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You are here: Experts > Home/Garden > Home Improvement/Repair > Roofing > Insulation / water vapors
Roofing - Insulation / water vapors
Expert: Brad Zacharia - 10/29/2009
Question
First let me thank you for all your input provided here as a volunteer. I'm a little confused by a recent answer you wrote which I've copied below. But in answer to a previous question, which I also copied further down below, you seem to say something different. In your recent answer your state that "Fiberglass insulation works by having trapped air in it" but in the other answer you say "If there is no trapped air then water can't condensate out of the air". Could you please clear this up for me?
You can't "keep the dew point outside". Any air you have has a dew point. It's the point at which if you cool the air to that point that you will get moisture to come out of the air. No matter what you build you will have air in there and if that air is cooled to a certain temperature you can get moisture out of it.
The air in the layers will not be sealed off from the outside world when you build this. Air will move in and out. You try to minimize it but unless you have a bottle with a cork on it don't think the outside air won't get in. On a humid day you have a humid house. You can try to cut down on the humidity with a dehumidifier in the house.
What you want to do is to try to seal off each layer so you do not have too much air flow between the layers but you won't get it perfect. Whether you have 2 feet of insulation or you do it in layers of different materials you will still have air in there. That air came from somewhere and has a dew point. But since air is not always humid then the air that migrates into the system will not be air at 99% humidity. It will be an average of air over time. If a rain storm comes along the air inside there doesn't instantly change to the humid air outside. By the time the air starts to migrate into the system the weather outside has cleared up so the air inside doesn't get to be too humid to begin with.
Fiberglass insulation works by having trapped air in it but it's not sealed air. If you flatten the insulation (like stuffing it in corners) it does not insulate well. It needs to be fluffy.
Most homes are insulated by just installing one layer of insulation between the inside and outside so by adding more layers can't make it worse. I think you're worrying about a problem that doesn't exists. Your main concern is to try to seal off each layer so they can work as a total system. If you get cold air between layers then you’ve defeated the purpose of one of the layers and it is doing nothing
11/30/2008 you wrote:
Water vapors or condensation happen when moist trapped air is cooled down. If there is no trapped air then water can't condensate out of the air. If there is trapped air between two layers of insulation then it will have a stable temperature and again the temperature of that air will not change so there will be no moisture problem.
It only happens if you take humid air and cool it down to its dew point.
Answer
I guess I was talking about 2 different things. The air that is inside the insulation is not necessarily trapped. The insulation is not air tight. The air can move in and out of the insulation but because the material is fluffy it will take time to move in and out.
Condensation happens when air that is humid is cooled down. That is how you get morning dew or fog. You could think of the attic as two different sets of “air”. One is the entire attic space and the other is the small or microscopic pieces of air inside the insulation.
The reason you ventilate the attic is so that “IF” you get humid air in the attic it can be swapped out for fresh air from outside. If there is a day that is humid and the attic fills with that humid air, and if the nighttime air outside cools to the dew point, the air tomorrow will be better and the attic air will be swapped out with low humidity air the next day. So the humid air does not stay very long since it is not trapped. Also, the attic will be warmer so the attic might stay above the dew point anyway as the outside is cooling down.
Another source for humid air inside the attic is humid house air from leaky duct, steamy showers or steamy cooking or humidifiers set too high. This humid air just leaks into the attic and does not get there by way of the insulation.
If you trap that air in the attic, AND IT IS HUMID AIR, you can get condensation. So you need 3 things to happen to have a continuing problem:
1. Humid Air
2. The air to cool to the dew point.
3. The air stuck there so the humid air can’t get replaced with dryer air the next day.
The air inside the insulation is trapped air if you think in terms of small time periods. The air does not move easily so it takes time for that air to move in and out of the insulation. But as the air in the attic is cooled down the air in the insulation does not move easily so it can’t carry the heat away. Sort of like closing your windows so the heat doesn’t get out yet you know over time the heat will get out.
The air closer to the living space will be less humid since it is a higher temperature.
So having trapped air does not necessarily mean you have a moisture problem. But you need trapped air to have a moisture problem. You can seal a bottle with dry air and it will be trapped but it will not be humid. But put humid air in that bottle and that trapped air will be humid all the time.
Open your windows on a humid day and you know it is humid outside and you might even see dew in the morning but you don’t see dew on your furniture. But the next day the sun comes out, dries up the air and then there is dry air outside. Your house will get dryer air in the house but you never really had moisture developing inside the house because the temps never went as low inside as they did outside.
But don't worry about small spaces between building layers.
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