The Basics of Roof Ventilation
Basics of roof ventilation, you’ve got to have an intake, usually at the bottom of your roof. You’ve got to have exhaust usually at or some place near the peak or the ridge or the top of the roof. Other basics of ventilation, when you vent your attic, you must look at the amount of the free air flow. Basically, you’re looking at the amount of holes or perforations or vents that are in that area where the air comes in and you want to make sure that you’ve got a similar amount of free air flow at your ridge vent. Usually at your ridge vent, I’m going to talk in terms of ridge, but then again it could apply to other types of vents as well.
Usually your ridge vent is going to be feeding 2 sets of soffits vents, 2 sets of overhang vents on opposing sides of the roof. You’ll add up the total amount of your sofa vents, your free air flow, venting products that you buy will tell you what the free air flow is. You want to make sure that the total amount of the soffit vents is very close to being equal to the total amount for free airflow through your ridge vents. You create this situation where air comes in, all the air that comes in can go out at the peak. as far as how much ventilation do you need, general rule of thumb is to look at the square feet of your attic and you look at how many square feet there are on the floor of your attic space.
Let’s say you come up at 3000 square feet on the floor of your attic, then you want to look at how much free airflow do I need for my vents and basically you want 1 square foot of net free air flow, one square foot for every 300 square feet of attic floor space. For example, on that 3000 square foot attic, you are going to need 10 square feet of net free air flow allowed through the ventilation of your ridge vent and your soffit vents. That net free airflow number is not the intake of soffits combined, its just one or the other. You want to have that much on whatever is the lower number of your soffits and your ridge vents.
Then again, hopefully your soffits and ridge vents numbers are about the same. The 300 square foot of attic floor space applies if you got a vapour barrier behind the ceilings on your home. I am going to explain vapour barriers in a second, but if you do not have a vapour barrier behind the ceiling in your home and that would be particularly a case of an older home perhaps a home prior to about 1970 or 1975. If you do not have a vapour barrier, then you’ll need to have one square foot of net free airflow ventilation per just 150 square feet of attic floor space. In my example, that 3000 square feet attic, you would need 20 square feet of net free air flow to serve as that amount of attic space. What is the vapour barrier? Well vapour barrier is basically and usually a sheet of polyethylene. A lot of times on home construction, before they put the dry wall up your ceilings, they will put this layer of polyethylene behind the dry wall. They do this on walls also. Basically, the purpose of that polyethylene is that what we call vapour barrier. It means that vapour which would be moisture does not pass through it. If we have this vapour barrier behind your dry wall, they’re never going to be a 100%, but they’re going to be very helpful in containerizing the moisture inside your home. Basically the theory is, or what we are accomplishing there is, we keep that moisture that is created or generated inside you home. we keep it inside the house, that vapour barrier blocks it and keeps it from getting into the attic. If does get in to the attic, we don’t need to vent it out. That is the reason that if you got a vapour barrier, technically, you probably need to less ventilate your attic than if you do not have a vapour barrier. if you do not have a vapour barrier especially again if it is an older home or if quite simply one wasn’t installed, then you’re going to need more ventilation in your attic because that moisture from the inside your home to a greater degree is going to end up in the attic and you need it to get it out there so it doesn’t cause problems like condensation, rot, mildew, mold and all that horrible stuff. Louisville Roofing Contractors do full roof inspections and quote free estimates.
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Louisville Roofing Contractors
624 E. Market St
Phone 502 208 3778