gable vents covered with insulation? (floor, roofing, Home Depot, window)

[Mary Ann789]

I am a caretaker of a few buildings where I work. One of these buildings was built in the mid 1930s so it's safe to say there is little to no original insulation. About 10 years ago, previous employees rewired the whole building, removed the siding, added foam sheet insulation and resided it with paperboard siding (don't get me started on paperboard siding).

Well, my question is about the gable attic vents in the L-shaped roof. All 3 are all covered on the inside with the sheet insulation.

Should I go up there with a knife, cut the foam sheet away from the gable vents and cover with hardware cloth to keep the critters like bats out of the attic? Within the next month, we are going to be blowing insulation up there. I've been reading other posts and it's sounds like this attic needs some air flow. There aren't any soffit vents. There is one metal roof vent.

[K'ledgeBldr]

Obviously from your description you got some work ahead of you- it's all wrong, and in more ways than one.
Do this to get you started-
Step 1- Calculate attic square footage
How: Multiply length of attic (in feet) times width of attic (in feet)
Example:
30’ x 40’ = 1,200 square feet
Step 2- Calculate NFA (Net Free Area) needed for this attic
by using the “1 in 300” rule
How: Divide attic square footage by 300
Example:
1,200 sq. ft. ÷ 300 = 4 square feet of NFA needed

Step 3 Convert square feet of NFA to square inches
How: Multiply square feet of NFA by 144
Example:
4 sq. ft. x 144 = 576 square inches of NFA needed

Step 4 Split the amount of NFA needed equally between
the intake and the exhaust
How: Divide square inches of NFA needed by 2
Example:
576 sq. in. ÷ 2 = 288 square inches of NFA needed equally for exhaust & intake
Remember:
Always have a balanced ventilation system. In no case should the amount of exhaust ventilation exceed the amount of intake ventilation. From this you can determine if the gable vents are adequate and the number of roof vents (turtlebacks) needed. Since you don't have soffit vents, the gable vents would be considered your intake- the turtlebacks, the exhaust. And a big affirmative on the hardware cloth for the vents.

[Mary Ann789]

Thanks! Just for clarification, using your example, I would need 288 sq inches of intake ventilation (measure of gable vents) and 288 sq inches of exhaust (turtle backs)?

[Mary Ann789]

I got some more specifics and I need opinions on options.


Building is 1084 sq ft. built in 1935 with a cypress roof

No vapor barrier so I used 1:150 instead of 1:300 = 520 sq in for intake and exhaust

Current intake and exhaust
Intake – 1 gable vent = roughly 250 sq in
Exhaust – 2 8in diameter turtlebacks = 100 sq in total

Building needs to increase intake by 270 sq inches and exhaust by 420 sq inches.

Options:
Intake
1. Soffit vents added to porch ceiling. Porch ceiling is just like a giant eave since it’s not over conditioned air (this is a guess on my part). If we add soffit vents to this area and leave the 1 ft gap between the porch “attic” and the main attic, that should allow the air intake to flow freely into attic.
a. 4 8x16 soffit vents = 224 sq in
b. 6 6x16 soffit vents = 250 sq in
c. 9 4x16 soffit vents = 252 sq in

2. The building has 3 total gable ends so we could add 4 round top gable vents instead of the soffits. HD says they have NFA of 50 each.


Exhaust
1. Replace 2 turtlebacks with 3 - 14 inch turbines = 432 sq in (Home Depot website says they have a NFA of 144 sq in each); this doesn’t equal 520 so it's not completely balance but better than what's there now

Do ridge vents work better than turbines? It's possible in the future during a reroof, we could go with that and use the turbines as a temporary fix.

One of the pickles of this is that this building is historical so we have to pass everything through the historian.

So ignoring the historical aspects, what our best route?

[K'ledgeBldr]

I like your approach- sometimes it's impossible to be exactly balanced. And there has been debate about that as well (some say there should be more of one than the other- I can't remember which- as the feud goes. It all sounds like hogwash to me if you have any understanding of fluid dynamics).

I'm not a fan of turbines- mechanical devices eventually break. And they can create vibrations and excessive noise. Keep a passive system. I think ridge vents do an adequate job but, I'm not really a fan of them either. I don't like the way they interrupt the roofline/ridgeline(s). Of course, you will have to contend with the historical aspects, so you maybe somewhat limited as to what will be allowed.

[Coldjensens]

TO your original question, it depends on your insulation principal. Spray on foam insulation is sprayed onto the underside of the froof sheathing, not on top of the drywall. Thus the attic becomes "conditioned space" and you do not want it vented any more than you would want to leave a window open in the winter time. What part of the attic is insulated? Is the insulation on the underside of the roof, on on the floor of the attic (cileing of the room below.). If it is on the underside of the roof, do not open up the vents.

THis is a new method of insulation and is not likely to be used 10 years ago, In fact, it woudl be very difficult to get it through inspection ten years ago. So you probably do not have this, but if you do, do not cut those vents open.

[Mary Ann789]

Nah, it's antique insulation just like the building

[Mary Ann789]

Okay here's what I think is the final question related to this

What happens if we do nothing about the unbalanced and inadequate attic ventilation and add R-37 blown fiberglass insulation to the attic floor?

[K'ledgeBldr]

Well it would probably be the actions taken by homeowners to cool themselves. In other words– they turn on fans, window airconditioners or central air conditioning systems. As the hot weather continues, these appliances run longer and longer putting undo strain on mechanical parts. Homeowners pay for all this added energy consumption in higher utility bills. A less obvious, but equally costly consequence can be the roof itself. Homeowners can’t see it happening, but over time excess attic heat can cause some shingles to distort and deteriorate. The result is premature failure of roofing materials – and perhaps a leaky roof. Once that happens, the cost of a new roof is the least homeowners can expect to pay.
They also may face added costs for structural and interior repairs related to water infiltration.
There are other smaller things but that covers the "big" stuff.