Attic Ventilation

In my area attics are ventilated by perforated aluminum soffits at the eaves and roof vents on the roof. Even though the entire perimeter of soffits is perforated, only those that are baffled in the attic pass any air.
My question is, how many truss spaces in the attic should have baffles? One out of every five? One out of every three?
Does anyone have a resource which clarifies this?

Generally 1:150 is the ratio, so forever 150 ft.² of attic floor space you should have 1 ft.² of ventilation.

Here is an excellent and oft-referenced article on attic ventilation.

Every rafter space requires baffles for proper ventilation of the attic. Continuous soffit ventilation is only as good as what it’s circulating. If your desire is to circulate the air in the dead space of the eaves’ projection only, then you should be good to go. To vent the attic and create airflow, open up the rafter spaces.
One last popular question is – do I need to add a baffle to each bay? In short – YES you should as each bay is open to the venting area & thus vented. If you have an older roof without venting in the soffit area, you may wish to install this for future proofing. If you don’t think you will go that route, capping that area off with foam board (to the exterior edge) & up the sheathing to the height of the insulation would be recommended.

1:300 ratio is ok as long as ventilation is split between soffit and ridge, the important thing is ventilating each rafter bay in a cathedral ceiling that is not open to other rafter bays.

The problem is that the 1:150 ratio is not convertible to the ratio of baffled:unbaffled rafter spaces, which is the only thing we see when we’re in an attic.

I’ve never seen every rafter space baffled. I’m lucky if it’s one in six.

It varies between standards, but more ventilation is better if I am advising a client, which is the only time I would be using this information.

The ratio 1:150 or 1:300 would apply to the output(ridge) side and you would have a slightly greater amount at the intake(soffits) to ensure positive pressure in the attic space. The size of the baffle openings would determine if they meet the required amount of ventilation. I’m not sure what you mean by it not being convertible as it is a simple ratio. Could you clarify that for us?

Well, what I’m saying is that the 1:150 or 1:300 is a good theoretical venting ratio but it’s not something we can readily calculate on an inspection.

With respect to the soffit venting, all we can really see is that some rafter spaces are baffled and some are not. So it would be nice to know what percentage should be baffled ie. every third space, every fifth space, etc.
If every space was baffled like a previous poster suggested then it would be a no-brainer but that’s never the case in my area.

I’m probably looking for a guideline that doesn’t exist but I thought I’d ask anyway.

It is dependent on the net free venting of the soffit/eave venting through the perforated soffit and the 1/150 or 1/300 formula referenced by others. To answer your question would require knowing the Net free area and the location of the upper vents in relation to the eave and the attic sq ft and the rafter spacing.

Now remember that venting is a system, not just a mathematical square footage equation.

I’m thinking that there is no simple way of determining the required number of baffled rafter spaces.

**Determining Your Ventilation Requirements **](

Yeah, that is a tough one to narrow down to a simple guideline. Most manufacturers average about 1.25-1.5 inches of depth in the opening their baffles produce and the width is usually either 10 inches(12" O.C.), 12 inches(16" O.C.) or 20 inches(24"O.C.).

To get one square foot of ventilation you would need 12 baffles 10" wide, 10 baffles 12" wide or 6 baffles 20" wide. Even if those rough estimates were accurate across the board, you would still need to figure the total square footage of the attic floor and divide by 150 or 300. Then you would still have to figure the ventilation provided at the ridge.

It’s just too much uncertain math to be doing during an inspection. You’re probably better off calling out poor ventilation with a conservative approach, meaning, if it could stand to be improved or is even remotely in question, go ahead and suggest they increase the attic ventilation. Vent Requirements

  1. Except as provided in sentence (2), the unobstructe
    d vent area shall be not
    less than 1/300 of the insulated ceiling area
  2. Where the roof slope is less then 1 in 6 or in roofs that are constructed with roof joists,the unobstructed vent area shall be not less than 1/150
    of the** insulated ceiling area **
  3. required vents may be roof type, eave type, gable-end type or any
    combination thereof, and shall be distributed
    a) uniformly on opposite sides of the building
    b) with not less than 25% of the required openings located at the top of the space, and
    c) with not less than 25% of the required openings located at the bottom of the space
  4. except where each joist is separately vented, roof joists spaces shall be
    interconnected by installing purlins not less than 38mm by 38mm on the
    top of the roof joists
  5. vents shall comply with CAN3-A93-M, “Natural Airflow Ventilators for
    The best baffiling system between rafter or top chord and the waxed cardboard.

I used wood myself. 2x4" blocks then sheathing.

I tend to agree.
There is not sufficient information on site to determine whether the number of rafter space baffles is enough. That being the case, it appears that determining whether an attic has sufficient ventilation is “beyond the scope of a standard home inspection”.

For all you anal retentive types(my kin) :), I did fiddle around with some of the numbers and found something interesting. I used about 15 different recent homes I inspected and here’s just a couple examples I looked at:

Example #1:

  • 800 sqft. house/attic floor 25’(gable)x32’(eaves)
  • 16"O.C. Rafters, 48 rafter bays
  • 1.5"x12" baffle openings

At 1:150, the required ventilation would be 5.33 ft.² or 768 in.². That would require 22 baffles installed which is nearly every other rafter bay plus a good ridge vent.

Example #2:

  • 1500 sqft. house/attic floor 30’(gable)x50’(eaves)
  • 16"O.C. Rafters, 76 rafter bays
  • 1.5"x12" baffle openings

At 1:150, the required ventilation would be 10 ft.² or 1440 in.². **That would require 40 baffles installed which is more than every **other rafter bay plus a good ridge vent.

Example #3:

  • 500 sqft. 2nd floor/attic floor 20’(gable)x25’(eaves)
  • 16"O.C. Rafters, 38 rafter bays
  • 1.5"x12" baffle openings

At 1:150, the required ventilation would be 3.33 ft.² or 480 in.². **That would require 13 baffles installed which is a third of the rafter **bays plus a good ridge vent.

At 1:300 with a vapor retarder installed, those numbers would be halved. There is some confusion as to how the net free ventilation is applied. Some sources split this between eaves and ridge. Others apply it equally to both.

Every house is configured differently, but the numbers don’t change dramatically for different floor plan designs, different rafter spacing or even different baffle manufacturers(with different baffle openings).

** For hip roofs, every third or fourth rafter bay is probably adequate. For gable roofs it takes a lot more, especially once you get above about 700 sq.ft. If at least every other bay doesn’t have a baffle, it’s probably not vented enough.**

Interesting Cameron, thanks.