Ventilation
QRFL
Mon Aug 29, 2005 9:01 pm
we always get homeowners that are asking about ventilation....
so far i got this the 1/300 formula minimum fha(federal housing authority) requirement
1 square foot of ventilation for every 300 square feet of attic floor
and that it has to be distributed 50% from intake soffit vents and 50% exaust
i see some post who say install "so many" vents but vents do vary in size and i see we need to pay attention to that...
i invite our fellow professionals to cover all the ventilation subject in this post and to use it as a link for future questions..
so far i got this the 1/300 formula minimum fha(federal housing authority) requirement
1 square foot of ventilation for every 300 square feet of attic floor
and that it has to be distributed 50% from intake soffit vents and 50% exaust
i see some post who say install "so many" vents but vents do vary in size and i see we need to pay attention to that...
i invite our fellow professionals to cover all the ventilation subject in this post and to use it as a link for future questions..
Master
Mon Aug 29, 2005 9:51 pm
I use the old one foot in and one foot out per 300 sq ft. or 1:150. either way it is the same thing.
G-Tape
Mon Aug 29, 2005 10:17 pm
I agree with Master. But I also can not see if the soffit vents are installed properly, so I always side with caution and add one more roof vent if it can fit on the roof.
QRFL
Mon Aug 29, 2005 10:52 pm
ok so can someone explain what does it mean to have one square foot of outtake per 300 square feet of atticc space???
i dont think it means to have a turbine every 3 squares of attic since the trubine is about 12" round
i dont think it means to have a turbine every 3 squares of attic since the trubine is about 12" round
Master
Mon Aug 29, 2005 11:50 pm
circumference times pi?
I know that the turtle type vent in my area provide 55 sq, inches of net free vent space...so you need three vents per 300 (144 sq inches in a square foot)
I know that the turtle type vent in my area provide 55 sq, inches of net free vent space...so you need three vents per 300 (144 sq inches in a square foot)
QRFL
Mon Aug 29, 2005 11:58 pm
ok here it is:
every vent comes with a net free square inches of out take.(depending on the model)
(for example) if we have 1200 square feet of attic space and the rule is to have" 1 square foot of open ventilation per 300 square feet of attic space" that would mean:
1200/300=4 we need 4 square foot of ventilation, now this in inches is:
4x144=576 (144=12 x 12) that means we need 576 square inches of net free ventilation area
FHA recomends a 50%/50% balance between intake and exhaust. so thats :
576/2= 288 we need a minimum of 288 square inches of intake and 288 square inches of exhaust on a home with a 1200 square feet area of attic floor
exhaust beign normally ridge vent or other vents.... and intake beign normally soffit vent... all that we need to do now is find out what model we want and how much airflow it allows..since most models come with a square inch report.. this formula should do it ..always trying to go above the minimum..
thanks for your help pride_roofing
every vent comes with a net free square inches of out take.(depending on the model)
(for example) if we have 1200 square feet of attic space and the rule is to have" 1 square foot of open ventilation per 300 square feet of attic space" that would mean:
1200/300=4 we need 4 square foot of ventilation, now this in inches is:
4x144=576 (144=12 x 12) that means we need 576 square inches of net free ventilation area
FHA recomends a 50%/50% balance between intake and exhaust. so thats :
576/2= 288 we need a minimum of 288 square inches of intake and 288 square inches of exhaust on a home with a 1200 square feet area of attic floor
exhaust beign normally ridge vent or other vents.... and intake beign normally soffit vent... all that we need to do now is find out what model we want and how much airflow it allows..since most models come with a square inch report.. this formula should do it ..always trying to go above the minimum..
thanks for your help pride_roofing
G-Tape
Tue Aug 30, 2005 1:48 am
And lets not forget, yes the type of vent is important, but what people tend to overlook is the size of the hole you cut into the roof decking.
Most of the time we have to recut the existing hole bigger, We see alot of 6"x6" holes, these holes need to be 12"X12" to get your 144 sq. inches.
8"X8" is OK also, but that is only 64 sq inches, you will need more vents if you cut them that big.
If you inspect the roof vents on the market, most will be questionable with a 12x12 hole cut into the roof deck. There will be very little room for nailing and it will be weak around the vent. I like to add a couple more vents and go with the 8x8 hole.
something to think about.
Most of the time we have to recut the existing hole bigger, We see alot of 6"x6" holes, these holes need to be 12"X12" to get your 144 sq. inches.
8"X8" is OK also, but that is only 64 sq inches, you will need more vents if you cut them that big.
If you inspect the roof vents on the market, most will be questionable with a 12x12 hole cut into the roof deck. There will be very little room for nailing and it will be weak around the vent. I like to add a couple more vents and go with the 8x8 hole.
something to think about.
Master
Tue Aug 30, 2005 1:49 am
In the 1 to 300 model, that is 1 per 300 in a balanced system, meaning one in AND one out. This is why it works the same as one net per 150.
QRFL
Tue Aug 30, 2005 2:17 am
yea Master that is correct you can use either model....G-tape it sound like you say if the hole is 12x12 you will get 144 inches of air-flow... thats not correct every model has a different exhaust rate here is a sample of some and notice that the size of the hole dont mean that will be the total exhaust ,, is what the vent has been tested for.. thats what they based it on not the hole..
http://www.awardmetals.com/product_vent.htm
what is important is that they cut the opening according to the specifications...
maybe i understood your reply wrong... if so my apology
http://www.awardmetals.com/product_vent.htm
what is important is that they cut the opening according to the specifications...
maybe i understood your reply wrong... if so my apology
G-Tape
Tue Aug 30, 2005 2:46 am
QRFL, you are half right about my post, I was just trying to stress that the hole that you cut is just as important as the vent you install.
just throwing a vent on the roof doesn't mean it will vent. you need to cut a hole in the decking.
just throwing a vent on the roof doesn't mean it will vent. you need to cut a hole in the decking.
SYS
Tue Aug 30, 2005 6:15 am
Pride knows vents like the back of his hands hell get down to the hair. He should post on this one for sure.
Master
Tue Aug 30, 2005 11:03 am
Its really simple.
SYS
Tue Aug 30, 2005 4:44 pm
I usually just put them in where the holes are cut. I don't play with the saw cause it growls (scarry)
