Soffits - Full vented or Partial?

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Old 06-04-15, 02:18 PM
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Soffits - Full vented or Partial?

Any pros and cons between full vented and partial? i currently have partial - replacing with my siding job. Also shoud your siding go up into soffit area. or cut and butt it off where soffits meet?
 
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Old 06-04-15, 02:49 PM
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You'll need to estimate your total required ventilation area and then estimate your existing high and low NFA (net free area).
The guideline is 1 ft² NFA for every 300 ft² of attic floor, but you must have a well air sealed ceiling and a vapor barrier of some kind. Lacking the vb and air sealing, you double the calculated results. The resulting NFA is then divided half high and half low.

It gets a little more complicated when you have something other than the basic high vent (ridge or gable) and low vent.

Not sure what you have for existing soffit vents and what your high venting is?

Bud
 
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Old 06-04-15, 03:16 PM
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I have a split level ranch, each of the two attics is 25x25 (625sqft). I have open soffits with partial vent soffit covers. I have snow country ridge vents and 20 inch gable vents at each end (total of 3). attic floor is insulated with faced and topped with unfaced to the top of joists. the roof is new with sythetic felt & ice barrier.

Is more vent openings better for icedams, and cooling in the summer? I live in hudson valley new york. What advantage can partial vents provide? im talking about the vinyl or aluminum soffit vent covers...
 
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Old 06-04-15, 03:34 PM
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Somehow we should convert all of that to the number of sq ft. but first, you say two attics??
Here's where I'm headed. A basic ranch will have ridge and soffit vents. It is the height difference between those vents that determines the air pressure that powers the venting. if that attic were connected to another attic with a lower ridge vent, it complicates the resulting air pressure. A picture would be worth a thousand words. http://www.doityourself.com/forum/li...rt-images.html

Now, a ridge vent is usually rated at 18 in² NFA per linear foot. Gable vents are about 50% of their length times width. And those existing soffit vents are (a guess) 50% of whatever you can estimate their length times width.

In most cases they advise having more low venting than high, but I want to be sure all soffit vents act as intake while all higher vents act as exhaust. Are those two attics connected?

Bud
 
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Old 06-04-15, 03:59 PM
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yes the 2 attics at connected at center of home. so total attic space is 1250 sq ft. total linear feet of ridge = 21x2roofs=42. soffits = 100 linear ft x 9 inches (partial vent), each of 3 gable vents = 2.5 sq ft x3. does that help?
 
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Old 06-04-15, 04:22 PM
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Are both ridge vents at the same height and same for all soffit vents?

We need? 1250 divided by 300 = 4.16 ft² of total vent area, but that should be doubled as your ceiling is probably not fully air sealed. That goes fro 95% of the homes out there. So, 8 ft² with half high and half low means you need 4 ft² high and 4 ft² low.

for high venting you have both ridge and gable vents.
42 ft of ridge vent times 18 = 756 in² or divide by 144 and we have (drum roll) 5.25 ft².
Add 7.5 ft² but derate that by 50% for NFA and you have 3.75 ft² for gables.

Total high vent area (NFA) is about 9 ft² or more than twice what you need.

For low venting, I'm not sure what the 9" refers to. If you have 100 linear ft of 9" venting, that is 75 ft² covered with small holes. Even at the lowest derating of 20% you have 15 ft² of NFA, which seems way too high. Do you know what is above those soffit vents. Sometimes they are covering a plywood soffit with some holes cut in them. Those hole then determine the actual vent area.

It is looking like you already have way more than enough ventilation, so that brings us back to why, you mentioned ice dam problems?

Bud
 
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Old 06-04-15, 04:27 PM
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What kind of siding and what kind of soffit you are getting, you didn't say which... aluminum soffit is generally better than vinyl.

..but if you have no wooden soffit or framing above the existing soffit, then the guys who install the soffit will want to use f-channel, which nails back to the wall. It's almost always best for the WRB to be installed as high as possible (to the rafters) then the f-channel should go on top of the WRB, and then the siding butts up under that.
 
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