Soffit Vent, Baffle and Windblock Retrofit


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Old 07-20-15, 06:28 PM
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Soffit Vent, Baffle and Windblock Retrofit

I am having new asphalt shingles put on my roof, with an Air Vent Shingle Vent II ridge vent. I am also having existing Masonite siding removed and house wrap and vinyl siding installed. Currently, I have 2-inch continuous soffit vents and gable vents. The gable vents will be closed off However, it appears that my soffit vents are blocked by both blown insulation and by boards that I think function as wind blocks... pictures are attached. My questions are: (1) Am I right that the soffit vents are basically clogged, (2) do I need to have baffles installed, (3) if so, between every rafter, (4) what is the best way to do that, and (5) would it make more sense for my roofing contractor or my siding contractor do that work, or will I need to hire a carpenter? My siding contractor had proposed removing the metal vents in the wood soffit prior to installing new vinyl soffit to maximize air flow... but that was without inspecting the attic first, and prior to my discovery that the soffit vents appear to be mostly blocked - hence my questions about baffles, etc. Any help/advice will be appreciated!
 
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Old 07-20-15, 07:01 PM
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Let's start at the beginning.
The amount of ventilation, the net free area (NFA), you need in the attic is partially dependent upon how well the ceiling (house to attic) is air sealed. There are specific steps required and most existing home never required those measures. If you wish to do the air sealing best to do it before putting the attic and insulation back together.

Without air sealing, your target for NFA is 1 ft² of vent area for every 150 ft² of attic floor. That target amount is NFA and is then divided 1/2 high and 1/2 low. Calculate your target for vent area and divide that high and low to determine how much soffit venting you need.

I recommend continuous venting with baffles and wind protection in every rafter bay. Some baffles will have a lower portion that folds down to block the incoming wind from blowing right through all of the insulation. The baffles will provide a space above them for air to flow and keep the insulation away from the bottom of the roof as well as out of the soffit area.

I see the wood blocks you mention, they should possibly come out as the baffles are installed. Nice summer job.

Keeping or removing the gable vents is optional, they do not interfere with soffit to ridge air flow, but since you are installing new siding and VA is not a heavy snow area, now would be the right time to close them off. But keeping them would be perfectly fine and they do increase the total attic venting.

As a note, some shingles will have a reflective property to reduce the amount of solar energy that warms the roof. The less heat that enters the attic the less you have to deal with.

That will get you started.
Bud
 
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Old 07-20-15, 07:40 PM
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Thank you, and follow-up questions re: ridge vents

Thank you Bud9051, your help is much appreciated!

I have some follow-up questions about my multi-level ridged roof and ridge vents (picture attached). I read that if ridges are more three feet apart in height, only the higher one should be ventilated with a ridge vent. To quote the article:[SIZE=3] "As the wind passes over the baffle, it creates low static pressure, drawing air from the attic. The faster the wind moves over the baffle, the greater the pressure it creates, drawing more air from the attic. Typically, wind moves faster at higher elevations, therefore, the higher ridge will be exposed to higher wind speeds. If the wind speed difference is adequate, the pressure at the higher ridge may be enough to pull air through the lower ridge vent.

If the ridgelines are more than 3 feet apart in height, we recommend that you separate the attics with a piece of plywood or poly sheeting to create two distinct attics. Once this is done, you can ventilate both ridgelines with exhaust vents and balance them with intake vents."

I assume the author meant that if you separate the attics with plywood or poly sheeting, you CAN then install a ridge vent on the lower ridge (right?). When my fourth bedroom was finished over the garage (the lower ridge on my house), a hole was made in the sheathing between the two attic sections so the flexible ductwork could be installed to the bedroom (picture attached). My questions are, (1) if a ridge vent is installed on the lower ridge, would the hole in the sheathing need to be sealed around the flexible ductwork in order to properly compartmentalize the two attic sections and prevent the upper ridge vent from drawing air in from the lower vent, and (2) based on the attached picture, does it look like the two ridges are close enough to cause the problem (it looks like the ridges are about 5 feet apart to me). And finally, a portion of the bedroom wall on the dormer side is slanted, with only insulation between the sheetrock and the interior of the roof deck. Should I have V-Baffles installed between the roof deck sheathing and the insulation on this portion? I have soffit vents here, too, but I doubt they're doing anything. Thanks again for any help and advice!
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Old 07-20-15, 08:45 PM
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Multi-level vents can be fun, both in determining how much each contributes to the overall ventilation and whether they will function as intake or exhaust. The determination for static ventilation (no wind) deals with their relative height and how they are connected. The opening you describe doesn't sound like it goes from attic floor to the ceiling, in which case we would have to calculate the transfer of pressure through the opening. The reality is, that would take far too much guessing to have any faith in the conclusion. An easier approach is to presume the wind is dominant when present, which it always is. When the wind is blowing you will have more than enough ventilation wherever it comes and goes.

When the wind is not blowing and you are relying on the temperature difference between inside and outside the attic, if all soffit vents are at the same height, then you want a lot of soffit vent area to ensure that both high and not so high ridge vents will act as exit points. Instead of the traditional half high and half low divide, you might want 60% low and 40% high. That ratio has other benefits we can't go into at this time. As a note, if you have excess ridge vent area, you can reduce that area to help meet the 60/40 ratio.

I have at times recommended partitioning, but always in relation to static venting, always assuming that wind driven ventilation is dominate when present.

If you could post a link to this quote I will review it further as I have not run into their 3' concerns. Use PM if you don't want to post it.

As for their concerns that the wind might pull some air in through the lower ridge vent, they seem to be forgetting the soffit vents will be affected as well by this pressure change and they should be pouring in lots of desired air.

My conclusion would be, lots of soffit vents, perhaps a reduction in total ridge vent area (would need to see all the numbers), leave the opening between attics as is, open a path to the soffits at each rafter bay and add baffles and wind blocks.

Bud
 
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Old 07-20-15, 09:20 PM
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Thanks again!

Thanks again Bud9051, PM on the way.
 
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Old 07-21-15, 05:39 AM
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I would go with ridge vents on both the short and tall attics and leave those openings where the ducts pass through as is for now. Since the opening between the attics is positioned low, the two spaces do not merge entirely together and thus will function somewhat independently. If in the future you determine they should be separated, which I don't think will happen, that is an easy change. Adding ridge vents would less so.

Redo the soffit venting so you will have continuous vents, baffles to maintain an air channel and keep the insulation out of the soffits, plus it can protect the end of the insulation from wind.

Air sealing and more insulation, along with some work on those flex ducts should go on your "to do" list. Flex ducts are notorious for nasty twists and turns that greatly reduce the air flow.

Thanks for the reference,
Bud
 
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Old 07-21-15, 07:47 PM
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Thanks Bud9051

Thank you Bud9051 - very informative and helpful!
 
 

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