Thread: Attic space ventilation

This carport had perforated soffit material around the entire perimeter. There were no out-take vents at the roof surface at all. My question is whether the perforated soffit vent material at the gable ends would act as outtakes as these areas are higher than the eaves at the sides, or would out-take vents still be required in the form of gable events, ridge vents or ports? Just didn't look right to me.

Thanks,

Eric

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It seems to me that this installation is like using eave/ soffit ventilation in conjunction with gable vents which is not recommended as it will disrupt proper airflow--- just my opinion

Usually if it doesen't look right, it isn't.

SECTION R806- ROOF VENTILATION- - R806.1 Ventilation required. Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow. Ventilating openings shall be provided with corrosion-resistant wire mesh, with 1/8 inch (3.2 mm) minimum to 1/4 inch (6.4 mm) maximum openings.- - R806.2 Minimum area. The total net free ventilating area shall not be less than 1 to 150 of the area of the space ventilated except that the total area is permitted to be reduced to 1 to 300, provided at least 50 percent and not more than 80 percent of the required ventilating area is provided by ventilators located in the upper portion of the space to be ventilated at least 3 feet (914 mm) above eave or cornice vents with the balance of the required ventilation provided by eave or cornice vents. As an alternative, the net free cross-ventilation area may be reduced to 1 to 300 when a vapor barrier having a transmission rate not exceeding 1 perm (57.4 mg/s · m2 · Pa) is installed on the warm side of the ceiling.

Example:
Attic space is 2500 square ft.
The requirement for soffit only ventilation is 1/150 of that= 16.67 sq ft.
Now, if there are gable vents or roof mounted vents (w/ square footage of vents that total 50% and not more than 80% of the total requirement) 3' ft or greater than the soffit or cornice vents, the 1:150 is reduced to 1:300 (half of the soffit only ventilation requirement).

So... under the above scenario (2500 square ft attic space) that requires 16.67 square ft of ventilation, there would need to be at least 8.33 sq ft of roof mounted or gable vents (minimum of 3 ft above soffit or cornice) to reduce the total requirement of 8.33 sq ft (total combination of soffit/cornice and roof mounted vents) to be adequate.

RR

it's not right. the gable vents are only venting the bay they are in. the enclosed attic area of the carport has no high end venting.

Brian,

I'm not sure what part of
"EXAMPLE" is eluding you.

RR

That house or carport does not have a gable vent. It only has the soffit vent.

I could never decipher that part of the code, but under the code that continuous soffit vent will most likely meet the code. I think!!

If it was me I would simply report that the soffit venting might not provide enough venting in the attic space. If it was me I would add a ridge vent to help with the cooling of the attic area.

Let's use Richard's example:

Originally Posted by Richard Rushing

Example:
Attic space is 2500 square ft.
The requirement for soffit only ventilation is 1/150 of that= 16.67 sq ft.
Now, if there are gable vents or roof mounted vents (w/ square footage of vents that total 50% and not more than 80% of the total requirement) 3' ft or greater than the soffit or cornice vents, the 1:150 is reduced to 1:300 (half of the soffit only ventilation requirement).

So... under the above scenario (2500 square ft attic space) that requires 16.67 square ft of ventilation, there would need to be at least 8.33 sq ft of roof mounted or gable vents (minimum of 3 ft above soffit or cornice) to reduce the total requirement of 8.33 sq ft (total combination of soffit/cornice and roof mounted vents) to be adequate.

Let us assume *no* reduction (which is not allowed anyway because this is only soffit vented).

Let's 'presume' that perforated soffit vent provides 10% net free vented area (pulled that number out of thin air as I feel it is on the conservative side, by quite a bit).

8.33 sq ft of net free vent area is required (for the above example)

2,500 sq ft of attic space, let's presume, again, that the house is square (which is not very likely, but presents the worst practical case scenario for the example), thus, the house is 50 feet by 50 feet.

Now, let us 'presume' that soffit is only 1 foot wide (also a more or less 'worst case' example).

There is 50 feet of soffit on two opposite sides, or 100 feet of soffit, times 1 foot wide equals 100 sq ft times 10% equals 10 sq ft of net free vent area.

I *did not* count the gable end soffits.

