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

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Forum topic by RonInOhio posted 01-12-2013 09:12 AM 18474 views 0 times favorited 8 replies Add to Favorites Watch
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721 posts in 3042 days

01-12-2013 09:12 AM

Excuse the long post.

I will soon be insulating my shed and have come up against a few issues I need to resolve. Was hoping someone with a knowledge of insulation and proper venting techniques could weigh-in on my situation.

I have some round two inch vents. I was thinking about using them in the gables for exhaust vents. Easy to install just drill a two inch hole in the gable and push the vent into it. The problem is , I’m not sure if the little 2” gabel vents I have will be sufficiently big enough.(They are not installed yet.)

The plan is to put one at both ends of the gable.

I have read where its just as bad to have gable vents too large, as not having any attic/gable vents. I suppose if design and proper venting calls for something like a 4” vent, I could put two in each end or cut out a 4 inch hole, but hopefully that won’t be necessary. A hassle. Don’t think I have a 4 inch hole saw attachment
and those are expensive.

Along with the gable vents I will have soffit vents.

Apparently a proper design rule is to have 1 sq. foot of inlet and outlet vent for every 144 sq. feet of attic floor. Or, 1 sq.inch of vent for every sq.foot of attic area. Do the soffit vents count into that total ? I assume they do.
I was under the impression that the soffit vents are an intake vent .

Not worried too much about the soffit vents as I will be using the vinyl soffit material which are perforated along there entire length and width.So there will be plenty of intake. Just not sure about the exhaust vents up on the gable ends.

So I need to calculate the area of my little attic (or space) that needs to be vented to determine if my gable vents are large enough. But more on that later.

For now, The illustration below shows how I plan on insulating the rafters and attic. The parts shaded in blue will be insulated. Additionally I will be installing batt baffles from the wall plates , up to where the collar-ties meet the rafter. These baffles provide about an inch or so of air space . The air space will remain cold during cold outside temps and so condensation won’t form inside and cause rot and ruined insulation.

Now moving into the little attic area . I will be insulating the collar-tie bays with at least 6 inches of insulation. Above the collar-ties I’m stumped. It would seem against logic to insulate the rafter bays above the collar-ties, no ? So as the illustration shows I don’t have those insulated. Is this correct ? I am probably over-thinking this but don’t want to screw this part up and end up with condensation , wet insulation , rot ,mildew or other damage.

My attic is a triangular shaped space. So I’m assuming I just find the area of that triangle right ? Been awhile since I used this math.

A=1/2 B*H

The shed is about 10×12 and the collar-ties only leave about a foot up there to the roof and maybe
5 feet across. Does this look about right or am I missing something ?

~ dimensions of attic.

The width of the collar-tie is about 5 feet.
The height from top of collar-tie to ridge beam
about 1 1/2 feet.
The shed is 12 feet front to back.

8 replies so far

View Howie's profile


2656 posts in 3101 days

#1 posted 01-12-2013 12:13 PM

Have you thought about just putting a ridge vent in?

-- Life is good.

View REO's profile


929 posts in 2252 days

#2 posted 01-12-2013 01:21 PM

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

View REO's profile


929 posts in 2252 days

#3 posted 01-12-2013 05:25 PM

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

View RonInOhio's profile


721 posts in 3042 days

#4 posted 01-13-2013 04:15 AM

@ 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.

View RonInOhio's profile


721 posts in 3042 days

#5 posted 01-13-2013 04:20 AM

@ 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.

View RonInOhio's profile


721 posts in 3042 days

#6 posted 01-13-2013 05:06 AM

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.

View REO's profile


929 posts in 2252 days

#7 posted 01-13-2013 07:26 PM

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.

View RonInOhio's profile


721 posts in 3042 days

#8 posted 01-14-2013 10:26 AM

“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.”


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

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

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