Proper way to vent a hip roof


#1

We’re going to have new plywood sheathing and shingles installed on our hip roof. We are basically starting over and can install any type of attic exhaust system we wish. However, perspective roofers have various opinions on how to properly vent a hip roof with soffit intakes. Here are the three options presented:

(a) powered attic fan and install a ridge vent
(b) ridge vent only and install additional soffit intakes
© powered attic fan only with temperature AND humidity control to work in the damp summer/winter.

I would think option (a) would turn to the nearby ridge vent into an intake instead of exhausting the attic air. However, hot air always rises and wants to escape towards cooler air (outside) so perhaps I am over thinking this.

I would think option © would be the most efficient when the fan is actually running. It would probably run quite often in the summer and winter. With a strong enough fan, I probably would not need any more soffit vent cut in.

Option (b) concerns me on days when there is little or no wind. I think on a hot day with zero wind, my attic temp. could become hot and cause my second floor to become hot even with the A/C running.

What should we do? I’m leaning toward option (b).


#2

Power vent + soffit or eave vent.


#3

ditto. what tar monkey said


#4

When the fan is not running, will that be enough static venting for the attic? It seems crazy to have one hole in the roof for the entire attic. Attic floor is about 950 sq. feet. There are twelve soffit vents sized 4"x16" on the hip roof.


#5

Mono,

You mentioned in one of your other posts about only having a very limited amount of soffit vents on your home.

Soffit fresh air intake is more vital to the attic air changes per hour than the exhaust venting and if you want to think of it in per centages, the most ideally recommended scenario is 60% Fresh Air Intake and 40% Hot/Humid Attic Air Exhaust. The perfect world results are not always obtainable though.

Since you are very concerned about the mold growth that has already occured, I will make sure you get the proper documentation in your hands to properly analyze the situation.

For instance, a 4" x 16" under soffit louvre vent only provides a grand total of 28 square inches of net free ventilating air flow intake for each vent. If you have 12 of these intake vents, you would currently have 336 square inches of intake venting. That would only be true though, if none of the vent louvres are clogged by any dust particles, cob webs. or attic insulation and also, if they have not been painted over, which would substantialy reduce the amount of intake NFVA flowage.

Typically, most soffit vents do not have the proper hole cut out in the first place and also have become restricted severely due to the other scenarios commonly discovered in the real world. They only perform to the rated NFVA if they are instaled correctly and Never get clogged. At best, they are usually only 50% efficient from the original rating.

Your upstairs section of your home is 30 foot by 40 foot. Lets do the actual math for the actual required “Minimum” amount of ventilation you will need.

30 x 40 = 1,200 square feet of attic floor space from corner to corner.

1,200 divided by 300 = 4 square Feet of attic ventilation required. This is the “Absolute Minimum” needed, and might need adjustments upwards, depending on the pitch of the roof or if their is no vapor barrier installed under the warm side of the insulation in the attic.

Translate 4 square feet into square inches, since that is how all of the ventilation products are rated. 1 square foot = 12" x 12" = 144 square inches.

4 square feet x 144 square inches = 576 square inches Minimum needed.

If you roof slope is above a 6/12 pitch through 9/12 pitch, then add 20%. If it from a 10/12 pitch to greater than a 12/12 pitch, then add 30%. This is to account for the additional cubic footage of air space in the attic.

If your attic is a steeper roof, like an 8/12, then add 20% for example. 576 x 1.20 = 692 square inches would then be required.

So, you at minimum realistically need 692 square inches.

Ridge line = 10 feet with a Shingle Vent II Ridge Vent with an internal filter and an external baffle providing 18 square inches per foot, so you can achieve 180 square inches of total exhaust venting.

You now need a minimum of 692 sq in - 180 sq in = 512 square inches more. This can be accompliched with continuous Fresh Air Intake Venting, such as the Smart Vent, by DCI Products, which provides 9 square inches per foot of intake. You would need just around 57 feet of intake venting to achieve this total NFVA.

This was with the asssumption that the old existing soffit vents were functioning at full efficiency though.

Another way to achieve the proper amount of air flowage instead of using the 10 feet of ridge vent, would be to use Only a Powered Attic Ventilator fan on the roof top, but definitely not using any additional roof top exhaust vents. They now have Solar Powered units, which provice a CFM rating of 800 cfm. Electric units actually cost more in the short and long term, due to increased energy usage and replacement costs for when they stop functioning.

If you have multiple types and placements of roof top exhaust vent products, they for sure would short circuit the entire flowage and you were NOT overthinking the scenario. I have case study stories analyzing this occurrence, which brings more problems than it solves.

Ed


#6

Ed, thanks for the response. I like the idea of using a power attic vent as the sole source of exhaust. I have one now and although I may not have enough soffit intakes, I notice a huge difference in the second level temp. once the power attic fan turns on (during the summer). It’s great on hot days with no wind. The attic fan kicks on and pulls cooler air into the attic thus cooling my second level.

If I go this route, I think I may have two options.

  1. install additional soffit intakes
  2. keeping existing intakes (clean the screens) and run an attic fan that’s CFM rated higher than necessary to make sure there is enough “pull” into the attic.

Either way, I like the idea of running a solar powered fan.


