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Old 10-04-2015, 10:52 PM   #1
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Attic Insulation / Vent


If modern builders always install the plastic/styrofoam vent chutes designed to staple where the truss rests on the top plate, doesn't this cause more intake ventilation than exhaust?

Or is this only the process because the true factor is determined by the square inches of soffit venting?

I don't see a faster/more economical way to allow venting but also prevent blown in insulation from closing the gap.
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Old 10-05-2015, 05:54 AM   #2
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Not at all.

Venting at the soffit depends, as you referenced, on the NFA rating of the soffit and the exposure.

Most homes, given the layout, should be heavy on the soffit/intake provided they are open and unblocked by attic insulation.
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Old 10-05-2015, 08:19 AM   #3
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If I have every truss space open for venting I calculated it would be 3-4 times more nfa than my ridge vent. I assume that wont matter too much as long as my soffit nfa matches the ridge vent nfa. Yes?

It seems silly to leave every gap open if I don't need to. It cost 2 to 3 bucks per plastic vent chute to keep insulation from blocking the gap.
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Old 10-05-2015, 08:35 AM   #4
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$2-3 per chute?

You doing this or having someone else do it?

You cannot have insulation in that area whether or not you need the venting.
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Old 10-05-2015, 10:05 AM   #5
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Myself, the cheap ones are $1 or so but I feel they will deteriorate. I have approximately 90 cavities that will need filled. I realize air needs to flow, but if I don't have a vented soffit in the vicinity of the baffle, I wondered if it would be more efficient to not use a baffle in that location. If I don't have the need for venting there, I can fill insulation further in that area. I do have a raised heel truss so it's not as much of a concern for the extra insulation right at the kneewall.

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Old 10-06-2015, 02:02 PM   #6
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"If modern builders always install the plastic/styrofoam vent chutes designed to staple where the truss rests on the top plate, doesn't this cause more intake ventilation than exhaust?"---------------- no, it is the size of the actual soffit opening minus the screening/louvers for your NFVA per unit.
The system should be balanced- 50/50, ---- not 60/40 or 70/30 as this leads to greater convective loops (and more moisture deposited) in the summertime. With more soffit inflow (cooler air) pushing the warmer (from room (house-stack effect) below/roof solar gain) air out the top exhaust, it would create a bottle-neck effect in the convective loop swirling around up there, getting even larger, robbing you of energy dollars spent.

As the baffles are to stop insulation from clogging the intake opening/help direct air stream along roof sheathing and stop wind-washing of the floor insulation; you could substitute rolled fiberglass in a plastic bag, thin plywood strips of acceptable height (though this may pickup incoming moisture and mildew) or rigid foamboard (optimum for reduced clearance over sloped areas yet meet code minimum energy rating- not in your case with raised heels). If the heel-height is enough with only blown-in, just add some housewrap stapled under the top chord framing and forget the baffles. You really want/need the vents in every rafter/truss bay to wash all the slope areas against moisture.

Gary
PS. may want to cover the blown (if FG) with 2-3" cellulose or housewrap to prevent convective loops
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Last edited by Gary in WA; 10-06-2015 at 02:05 PM. Reason: sp
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Old 10-07-2015, 08:14 AM   #7
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Awesome, thanks. I did find out a larger concern is keeping critters out of the soffit area. I guess it is common for them to get into the soffit and thru a baffle and nest in the attic. I'll adjust venting nfa with the amount of vented soffit.
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Old 10-07-2015, 09:06 AM   #8
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I see a problem with some of the venting theory. Cold air doesn't push warm air up but only replaces that warm air that exits by convection so ridge to soffit ratio doesn't matter.
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Old 10-07-2015, 12:55 PM   #9
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Quote:
Originally Posted by SeniorSitizen View Post
I see a problem with some of the venting theory. Cold air doesn't push warm air up but only replaces that warm air that exits by convection so ridge to soffit ratio doesn't matter.

Six years ago I thought the same thing... researching the subject further I found: #2 and #7 here; http://learn-science.20m.com/tmyths.htm#myth2

Read the concluding Q&A here; http://usatoday30.usatoday.com/tech/...derquest_x.htm

Gary
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Old 10-07-2015, 07:58 PM   #10
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Per the home made housewrap baffle....would that not deteriorate over the years and cause a future headache over a plastic baffle? I like the idea of saving on funds but wonder if it may bite me in the future.
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Old 10-07-2015, 11:21 PM   #11
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No idea what would shorten its life in your attic... not moisture, air, staples rusting (use galvanized), temperature- within limits... why would it fail? Conduction energy loss would be slightly more than a thin foamboard one... you only need them at the bottom to prevent wind-washing- now I'm repeating...; http://www.philwrites.com/AW_Manual_12_1_all_LR.pdf

Someone else also wrote about it a few years after I used it on this forum; http://www.finehomebuilding.com/item...lation-baffles

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Old 10-19-2015, 07:05 AM   #12
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Thanks for advice. It seemed to be dead end with me.
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