutes.
How to fix attic moisture problems
The solution is extremely simple in theory – don’t allow warm, humid air from the house into places where it will cool off and condense.
The most important first step is to vent that damned bath fan directly out through the roof. Check every bath fan and ensure that it’s vented, through insulated duct, up and out the roof. Not out the soffit. Out the roof. Bath fans vented into the attic are almost guaranteed to destroy your roof. So if Bubba, your friendly handyman, installs a bath fan and vents it into the attic, his incompetence will probably cost you about $20,000 to replace your roof. And another $20,000 when you have to replace it again. If after reading this, you still insist on venting a bath fan into your attic, well, you deserve what you get.
Next, wherever you have attic access, install an air-tight gasket. I wrote an article showing how on my other site. You can also buy a commercial product from ESS Energy Products. I’ve installed several of these and they’re great. Top notch. Much better than anything else I’ve seen for this purpose.
Another big problem with many homes are access ports/doors in knee-walls. Those are the walk-in attic spaces next to an upstairs bedroom. Since they’re indoors, people usually install interior doors that don’t have weatherstripping or insulation. Horrible. Really, these doors should be just like your exterior door – insulated and air tight. But if you don’t want to (or can’t) install a real door, figure out some way of air sealing the opening. You might make an inner hatch with foam board or something else. But the key is, air-tight.
Beyond that, you’ll want to seal up all the other air leaks into the attic. Electrical boxes. Recessed lights. Ceiling fans. All of these are sources of moisture that can lead to roof damage. But start with that bath fan. That will put a thousand times as much moisture in the attic as these other things.
What if moisture problems persist?
Here’s where it gets tricky. Keep in mind that there are millions of homes with insulation pushed up against the roof where it’s NOT a problem. Why? Because the warm, moist air from the house isn’t getting up there. No house air, no moisture problem.
Second, most of the houses built like this were built by people who actually understood what they were doing. Look at older homes and they have well vented attics. Not with ridge vents and soffit vents, but with big gable vents. Or, instead of tight plywood roofs, they had nailing strips that allow a lot of fresh air to flow, flushing out the moisture. But as people built tighter homes, for some reason they built tighter attics. The worst case scenario is the “dead air space” with a bath fan venting into it. A dead air space is an attic space with no ventilation at all. No gable vents. No soffit vents. Nothing but air leaking from the house through an access door, carrying moisture into the walk-in attic. This is pretty much guaranteed to lead to serious moisture problems.
The solution? These spaces need good balanced ventilation. A place for air to get in and an equal sized space for air to leave. In a knee-wall attic, that means two gable vents, one on either side. That allows the cold, dry air to flow through the space and flush out the moisture that gets in there. Or, you can use a system like Cor-A-Vent that gives you proper soffit to ridge vent air flow.
An alternate solution – closed cell spray foam under the roof deck
This solution always causes some people to freak-out. Literally. I’ve seen roofers heads explode when I suggest this. The problem is, people remember half-truths like “always ventilate your roof deck or it will rot.” You know better because now you understand why rot happens.
Remember the physics of moisture. Moisture problems occur when warm, humid air comes in contact with a cold surface. If warm air can’t come in contact with cold surface, you have no condensation problem.
The idea here is to prevent warm air from ever touching the cold roof. If you spray foam that is not water permeable (closed cell spray foam) to that roof, there is no way the moisture can accumulate.
Disclaimer – this can be done incorrectly and your roof can still rot out. Also, if you have a roof leak from above, you may not notice it because the foam will hide the leak. Also, in very cold climates, it is better to allow soffit-to-ridge venting between the insulation and roof deck in order to reduce the chance of ice dams.
To repeat – the laws of physics tell us that condensation can not occur if the water vapor is prevented from coming in contact with a surface that is cold enough for the water to condense.
References showing various attic insulation practices:
- FAQ – Conditioned attics. BuildingScience.com short article on insulating under the roof.
- Crash course on roof venting – BuildingScience.com longer document covering a variety of attic insulation methods.
If you understand the basic principle of how condensation occurs, you can avoid many of the moisture related problems that lead to horrible mold and problems in homes. Just remember your bathroom window – warm, humid air coming in contact with cold surfaces equals water condensation and condensation can lead to mold and wood rot. Now, go check out your attic!