Insulating and venting a part hip roof
Renovating our attic to make it a living space (zone 5, 2×6 rough cut douglass fir, balloon frame, ~100 y/o), and we followed some basic suggestions from our roofers and carpenters working with us on the project: rafter vent, insulation, vapor barrier, drywall. We were about to put up the drywall, and checked that all is working fine, except, of course, those rafter bays that don’t connect to both a soffit or ridge vent… mold, of course. Cleanup is underway (vinegar, orange oil, borax; considering mold-resistant paint once it’s dry).
The question we have is, is it REALLY OK to drill 1/2″ holes in our rafters to create vents for the rafters on the hip?
Some say no, it weakens the framing (except this doesn’t make sense since we’re expected to do the same running electric).
Some say keep it centered along the width, to maximize strength, and while insulation will cover holes, it’s permeable, and we’re using rock wool, so breathing will happen.
Some say within the 1″ vent area, so there is direct communication from rafter bay to rafter bay and eventually to soffits and ridge vents. Here’s an example of what one contractor did: https://youtu.be/nbuCgjjVngs
Back to our question (is it REALLY OK to drill 1/2″ holes in our rafters to create vents for the rafters on the hip?), we’re thinking, similar to the video, two batches of 4-6 holes, one toward the top and one toward the bottom of the hip rafters, with maybe a few between spaced evenly along the length. But before we start drilling, is this really OK?
Aside from reconstruction, what other options do we have without tearing down the roof?
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Replies
ecopgh,
Unfortunately you won't get sufficient air-movement along the circuitous path it needs to take through each roof cavity, through the holes, to the vents at the peak, to keep the roof safe. Drilling holes is something people in your situation often try as a remedy and it rarely if ever works.
At this point your options are limited. You could try one of the proprietary vents designed for hipped roofs (something like this: https://www.gaf.com/en-us/products/cobra-hip-vent), or go with one of the unvented options in this article for that portion of the roof: https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
ecopgh,
What about a fascia (or possibly soffit depending on the construction) vent and keeping a gap above the insulation up to a ridge vent?
Most of these rafter bays in question (almost all are north facing, on or adjacent to the stairwell wall) reach either a ridge vent, or a soffit, but not both. A couple smaller ones touch neither a ridge vent nor a soffit. Thinking, because this is a balloon frame had plaster and lath walls (before the demo), the gaps between lath and sheathing (1x planks) was enough of a pathway for air to circulate. Also wondering if my timing of this work has anything to do with it. Should this be completed when it's warmer?
Do the hips have hip vents on the top, similar to the ridge vents? I think there have been some discussions on here about similar roof structures as what we are possibly describing: https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
With regards to drilling through structural members, there are requirements spelled out in the IRC with regards to the size and location of holes through structural members and from the sound of what you are describing, that will likely not meet code if you drill through structural roof members, especially near the top of the beam, so I would double or triple check before doing any drilling on anything structural. Typically one can only drill through the middle third of a beam but you'd want to double check before doing that.
Can you add some soffit, ridge, and hip vents where required?
ecopgh,
"wondering if my timing of this work has anything to do with it. Should this be completed when it's warmer?"
Over the course of each year those same conditions will reoccur, and the moisture, which has no effective path to be exhausted, will accumulate in those dead rafter bays and cause damage.