Even so, with those 'most likely worst case' presumptions, there was more than sufficient soffit ventilation to meet code.

Jerry... and that would be a wrong assumption.

Using the example and the (right) numbers you used, the 10 square feet would be short of the required 16.67 sq ft-- not 8.33 sq ft (which the 10 sq ft would be more than).

You can only use the 16.67 sq ft *IF* the attic has only soffit/ cornice vents. Remember, the requirement states that a minimum of half (50% or more) of the required ventilation has to be 3' ft above the soffit/ cornice before the 1:300 ratio can be used.

The 1:300 would reduces the TOTAL requirement to 8.33 only if the 50% or greater ventilation requirement is 3' ft (min) above the soffit (gable vent or roof mount vents) is present.

Rich

Does anyone have a chart of applicable net free area for different types of attic vents?
I would thing it varies greatly by manufacturer but a ballpark for each type vent would be helpful.
I would guess stamped soffit vents would be maybe 50% free.
Wind turbines maybe 90%.
Standard vent boxes for sloped roof vents maybe 70%.
Louvered gable vents maybe 70%.
Perforated Hardi board? 10%?
What figures are out there?

Originally Posted by Richard Rushing

Jerry... and that would be a wrong assumption.

Correct. I goofed. I used what I thought I read in the example without doing the math myself - my bad. I now went back and re-read it and see I pulled the wrong number out.

As I said, "Let's 'presume' that perforated soffit vent provides 10% net free vented area (pulled that number out of thin air as I feel it is on the conservative side, by quite a bit)."

Let us now make another presumption ... (hopefully I don't come out looking as stupid as in the previous post)

That photo shows the soffit which is about 50% solid and 50% vented (based on the one photo and one angle, I'm just guessing - again - ), then let's presume the vented area is 1/3 net free area, again - just numbers out of thin air, those holes *look like* they are less than 1/2 of the vented area, so I'm going with 1/3 - guessing, again). 100% time 50% equals 50, 1/3 of 50 equals 16.667% of the total 100% area.

So, if my "worst case square house" would just make it at 16.67 sq ft net free vent area based on that 100 sq ft total soffit area. Also, remember, I *did not* include any gable soffit vent area.

Okay, so it looks like I'm just playing with numbers, and I am, the point being (forget that my 'worst case conservative guess of net free area' means diddly squat ) that one must do the numbers.

Take a house which is not square, instead of 50' by 50', make it 60' long, which makes it 41.7' wide. Now, though, you have 120 linear feet of soffit vent.

Now, make the soffit a more reasonable 2' wide instead of 1' wide, you now have 240 sq ft of soffit.

NOW do the math.

The point is, you've got to at least roughly do the math to make a judgment, regardless whether it's perforated soffit or spaced out soffit vents.

Okay, so my previous post was all screwed up ...

Quote: "The point is, you've got to at least roughly do the math to make a judgment, regardless whether it's perforated soffit or spaced out soffit vents."

Agreed.Usually, the only time this is calculated is when the archictect is designing the home, late at night and after a few glasses of Merlot.

After that... no one ever bothers.

Rich

Originally Posted by Jim Luttrall

Does anyone have a chart of applicable net free area for different types of attic vents?
I would thing it varies greatly by manufacturer but a ballpark for each type vent would be helpful.
I would guess stamped soffit vents would be maybe 50% free.
Wind turbines maybe 90%.
Standard vent boxes for sloped roof vents maybe 70%.
Louvered gable vents maybe 70%.
Perforated Hardi board? 10%?
What figures are out there?

Jim,

I just did a quick search (to compare to my latest previous post above) and this ( https://www.alcoa.com/alcoahomes/Pro...=2&styleId=830 ) material is 12" wide, has three sections, with 2/3 being solid and 1/3 being perforated, and, of that 1/3 which is perforated, there is 4 sq in per linear foot.

Figure that this would need to be installed perpendicular to the house (as shown in the photo on the linked page), assuming a 2 foot wide soffit. that means that each linear foot of soffit would provide 8 sq in net free vent area. 144 divided by 8 equals 18, which means that it takes 18 linear feet of soffit to provide 1 square foot of net free vent area if the soffit is 2 feet wide.

