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How much should a roof breath?

GregoryPaolini's picture

I'm in the process of insulating the roof/ceiling of my woodworking shop - And while I know a bit about building, Insulation is not one of my strong points.

So, here's a bit of background info on the project:  My shop is in Western North Carolina.  Summer temps get into the low to mid 90's, winter temps can drop into single digits, but generally remain above freezing.  The sun here is intense!  The building is 24x32, with scissor trusses on half of the building, and attic trusses on the other half, space on 24" centers.  The roof has asphault/fiberglass shingles, and a full length ridge vent.

I'm pricing the job to have a pro do it - BUT, if I attempt to do the job myself, I'm concerned with how much my roof should breath.  When I did the siding & soffits, I used vented soffit for the whole project.  If I install the insulation, I know I'll need some baffles to provide inlet airflow, and not block off the soffit vents.  My big question is, should I put baffles between every truss, or every other, or every third, etc, etc, etc...

Thanks in advance for any input - You guys/gals have always helped me out in the past - And If I can return the favor with providing some answers to woodworking questions, I'm your man



Gregory Paolini

Custom Furniture, Cabinetry, and Woodworking Instruction

Gregory Paolini

Custom Furniture, Cabinetry, and Woodworking Instruction

(post #116166, reply #1 of 6)

Glad to hear you, Greg.

That topic you approach has had many varied opinions revealed over the years of which it has been debated, at times in fairly loud tones. :o)

A wealth of knowledge can be gained by familiarizing yourself with the advanced search feature found in the upper left corner of the homepage

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beneath the FineHomebuilding blue masthead.

Placing keywords in the search engine such as 'roof venting' will bring a slew of past posts regarding the topic.


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- from Dinosaur, 10/12/2006

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Edited 9/24/2009 9:52 am ET by rez


(post #116166, reply #2 of 6)

Convention is to vent between every rafter (or truss).  I've never seen it done otherwise.  Seems as if you don't, you could have pretty severe surface temperature differentials across the surface of the roof (vented areas being a bit cooler).  Don't really know if this matters though.

(post #116166, reply #3 of 6)

If you've got the vented soffit then you should put baffles in every bay, to the extent practical. The more the merrier.

Note that the baffles don't have to be the plastic things you buy at HD -- anything that will create the space (without allowing insulation to leak by) is OK. I used old corrugated plastic political yard signs, fastened to the bottom of the top chord of the truss.

If for some reason venting every bay isn't practical (eg, because the ceiling is already up and it's a PITA), you should attempt to distribute the venting evenly, and get as far into the corners a possible.

As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #116166, reply #4 of 6)

As REZ says, vent or no vent has lots of past discussions on it. It can be done either way. Non venting I believe relies on the concept of not allowing penetrations of the ceiling to transfer moisture into the 'attic' (or roof rafter space). That being said and being a shop (dry shop?), you may not have issues like a house has ... e.g. lots of J boxes, recessed lights, plumbing penetrations, ductwork, etc. You may have some J-boxes, I suppose, unless you work in the dark like I do ;). You could seal the boxes or run surface wiremold for the few ceiling mounted lights, etc. That's the way I've always understood the 'hot roof' approach (not sure if that is what they call it).

If you choose the conventional vent option, I'd vent all rafter bays using whatever method seems to work. You could make your own cardboard baffles w/ e.g. refrigerator boxes, etc. and a template. The scissor trusse area should be less hassle as you only do the narrow spots near the eave. If the insulation is the right thickness, you may not have to baffle the whole length of the rafter.

If you e.g. use 12" insulation in a 12" deep space, then you need the added help. You might take some heavy twine and crisscross on the bottom of the top chord of the truss. Easy to do and install maybe and be pretty effective. I've seen this done for supporting insulation under floors when you can't use the paper back flange of the insulation to support it. If the chord is e.g. 2x4 on edge, you have leeway for insulation to bulge into that space a bit w/out being detrimental to air flow.

When I insulated my house, I found my contracted bid to be about as much as my cost of the materials itself ... why would I DIY ... I HATE installing fiberglass insulation. While I like cutting it and tucking it in, the itch and the dust just drive me crazy! I gladly paid my contractor. Since I was there doing other things when they did it, I was able to nudge them to do a little better job ... but they were already doing pretty well. Many insulators tend to be sloppy in cutting and installing, so you don't get a tidy job. Why tidy? Who cares, you don't see it. But it affects thermal performance. Cuts +14" minus 0. Tuck it to fill the cavity and fluff out to level the inside surface.

There ain't NO free lunch. Not no how, not no where!

(post #116166, reply #5 of 6)

Thanks to all for the quick answers, and for pointing me in the right direction to find answers that are already out there.  I feel a little more knowledgable and confident about the project.  Not sure If I'll do it, or sub it, but atleaset I know a bit more.

I'm off to the workshop!



Gregory Paolini

Custom Furniture, Cabinetry, and Woodworking Instruction

Gregory Paolini

Custom Furniture, Cabinetry, and Woodworking Instruction

(post #116166, reply #6 of 6)