Calculating ventilation: Check my numbers please


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Old 06-30-15, 07:53 AM
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Question Calculating ventilation: Check my numbers please

Hey all,
My 1967 rambler needs some roof ventilation. The only upper ventilation come from two gable vents that measure 1'x2'. Lower ventilation come from 9 8"x16" soffit vents. Total attic area is 2384.
I've been trying to figure out which calculation to use; 1:150 or 1:300. Some sites say to use 1:300 if...
  1. I have a vapor barrier (I don't)
  2. Upper vent percentage: Not less than 50 percent and not more than 80 percent. (I intend to do 60/40 where 60% of vents on soffit)
  3. At least 3 feet above eave to roof vents (This would be less than that)

So, it appears I should be using 1:150 BUT then I come across a site that says I should be using 1:300 since my home is so old and less energy efficient.

If I do the 1:150 calculation, the results say I need to put in a ton of roof vents
(2384 / 150) * 144 = 2288.64 NFVA sq in.
Lower Vent NFVA: 60% of 2288.64 = 1373.184
Upper Vent NFVA: 40% of 2288.64 = 915.456
I chose a typical roof vents with a Net-Free Area (Sq. In.) of 50. Not taking into account the gable vents... 915.456 / 50 = 18.3. I could be wrong but that calculates out to a vent every 30 inches. That seems like a lot of roof vents.
On the bottom, soffit's are about 56 nfa so 1373.184 / 56 = 24.52 or about 12 on each side. I currently have 9 total.

So, I think the 1:300 rule seems much more reasonable but since I've gotten mixed messages about how to actually calculate that, I'm a bit confused.
 
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Old 06-30-15, 09:51 AM
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I just want you to know that the people who wrote those rules you have been reading don't have a clue as to where the numbers come from. The 1/300 guideline has no research behind it (citation available) and was found to have been arbitrarily selected back in 1942. It has persisted due to a lack of any other guidance and is now carved in stone and still has only a history of "kind of" working. So, unless an inspector is hovering over you with a micrometer, close to any degree, will be just fine.

Are you on the coast or inland with snow and cold?
Do you have an exposure that will provide a lot of wind for air flow?
Roof vents and not a ridge vent?
Sounds like a very low slope?
Air sealing between house and attic is more important than a vapor barrier.
Paint may have the vapor barrier (vapor diffusion retarder) issue covered.

I would lean towards the 1/150 due to what seems like a low slope. Air flow due to temperature differences is directly related to the difference in height between the roof vents and soffit vents.

I'll re-read your numbers, but wait for answers from questions above.

Bud
 
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Old 07-01-15, 08:20 AM
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Seattle area. I don't think I have to say that it's "moist". 39 inches per year. Average temp is 53 deg F.
There are trees around but I'm not in a forest so wind does hit my home but we get very few windy days.
Yes, roof vents. I don't feel like installing ridge vents.
I'm not sure I know what you mean by this statement, "Air sealing between house and attic is more important than a vapor barrier."
 
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Old 07-01-15, 09:29 AM
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In a very moist climate it is very likely that the attic ventilation will be bringing in more moisture than what is passing from the house to the attic. An option for some is an unvented attic, especially if there are heating and ac ducts up there and/or equipment. Putting all of the ac equipment in the attic is common down south. But, since your attic is already vented and I assume no equipment up there you will have to do the best you can with venting. Unfortunately I don't have a lot of experience with your climate so will have to guess a bit.

Where that moisture presents a problem is when the roof over that attic full of hot humid air is quickly cooled in the evening or sudden change in the weather, cold shower. If the bottom of the roof deck drops below the dew point before the hot humid air can be vented out or diluted by cooler incoming air then there is the potential for condensation.

Although it was the ventilation that brought the humid air in, it will be the ventilation that takes it out. So, move your venting as much as reasonably possible towards the 1/150.

As for avoiding the ridge vent, they are easy to add and they are in the highest position possible, unless you add a cupola.

"Air sealing between house and attic is more important than a vapor barrier." In your climate they do not place a strong emphasis on the need for a vapor barrier, unless the codes are requiring it of you have extremely high moisture sources in the house. Under normal conditions, just the painted drywall will be enough.

If you air condition heavily, the need for a vapor barrier would actually move to the outside of the framing to stop that humidity from reaching the cool inside drywall.

Running long so I'm going to post and let you ask anything I missed.

Bud
 
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Old 07-04-15, 08:28 AM
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Our climate in Washington is temperate. If it's hot, which doesn't happen too often, rain is very rare, so I wouldn't expect that kind of moisture situation. I you get moisture in the attic, it's most likely caused by a leak. Since the Seattle area is near the water, there is some humidity when there is heat but I doubt it is bad as places down south. I've been to Florida a couple times and that was THICK air.
I got onto Google maps just to see what some other houses had as far as the number of vents for a similar sq footage and there were around 5 - 6. I'm not taking into account any gable vents and I can't tell how many soffit vents but who even knows if THAT is correct.

Yes, the furnace duct does extend into the attic but we have no A/C. It would be nice to have but since we don't generally have hot weather, we usually just open the doors and use fans.

Ridge vents seems like a lot more work and expense. Also, that seems like an entirely different calculation.
 
 

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