In the example in my last post, that would mean a house which was 60 feet long would only have 6.67 square feet net free vent area with this type of 2/3 solid surface. This style would be insufficient for the example given.

However, this ( https://www.alcoa.com/alcoahomes/Pro...=2&styleId=831 ) style has 13 sq in per linear foot, or 26 sq in per 2 foot wide soffit, or 3 times the net free vent area of the previous style, which would then net out at 6 linear feet of soffit being required to provide 1 square foot net free vent area.

With this style, the example would provide 20 sq ft net free vent area, and would be more than sufficient for the example.

The style shown in the photo was neither, but would be about half the latter style above, or about 13 sq in per linear foot of 2 foot wide soffit, and probably would not make it to the required minimum net free vent area either (half of 20 sq ft is 10, and 16.7 sq ft is needed for that example).

Originally Posted by Jim Luttrall

Does anyone have a chart of applicable net free area for different types of attic vents?
I would thing it varies greatly by manufacturer but a ballpark for each type vent would be helpful.
I would guess stamped soffit vents would be maybe 50% free.
Wind turbines maybe 90%.
Standard vent boxes for sloped roof vents maybe 70%.
Louvered gable vents maybe 70%.
Perforated Hardi board? 10%?
What figures are out there?

Table 2 in the APA publication form number X485 is a chart offering guidelines on some generic types of attic vents and coverings. Also offers some guidelines on how to calculate ventilation.
http://www.apawood.org/level_c.cfm?c...TOKEN=82457428

richard,
your example is fine! i am thinking along the lines of what scott said. i also agree totally with your picture and the way it shows high and low venting. if the upper venting is not three feet above the soffit venting then 1/150 must be provided.providing the full amount of venting at the soffit or eaves only can have detrimental effects to the roof coverings. how many times have you seen curled edges on comp roofs towards the peak of the roof and discovered that there were only soffit vents?what was your opinion on why the shingles were fine until half way up the roof? did you attribute the product failure to high temperatures in the attic due to inadequate design of the roof ventilating system?

Originally Posted by brian schmitt

how many times have you seen curled edges on comp roofs towards the peak of the roof and discovered that there were only soffit vents?

Soffit venting "only" is okay.

Just the formula is different. 1/150 instead of 1/300

Eric,

ESB: Research Exposes Attic Ventilation Myth

Michael,

Did you notice that information was 15 years old?

Not saying it is 'bad information', simply that it is 'old' information.

Cross ventilation has been in the codes from long before that.

This is from the current IRC: (underlining is mine)

- SECTION R806
- - ROOF VENTILATION

- - - R806.1 Ventilation required.

Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow. Ventilating openings shall be provided with corrosion-resistant wire mesh, with 1/8 inch (3.2 mm) minimum to 1/4 inch (6 mm) maximum openings.

That is the big thing stated in that article - 'cross ventilation'.

That's been in the codes since before I can remember - and it is still good practice.

How many times do you find soffit vents and NO cross ventilation? A lot, right?

Jerry,

Just making the point that soffit vents alone, if there is an unobstructed air path across the under roof space, could provide substantial ventilation - which is not what we would expect given the typical "thermal buoyancy" model on which most such illustrations are based.

That research may be 15 years old, but there are the only comments on the matter I'm aware of based on experiment rather than speculation. Rose has continued to research and write on the tropic of attic ventilation, here's a useful survey of the state of knowledge re: attic venting he co-authored in 2002:

http://www.fpl.fs.fed.us/documnts/pdf2002/rose02a.pdf

I believe the Florida Solar Energy Center (or whatever they are called) did similar experiments years ago too.

I know that all of South Florida is a big believer in soffit venting ONLY.

Gable end vents are too risky for hurricanes, and after having been in many attics after the hurricanes (even years later) you can see the insulation blown in toward the center from the soffit vents.

Down there (not knowing what research was done), it was acknowledged that soffit vents work, but must have cross ventilation, which is the part that people forgot and got screwed up.

I was acknowledging that the research you linked to was still good, old or not, and I was stressing (trying to anyway) that "cross ventilation" is a must for soffit vents to work, and, really, for any ventilation to work well.