Roof vents

[kevinO]

Hello. Still working up plans for a workshop. I'm spending lots of time thinking and planning to avoid botching the job as I have zero experience with anything like this. BTW, thanks again for your help a while back RE pole barn tips!

I'm drawing up plans for this shop and have a question or two about venting.

I am going to use 2x12 rafters from the side walls to a center beam. Will put 6" or 9" insulation between the 2x12's and then deck the underside. I may eventually heat the shop so I know that I need to vent the ridge. Since there will be no way for air/moisture to travel past the 2x12's, I'm assuming that I need to vent the entire length of the ridge.

We have a local Mueller outlet so I will use R-panels from Mueller for the roof. They have a couple vent options at their web site .

So I need to vent the entire length of the ridge? Wow, that'll be expenseive!

Questions about these (or similar) vents:

Is there a way to butt them up to each other so they don't leak?

Do you attach these vents down by simply screwing them to the R panel, or do you need to also hit a roof purlin underneath?

How big a gap needs to be left at the ridge between the R panel sheets coming up from each side of the roof?

Is there any better (or cheaper) vent options for the ridge of a roof with no attic space?

Thanks for any tips you can provide!

 

[cowboydoc]

They make a product that looks like steel wool that you just roll out on top of the sheeting and the ridgecap sits right on top of this. It's a third the cost of that vent and it has worked great in our barn.

 

[phantom309]

Along these same lines (?), how is it that traditional metal buildings don't have any vents? They place the insulation on top of purlins and then place metal roof directly on top. Is the "directly on top" (no air space) the critical distintion? I've seen plenty of barns/shops put up with insulation directly underneath metal roofs with no vents.

Could keven skip the vent if he totally filled the 12" (approx) area between his ceiling decking and the R panels?

Mark

 

[DavidJ]

Richard is correct, the mesh works great.

You only need to leave about a 1-1/2" gap down from the ridge opening on each side.

As for fastening them; they just screw to the sheeting and you can seal the lapped joints with caulking to prevent leaks.

 

[cowboydoc]

They do have ventilation you just don't see it, at least if the building is done right. Some of them also have the ventilation under the eaves as well. It is usually that mesh type or they have the ventilators on top.

 

[boustany]

You don't have to run the whole ridge. The recommended ration is something like 150:1 ceiling area to vent area, with the vent area split 1/2 roof and 1/2 eves. (so 300:1 roof)

I used ridge vents in my house and I asked the manufacturer if a lower ratio (more roof vent) would be better. They answered, in general, the more vent area the better, so I have continuous ridge and eves vents and it has worked out really well.

I checked the website you posted. That is one really expensive ridge vent. Home Depot sells a similar thing for about $25 for 10 feet, and I used a plastic vent (covered with shingles which was around the same price, but sold in longer rolls.

The problem you may face if you use 2x12 rafters (vs. trusses) if you put insulation between them you could block the ventilation path unless you use those thin styrofoam sheets that keep the insulation away from the roof deck.

Without airflow, your roofing will cook in the summer and you'll get ice dams in the winter. Since you are using metal, neither of these should be problems (maybe the ice dam would be) but its just how its supposed to be done.

 

[mahlers]

As was said earlier, the roll-type ridge vent would be a good way to go. I've used Cobra ridge vent quite a bit, and it works great. It's extremely easy to install, you can barely see that it is installed, and it seems to vent better than other options. It's not too expensive - somewhere around $50 for a 50-foot roll. You can go to GAF's website at GAF.com and do a search for it if you're interested in more info.

 

[bobodu]

Like the man said,you will have to keep the insulation away from the sheeting for airflow.Plus,should you decide not to go with a full length ridgevent,you will need to drill some holes top,middle and bottom through the rafters for cross ventalation.

 

[Cliff_Johns]

Another thing you can do, is to use the little springy metal holders that pop into place between the studs and hold the insulation an inch or two from the roof (just springy pieces of wire, really). Then, about a foot below the ridge line, run some collar ties and use that as your ceiling creating a air exchange area at the peak. This allows the air from any joist bay to flow out anywhere else and improves circulation. You can use regular mushroom vents or the roll kind to give the flow somewhere to go.

Air tends to vent out better on the leeward side, so having exchange between the windward and leeward side joist bays is useful.

Cliff

 

[kevinO]

Thanks mucho for all of the great tips. I like the roll type "breather" material and think that I'll go with that. Should be a huge saving$ compared to the Mueller vents.

Here's a dumb question. What is the reason for needing vents, and why don't the walls need to be vented?

For what it's worth, this shop will not be heated regularly. Just every now and then (at most six to eight hours per day on the weekends).