Calculating Attic Area, Corresponding Gable Vent Size,And Methods of Insulating Shed Roof

Have you thought about just putting a ridge vent in?

-- Don't rollerskate in a buffalo herd

I think the standard is 1 square foot of free air to 100 sf of AREA not volume. the soffit vents dont count as venting they are to let air in at the bottom so it can go out the vents at the top. Free air area is not the same as the opening size of the vent. Do not let the insulation completely fill the gap between the roof sheathing and the ceiling. allow a small gap on the roof sheathing side or you will trap moisture in the roof and raise cain with the sheathing. in your case with the small roof area the ridge vent would work the best. on very large roofs they do not provide enough airflow

so you have 120 square feet of space. one turtle vent, ridge venting or about one square foot of free air space.

@ Howie. I’m almost home free. All I need to do is figure out the size of my gable vents and
its done. Not really interested in all the work involved and tearing up the roof in January to put
in a ridge vent.

As long as I get the right size gable vents, it should vent fine with two gable vents of the right size.
long.

@ REO I think I see what you are saying.

Once I put insulation in the collar-tie bays, there won’t be a lot of open space between the ridge and the ceiling. There will be some open space in the attic and the air space in the rafter baffles, but not a lot. I think its less than
70 sq. feet. I need to do some calculations to try to nail that number (free air space) down close.

1sq ft. of venting for every 150 sq. ft. of attic floor is a good rule of thumb
I understand. According to Stan Skarbeck , a so-called expert of Home and Garden
Home Improvement repair, the soffit vents count as ventilation even though they are
by design, intakes.

Anyway I will post a link to his post about attic ventilation in case someone else
stumbles upon this thread and needs a reference of some sort.

Stan Skarbeck Home Improvement Expert

In my case, each 2 inch round vent has 3.141 sq. inches of venting area. Area of a circle is Pi x Radius Squared. So two are 6.282 sq. inches of vent area. My attic space is less than 75 sq. feet so I should be ok with (2) 2 inch gable vents.

Square feet is area. above the collar ties you have 60 square feet of area. you have 45 CUBIC feet of volume. ventilation is based on the AREA of the attic space which is from soffit to soffit not just where you have a space that you consider an attic. taking it to an extreme if there was no attic space then there is no need for venting? that is not the correct way to look at it. you have 120 square feet of area that needs to be vented. I increased the area of venting needed because you have restricted flow in the legs of the attic area and it may not vent properly if normal guide lines are used. I have had this particular problem in a barn style shed that was finished off by the homeowner. It had vented soffits but the flow restriction in the lower section of roof defeated the flow of air to the point that it was inadequate.

“It had vented soffits but the flow restriction in the lower section of roof defeated the flow of air to the point that it was inadequate.”

@REO

Did that particular building have baffles going from attic to the soffits in the rafter bays ? Or were there any
baffles at all in the rafter bays ? As you probably know, the baffles create an air space to
help prevent condensation. But the gap also creates a path-way for cross-flow ventilation from the
soffit to the attic vents.
Without baffles there would be little to no cross flow. I’m not questioning what you are saying.

Just making sure I understand the situation was indeed similar. And I appreciate the feedback.
Thanks. I’m reluctant to go too big on the attic vents because that defeats the purpose.or so I have
read.

Anyway , you got me thinking maybe I should go with at least a 4”vent or larger in each gabel end.