No Roof Ventilation in Stairwell?
Hi there, we’re at the point of installing insulation (5a) and are planning on dense packing cellulose for the roof. We’ll be doing 2″ baffles for ventilation from soffit to ridge. (Hoping to contain the cellulose between the baffles and Insulweb+strapping)
There is a 4ft section in the trusses that required hand framing for the stairwell but we’re not sure how to go about ventilating it? Our inspector said he just wouldn’t worry about that section. We suggested maybe we could drill 1″ holes across every 2×4 and the inspector said “Yeah, you could do that.” So I’m coming here for ideas on what should actually be done. We’re worried that drilling multiple holes in each 2×4 will compromise the integrity of the framing but also worried that the said holes might not even be enough ventilation?
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Replies
WCM4591,
To get enough ventilation to be useful you need more that 1" holes in the horizontal 2"xs.
My suggestion would be to drop the 2"xs by 1 1/2" and run a 2"x4" on the flat at mid-span to form a vent channel.
No overhangs at the gable ends?
We have 12" ladder framed gable overhangs. By dropping the 2x4s by 1 1/2" do you mean sistering/scabbing down?
WCM4591,
I would have used 2"x4"s for the horizontal members dropped down 1 1/2" to form the vent cavity. If it's already built, I would suggest going with Akos' suggestion to cut large slots for venting in the 2"x6"s you used. If you just reduce their depth by 1 1/2" they are still effectively 2"x4"s, which is adequate to span 4 ft.
I'm assuming this is a garage or workshop? If so, moisture mitigation isn't as important, and you won't need as much insulation, but I'm still not clear on the whole roof insulation strategy. If you are using 2" baffles, and the top chord you are insulating is just a 2"x6", you will only end up with about R12 of cellulose. That's not very much.
It's a "residential post frame" build. We will have to scab/sister the trusses down in order to get r-30 (~ 9") that extends over the wall assembly. Also debating possibly just using extra netting or house wrap to reach the 9" instead of extending trusses down. This was the best option we could figure out in our budget range.
Midwest manufacturing (who we bought the trusses from) specified to use 2xs in joist hangers at 24oc to match the chord sizes for that section. I tried attaching the photo in the original post above but it just shows me a piece of paper. But if notching out the slots is the best way to ventilate that section, that's what we'll plan on. Thank you so much!
WCM4591,
That's often the problem with truss layouts. Their concern is to make sure everything stands up, not worry about how it gets insulated or air-sealed.
Speaking of air-sealing... Take the time to think though your roof assembly before going ahead. You need a continuous air-barrier on the underside of the insulation. I'm not sure how you would do that while extending down the cavities with just netting or house-wrap to get more R-value.
I did not know we needed an air barrier under the cellulose actually. The inspector thought the netting would be fine but I want to make sure we're not going to have issues down the road of course. We do plan on finishing the attic with drywall in the future but we won't have those funds any time soon. The attic floor will be completely t&g osb (including the triangle area), will that make any difference?
What's your suggestion on the best air barrier to use in this situation? I've seen a bit about intello plus but not sure if that's any better.
Your inspector is correct if that was a standard roof with an overhang. The ladder framing can be vented at the gable end with soffit vents, you won't even need any vent channel as long as the outside edge of the insulation is exposed.
If the structure is up, the simplest would be to set your circular saw to 1.5" depth make a couple of cuts into the top edge of the framing and knock out a slot for a reasonable vent channel. Assuming those are 2x8 or so, the span there is small enough that left over 2x depth should be good to support. I would not try to dense pack this area, high density batts so you don't have to worry about wind washing and baffles would be much simpler.
The gable end's overhang is just ladder framed with no soffit ventilation unfortunately.
The structure is up but the top chord of the trusses is only 2x6. Would a cut like you mention for the vent channel still be acceptable here?
I would just use spray foam with low gwp
We would love to spray foam but it's unfortunately out of our budget
I would find the money somewhere else in the project. You only need 2-3 inches and that space you are talking about should only put you at like $5-700. Sometimes insulators have minimum of 1000. I think you will find it’s well worth it over having to go back and fix the roof if it rots. But either way, if you go non conventional assembly my suggestion would be to install some monitors against your roof deck to early detect issues