Mobile Home Roof - Under vented?

I have a 1,560 square foot cathedral ceiling double wide mobile home.

using this vent calculator set at 300,
I need 5.2 sq ft x 144 = 748 sq inches
or 374 sq inches on each side?

It is my understanding that the intake and exhaust need to be balanced,
It is 374 total, so that is 187 intake and 187 exhaust, on each side.

There are currently 4 soffit intake vents on each side under the eaves.
Each one measure 9.5 x 24 = 228 x 4 = 912 intake

The exhaust are square vents, 4 on each side, 8 total.
I cannot measure them, see picture above, but I found replacement vents that look like mine.
Galvanized painted black - Overall dimensions: 15 1/2" width x 15 1/2" length x 4" height. 9" width x 9" length outlet with net 50 square inches of ventilation.

They are 50 sq inch x 4 = 200 exhaust.
that almost matches the 187.

But the intake soffits are much larger than the 187.
Could it be that the soffits allow much less air in?
228 x 4 = 912 intake

374 Needed (187 exhaust + 187 intake)
200 exhaust + 912 intake = 1,112

So this does not see like a balanced system?

Install 62 Ft of Ridge venting
60 foot long house x 12 = 1,080 sq inch ( assuming a 1.5 inch gap)
Would this 540 each side?

EDIT - Just looked up current building code for San Jose CA, where I live
and it is 150.

& 6ea O Hagin vents, 3 each side, for In-Takes for proper attic ventilation
O’Hagin Tapered Low-Profile 72.0 sq. in NFVA
72 x 3 = 216, close to the 187.

But why would you need these when you already have more than enough soffit venting?

You have a lot of time on your hands.

So basically with roof venting, you can have a larger intake area than exhaust area.

I’ve read it was about 60-40?

This is going to be my last post at or nearly my last. I’m deciding on my contractor. It has been very educational.
The last time that I spent a lot of time on a discussion forum that was when I was buying a car about 10 years ago.

Ironically I’m going with the roofer salesman who gave me the first bid.
He actually had to change his bid around three or four times to come up with the one that I’m going with now.
If I hadn’t done all this research I probably wouldn’t have used him.
When I called up the referrals that the company supplied, most of the customers were extremely happy with the company. I think the salesman had to go back to the owner to understand how a mobile home is different from a regular home, and modify the bid to reflect that.

It doesn’t matter if one of the vents types is larger since the airflow will be limited by the smallest.

How much of a difference can a roof venting system that is based on the 150 number being improvement over the 300 number?

The most effective venting solution would be to eliminate your exhaust vents and install a power vent with a humidistat.

If you have a low pitch roof there is not a large volume of air in the attic space to be vented.


And make sure you put the right size power vent in. Bigger is not better in this case. Too large of a power fan can cause problems.

1 Like

How do you calculate power vent CFM needed
compare to passive ridge vent square inches?
in a cathedral ceiling?

A power vent won’t work for your situation.

1 Like

small volume of air?
Maybe only 1/2 inch, between the insulation and the roof deck?
My roof only looks like it is about 4-6" thick.

That can’t have much air in it.
There is a board attached to the front that is about 2 " wider for cosmetic reasons?

As an alternative to asphalt, why not do a TPO or similar foam plastic roof?
Half of my older neighbors houses are some type of white roof on mobile homes from the 70s - 80s.

Why exactly won’t a power event work in my situation?

What about replacing the four 50 square inch air hawk vents with a larger 150 square inch vents?
My total intake soffit intake is about 900 square inch.

Vaulted ceilings don’t usually have much common attic space so for power vents to work on vaulted ceilings you need to place one between each rafter.

You should use ridgevent.

On my cathedral ceiling there’s about 12-16?,(just a guess) inches of attic space at the top of the ridge and probably only a few inches at the bottom. I can tell this by looking at the skylight.

My mobile home is very iffy for a ridge vent, and it was also a 312 pitch which other roofers have said is a problem for leaks.
Could I just replace those 50 square inch turtle events with larger 100 to 150 square inch passive vents?