Attic soffit issue


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Old 12-07-15, 04:11 PM
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Attic soffit issue

hello. so I recently purchased a new house with a nice large workshop on the property which I am in the middle of fixing up. i want to insulate the attic one one half of it. I know I need to install rafter vents so the insulation doesnt block airflow, however when I went up there it looks like for whatever reason the previous owners sealed it right up intentionally (no insulation whatsoever in the attic yet, only a bit of old r-12 in the walls)

as you can see in the picture, they sealed the area with foam (but of course there are large gaps around doors etc... go figure)

but I need the attic to breath from these to the vents on the peak correct? my question is what is the best way to fix this? can I get a long wood drill bit and put a bunch of 5/32 holes in the wood so it goes from outside through the holes in the wood to the attic rafter vents? or is there a better way to do this?

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Old 12-07-15, 05:01 PM
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The soffit intake and the ridge exhaust are supposed to be roughly equal in the amount of sq. In.of free area. I would suspect that is why some are blocked off. You probably have others that are open. Not every rafter bay gets vented or you would have too much air moving around in your attic. Too much soffit ventilation and it can actually blow a lot of insulation away from the edges of the house.

If you find you need to remove a few, knock them out with a hammer. For goodness sake, don't drill holes in them!
 
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Old 12-08-15, 09:23 AM
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Name:  shop.jpg
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Size:  13.8 KBit looks like its sealed the entire length. at least where I am insulating.

its a 35x70 shop, (forgive my pathetic crayon style picture) the first 35 feet of the shop they "finished" (so the bottom half of the first overhead square picture on the left) and by that I mean they put r12 in the walls, and put a drop ceiling down on 12 feet of each side and left the middle 11 feet much taller, so they could fit larger equipment inside I assume.

so I am closing up the section on the one side under a dropped ceiling (area in pink) and insulating the attic there and the walls around the approx 420sq feet section so I have a insulated work space for the winter. so for the full 35 feet of area they had finished, is sealed with foam like in the pictures on one side only (the green line) no where else in the shop is, but that 35 feet is fully sealed the entire length., past that for the other 35 feet on the back half of the shop its wide open, but that's also outside the section that im finishing

now the back half of the shop, has no drop ceiling, no insulation anywhere, you can see the room rafters and some nails sticking through from installing the roof, the wall joists are open from the inside and its old lumber on the outside for sheathing that you can see between cracks etc almost like a barn. the attics in the finished side are all connected and fairly open to each other. I plan on "sealing" them up enough so birds cant get in the insulated side I am working on with grating or something that allows air to pass through still.

so since everything else is open do I even need to do anything for airflow on the one side (green line) and install rafter vents? and if so as you can see from the picture in the first post they have 2x4s installed between the rafters, are you suggesting I knock out a few entire 2x4s? leaving large holes for airflow? I guess I could grate them as well to keep birds from entering there if need be. , although with them being nailed in I couldn't imagine them being easy to get out from such a precarious position, and if so, how many do I need to remove? at least it sounds like if I do that I wont need as many rafter vents, I was assuming id need one between each joist.
 

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Old 12-08-15, 06:01 PM
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In a residential home the recommended ratio for ventilation is somewhere around 1:150. 1 sq ft of ventilation for every 150 sq ft of attic space. That's a general theory, not a rule. So if you have 420 sq ft of room you are heating and finishing, you theoretically would need < 3 sq ft of ventilation, with half of it being soffit intake, the other half being exhaust. Since 1 sq ft is 12" x 12", you can see that you don't need to open up "every" rafter. Can't do the math for you since I'm not there with a ladder and tape measure.

If you think it is easiest to drill holes, for the sake of putting screen over the holes, I would suggest using a 3" hole saw, and figure out how many holes you need. Otherwise just cut out every third or fourth piece of blocking, whatever it takes to get the right amount of sq ft of intake.

Also keep in mind this isn't a house, its a workshop. I doubt you will be heating and air conditioning it 24/7, 365 days a year. You probably won't need to achieve the comfort level a house requires. As such, there might not be anything wrong with completely ventilating each rafter bay to maximize ventilation if summer heat is what you are trying to alleviate. Insulation alone doesn't completely block the heat. If winter comfort is what you are after, then you certainly don't want too much ventilation. That would just speed heat loss.
 
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Old 12-08-15, 07:12 PM
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Thanks, that helps a lot
 
 

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