metal roof venting

MarkAndDebbie · May 22, 2007, 05:43:39 AM

I keep reading that metal roofs are self venting.
Like Jimmy here
http://www.countryplans.com/cgi-bin/yabb2/YaBB.pl?num=1146774851

But others
http://www.countryplans.com/kuhn.html
http://www.countryplans.com/nicolaisen.html
put in vents and leave a gap at the top of the insulation.

Purlins would obviously vent out well, but what about sheathing? The plans have vent blocks, but they are written for multiple roof types.

glenn kangiser · May 22, 2007, 09:15:32 AM

The sheets have large ribs every 6 - 8, or 12 inches usually depending on the style. Also a few small ribs. They will vent from bottom to top with only the flats contacting the sheathing or purlins. These would vent under the ridge cap if not sealed with rubber closure strips.

An exception would be pan decking with a standing seam. It pretty well sits flat all across with nearly no space at the standing seam.

May 22, 2007, 04:51:25 PM

So no need to vent the space between the ceiling and roof sheathing? I can seal it up fairly tightly (even if it's not really tight like iceeen - sorry about the spelling)?

glenn kangiser · May 23, 2007, 12:55:26 AM

I don't see a problem with that -- The most likely place for moisture to condense would be vented on the back side of the sheeting - vapor barrier inside should keep inside moisture from getting to the back side of the sheeting. Shouldn't cause problems.

Check with your metal supplier for any other recommendations they may have. If they say something different let us know too.

OldDog · May 23, 2007, 06:29:13 AM

I used foam closures with vent screens made in at the ridge cap.

Venting with Bug screening.

May 23, 2007, 06:10:48 PM

When we re-roofed with 26 ga ribbed metal the roofers cut slots in the OSB sheathing at the ridge and used foam strips with "screening" that looks like coarse 3M sanding pads under the ridge cap.

Daddymem · May 23, 2007, 06:27:02 PM

Any info on those screens? We are a little concerned about wasps getting in under our roof.

MountainDon · May 23, 2007, 06:38:34 PM

My roofing materials came from Metal mart. They sell to anyone with money.

http://www.metalmarts.com/

The material came in strips the width of the panels, with a peel off self adhesive that secured it to the roofing material. The space between the tall ribs are filled with this 3M material... on second thought it's about a medium grade.

The attic air vents out the slots cut in right near the peak and can vent out the ridge cap. I've been up there on a warm day and could feel the hot ait exiting.

PEG688 · May 23, 2007, 09:11:19 PM

QuoteSo no need to vent the space between the ceiling and roof sheathing? I can seal it up fairly tightly (even if it's not really tight like iceeen - sorry about the spelling)?

I've read this post a few times and still can't figger out if your talking about a attic space , or venting between the metal roofing and the plywood sheathing???

There is a difference in what your, or maybe be, :-/ :-? asking :-/

So is it a attic you want to vent ?? Or for some reason are you confusing the two , or at least confusing me

:-/ :-?

May 24, 2007, 08:36:58 PM

QuoteI've read this post a few times and still can't figger out if your talking about a attic space , or venting between the metal roofing and the plywood sheathing???

There is a difference in what your, or maybe be, undecided Huh asking undecided

So is it a attic you want to vent ?? Or for some reason are you confusing the two , or at least confusing me Grin undecided Huh

Let me try again. I am going to build the Victoria as a 16x32 with a half loft (or the 20x30 with a half loft). We are going to do plywood/osb sheets on top of the rafters. Roofing felt on the OSB. Metal on top of the roofing felt. under the rafters we'll do beadboard. Insulation on top of the rafters between the beadboard.

Between the beadboard and the OSB - do I need a gap? I know that the moisture will condense on the underside of the roof, but will I have moisture I need to vent under the OSB. If so, I assume I need both vents in the blocking (like Mickey's - no sofit) as well as some gap at the top of the ridge between the ridgebeam and the OSB.

Thanks - was that better?

scottb · May 24, 2007, 09:01:38 PM

yes, drawing is correct, except you should put something on osb sheathing under metal. If this is 6 inch or more truss consider spray foam insulation.

MarkAndDebbie · May 24, 2007, 09:09:03 PM

Quoteyes, drawing is correct, except you should put something on osb sheathing under metal. If this is 6 inch or more truss consider spray foam insulation.

Like roofing felt - or something more? BTW - I'm in Georgia - basically no snow.

PEG688 · May 24, 2007, 10:42:20 PM

Conventional you would vent above the insulation to the under side of the OSB, you can not conventional insulate and trap that / those rafter bays , air flow will need to from the soffit / bird block / vented anti rotational block up and between the insulation and the roof sheathing and out a ridge vent of some sort.

There is a spray type Iso something or other , Amanda, John , Glenn fill in the something , that you can do trapped bays with , it is not home owner project , IMO the spray stuff .

But you should vent that space ,

Metal , 30 lbs felt , OSB , air space (at least 1" ) (((that can be keep open they sell baffles to insure you don't pull to much insulation in the bay , sure you could still crush that if you tryed ))) , then R-30 , or 38 depending on your local code , if code is in play, then your 4 mil visqueen , then bead brd.

That how I see it,

G/L PEG

BTW nice graphic / puter work 8-)

glenn kangiser · May 24, 2007, 11:22:16 PM

Sorry PEG -- Microsoft did an update on my computer and I was locked up for a bit --- Hope they are done with that.

Icynene is Okie BoB's favorite insulation - has to be sprayed on by pro's.

May 25, 2007, 05:18:03 PM

Ribbed metal vents self. Humid air will rot wood, spray insulation has such high r value, heat will not come through. This quote" High density spray foam insulation due to its impermeability properties can be installed directly under roof decks in any climate zone without any additional provision for vapor diffusion resistance - including Climate Zones 5 or higher (see Map 1)." is from this page http://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation/view?searchterm=metal%20roof here is your climate http://www.buildingscience.com/documents/profiles/designs-that-work-mixed-humid-climate-atlanta-profile/view?searchterm=metal%20roof here is insulation link; check out both semi-rigid and rigid biobased 2000 http://www.biobased.net/products/index.php there are other foam brands just search.

PEG688 · May 25, 2007, 05:54:32 PM

Ya so what your point?? Is that what markanddebbie asked?? I'm still not sure that roof detail is codable , it appears the rafter is to small to provide for R-30 plus space and it appears to be batt insulation in the photo.

Yes the ribs would vent between the metal and the sheathing , but they would not vent the rafter bay space , and humid air , if it humid enought / sets up a dew point would in time allow mold to grow, once that starts rot is not far behind with the $hitty wood in use today.

scottb · May 25, 2007, 09:01:36 PM

My point is with spray foam insulation, there is no rafter bay space to vent. Yes rafter has to be large enough space for r-value. That building science document 'attic vent' had pictures of not venting attic and venting attic. depends on materials.

PEG688 · May 25, 2007, 09:57:53 PM

QuoteMy point is with spray foam insulation, there is no rafter bay space to vent. Yes rafter has to be large enough space for r-value. That building science document 'attic vent' had pictures of not venting attic and venting attic. depends on materials.

That what I kinda thought you meant, and that is right we've used it two or three times in remodels where nuttin else would even come close . It's spendy and not DIY friendly around here no one rents that equipment , YMMV.