#7

Remember though, that when a powered unit kicks on, it is also removing the conditioned air from the interior treated occupied spaces, which therefor, substantially increase your utility bills, not necessarily from the PAV, because that can be solar, but through the additional cost of continualy replacing the conditioned air from the interior.

Going with too much “Pull” in the attic is not a good thing, but if the intake ventilation system is supplemented correctly, then it would be pulling from the primary source intended, rather than the interior.

Ed


#8

[quote=“ed the roofer”]Remember though, that when a powered unit kicks on, it is also removing the conditioned air from the interior treated occuo\pied spaces, which therfor, substantially increase your utility bills, not necessarily from the PAV, because that can be solar, but through the additional cost of continualy replacing the conditioned air from the interior.

Going with too much “Pull” in the attic is not a good thing, but if the intake ventilation system is supplemented correctly, then it would be pulling from the primary source intended, rather than the interior.

Ed[/quote]

Good point. I just walked around outside and looked at the soffit vents. There appears to be enough room double up and have an additional twelve soffit vents installed by a roofer. It’s looking more like a PAV and additional soffit vents is the way to go. I’d prefer the PAV be controlled by a humidistat as well so it can run in the winter.


#9

I walked to a nearby subdivision this weekend and took note to the roofs on the homes. Many of these homes had hip roofs and are considered high-end homes built about 8-10 years ago. All the homes with hip roofs had soffit intakes and static roof vents. None of them had power attic fans. One big difference between these roofs and my roof was that these other homes had roof vents on three of the four sides.

I noticed my current roof only has roof vents on the back of the house. However, there is easily room of additional vents on the sides of the hip roof as well. If I had roof vents on three of the four sides, I could achieve somewhere between 400 – 700 sq. inches of venting on the roof depending on which roof vents were selected. It appears roof vents and some additional soffit vents could work as well.


#10

[quote=“ed the roofer”]Mono,

Ridge line = 10 feet with a Shingle Vent II Ridge Vent with an internal filter and an external baffle providing 18 square inches per foot, so you can achieve 180 square inches of total exhaust venting.

You now need a minimum of 692 sq in - 180 sq in = 512 square inches more. This can be accompliched with continuous Fresh Air Intake Venting, such as the Smart Vent, by DCI Products, which provides 9 square inches per foot of intake. You would need just around 57 feet of intake venting to achieve this total NFVA.

This was with the asssumption that the old existing soffit vents were functioning at full efficiency though.

Another way to achieve the proper amount of air flowage instead of using the 10 feet of ridge vent, would be to use Only a Powered Attic Ventilator fan on the roof top, but definitely not using any additional roof top exhaust vents. They now have Solar Powered units, which provice a CFM rating of 800 cfm. Electric units actually cost more in the short and long term, due to increased energy usage and replacement costs for when they stop functioning.

Ed[/quote]

I thought the goal was to have a 50/50 balance of intake and exhaust? If I went with your idea of a 10ft. ridge vent (180 sq. in.) and a Smart Vent (513 sq. in.), that’s like 26% exhaust/74% intake. In fact, with my existing soffit intakes, the ratio is even more unbalanced.

Am I missing something? It seems like the only way to have a 50/50 balance by installing a power attic fan OR as many static roof vents I can fit on all three side of my hip roof.


#11

[quote=“Mono”]
I thought the goal was to have a 50/50 balance of intake and exhaust? If I went with your idea of a 10ft. ridge vent (180 sq. in.) and a Smart Vent (513 sq. in.), that’s like 26% exhaust/74% intake. In fact, with my existing soffit intakes, the ratio is even more unbalanced.

Am I missing something? It seems like the only way to have a 50/50 balance by installing a power attic fan OR as many static roof vents I can fit on all three side of my hip roof.[/quote]

I have a similar situation with a hip roof. There is only 10 feet of ridge for a ridge vent, which would break the “50/50” balance rule of exhaust to intake. Ed, can you elaborate?


#12

[quote=“RD”]

[quote=“Mono”]
I thought the goal was to have a 50/50 balance of intake and exhaust? If I went with your idea of a 10ft. ridge vent (180 sq. in.) and a Smart Vent (513 sq. in.), that’s like 26% exhaust/74% intake. In fact, with my existing soffit intakes, the ratio is even more unbalanced.

Am I missing something? It seems like the only way to have a 50/50 balance by installing a power attic fan OR as many static roof vents I can fit on all three side of my hip roof.[/quote]

I have a similar situation with a hip roof. There is only 10 feet of ridge for a ridge vent, which would break the “50/50” balance rule of exhaust to intake. Ed, can you elaborate?[/quote]

There isn’t much you can do. I ended up have a ridge vent installed and increasing the soffit intake. There is no way to achieve 50/50 in my (or your) situation. The best you can do is increase the cool/fresh air intake by adding soffit intake area and make sure your attic is properly insulated. My attic does get very hot when the sun is beating on it and there is no wind. However, my two story house only has a 2-3 degree temp. variance from upstairs to downstairs when running the A/C. If I had two HVAC units, it would be equal, but I don’t. It’s pretty tough to keep the upstairs the exact same temp. as the downstairs because heat rises.


#13

etr must be a fast typer.

gweedo.