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I have a 1920s building with a gable roof that has a 10' peak. I'm going to finish the space and had an engineer detail the specs for structural safety, but I'm stuck on the appropriate way to provide venting with what I have.

The space has 8 static vents (not a ridge vent, unfortunately) that are about halfway down the roof, but no soffit intakes - there's simply a solid board under the eaves of the house on all sides (no visible openings). The previous owner had blown-in insulation put in about 20 years ago, but other than that there's no insulation.

I'd like to drywall all the way up to the peak (around the collars that I need to install, which will remain visible) and plan on insulating all of the cavities in the rafters, including those behind the knee walls that I'll install.

I'm doubting there's really any "venting" going on in the attic as-is, and I know it isn't as simple as installing baffles since there's nowhere for air to flow. How should I go about this?

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  • Welcome to Home Improvement. Is this going to be commercial or residential space? Where in the world are you? How thick are your rafters? How much insulation do you need/plan on? Please edit as much detail into your question as possible to get the best answer you can. When you've done that, take the tour and browse the help center to learn how you can get the most out of this site while you're waiting for an answer. – FreeMan Jul 21 '20 at 22:53
  • This is a residential space in Michigan. The engineer says that the existing 2x6 rafters (24" OC) will be fine to hold the required R19 insulation that I need - planning on using batting. – Sean Jul 22 '20 at 0:25
  • That's good info, Sean. Edit it into your question so others can find it easily. Not everybody reads all the comments. – FreeMan Jul 22 '20 at 0:54
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The Code says Attic Ventilation shall be “cross ventilation “ and shall be 1/150th the area that is to be vented. (See ICC R806 Vents.)

However, there is one exception: The attic ventilation can be reduced to 1/300th the area to be vented provided one of the following items are met:

  1. a Class I or II vapor retarder is installed on the “warm-in-winter” side of the ceiling, or

  2. not less than 40% and not more than 50% of the ventilation is located in the upper portion of the attic. The upper portion is defined as: “Upper ventilation shall not be lower than 3’ below the ridge or highest point of the roof. “ (See ICC R806.1.2.)

Currently your vents satisfy the “cross-ventilation requirement, but when you cover the roof rafters you will no longer have cross-ventilation.

Also, the Code says you shall maintain a 1” clearance between attic insulation and framing.

Where you live, R-19 batt insulation is grossly inadequate. (In fact, the 6” batt insulation will completely fill up the 2x6 joist space and you cannot vent it.) I’d use R-30 insulation (10” thick), but you’ll need to sister 2x12’s to the existing 2x6’s so you’ll have about 1 1/4” air space above the R-30 insulation. You’ll also need to provide soffit vents and a ridge vent (or provide a means for the air to cross the ridge.)

So, you can see there is clearly a benefit to having ridge vents, soffit vents, etc. , but in a proportion that allows COMPLETE CROSS-VENTILATION above your new insulation.

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  • and this is why location and rafter size are important! – FreeMan Jul 22 '20 at 12:41

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