Cold vented attic - to membrane or not

Hi folks.

I have a cold, vented attic in my SB plans.
The one where wind "howls" thru it.

The insulation is on the floor and there's no traffic as it's uninhabited.
Just a small hatch from the outside for inspection / servicing.

The actual roof is rafters with battens over them and metal roof above (no sheathing).
Something roughly like this:


There are soffit vents plus ridge vent for the entire length of the gable roof.
The ridge vent is some form of membrane that's permeable to air but not to water / snow.


Now, it seems over here people can't / don't know how to build anything unless it's engulfed in plastic sheets / membranes.

It was recommended to me that i place a membrane of sorts above the rafters and below the roof supporting battens /counterbattens.
I asked why since i was curious (not really) on the arguments.
I can find some pro's and cons.

	Pros	Cons
Without membrane	Good air circulation all over the attic	If something happens to the ridge vent or the metal roof, water can drop to the insulation below - bad
	If the above negative happens, you'll know very soon due to the ceiling wet spot (unless you have a waterproof layer there)
	Inspecting everything is easy as everything is in sight
With membrane	In case something happens to the roof or ridge vent, water will flow above the membrane to the gutter	You can't inspect what's going on above the membrane as it's not transparent
		Less air circulation in the attic, most between roof and membrane

Any experience / ideas about this ?

Seems to me, since your effective roof for insulation and holding warm air in your house is the floor of the attic, then I don't see why a membrane is needed in the outer roof. Are there old roofs standing fine after 100 years with a similar structure to yours? That might be a good argument for no membrane.

Hey Rebecca,

Well, ALL roofs traditionally built (meaning older than cca. 20 years) are this way.
It's just that they were done in ceramic roof tile with cemented ridge caps.
Ventilation took place thru the gaps between tiles.
For these roofs, rain penetration potential was higher but they held pretty well.
They have very low water penetration at the ridge though (ridge cap was cemented).

For a metal roof, chances of water getting thru the seams are very slim.
However, water could get thru the ridge vent, since i'm using a cold, vented roof.
The ridge vent band i showed i think has very little chances of water / snow getting in but it also has less ventilation ability.
The ridge vent below is considered HI-END but i'm not sure wind driven water / snow can't get thru.
You'd have to trust the brand and marketing mumbo jumbo.
I don't have access to track record data of this band in similar climates.

The membrane looks to me like a new product on the market because someone (or a group) wants a piece of the market share. That's what things like that appear to me to be, just like new garden tools for "easier weeding". Great examples of new products which aren't any better than what they're trying to replace and are completely unnecessary are the tools advertised on tv during thanksgiving and christmas targeted at women as a gift for their man. Like the "screwdriver" with gears in it and you hold the shank in one hand and turn the handle back and forth with the other hand to drive a screw. It's reinventing the mousetrap. Someone had a clever idea, and wanted to make money but it's not at all practical. I'm willing to bet those screwdrivers with gears in them sit in the back of the toolbox and guys still reach for the good old fashioned traditional screwdriver. People have been building roofs and keeping water out and air circulating successfully for millennia. A part of industrial western "progress" is new inventions which really don't work better than what has been around a while and is tried and tested and proven. I hate it when I see what appears to me as capitalists preying on unsuspecting consumers who don't know any better. Sorry if that seemed like a rant. That's my two cents.

My superstitious fear is that a lot of these membrane products are just a means to trap moisture in places were it will cause rot, mold, and ultimately an unhealthy decaying house.

James, Tyler, are you referring to the "ridge vent" band or the membrane to be put between rafters and roof covering ?

If it's the latter, i agree 100% with you.
It only makes sense for modern construction where designers don't quite understand the nature of things and builders almost never.

In my situation, i'm the designer and builder.
After talking some to the architect and structural engineers, i figured they are on a different planet than reality.
They just know one or two things (or they think they know it) and use those as a solution to everything.
They take care of the "looks" and of the structural integrity (as these seem to be the ONLY thing that REALLY matter around here) while i'll make sure that the darn thing actually works, and if i manage to pull it off, i want it to work spectacularly well.

So, to put matters into perspective.
I have a gable roof, 3.2/12 (15 deg) on one side and 3.9/12 (18deg) on the other.
Area is about 1500 sqft (150sqm) on one side and 1200 sqft (110sqm) on the other.
Covering is corrugated metal roof in a light color.
I wanted metal since it works at these low pitches, is lighter, and can come in very light colors to decrease attic heat buildup during summer.

For this much surface area i need a ridge vent with lots of venting area to have proper ventilation.
And at these low pitches, wind driven water and snow can be a problem so it must resist those too.
Most ridge vents spec a minimum pitch of 3/12 or 15deg so i'm covered.
It's the venting area provided and weather resistance i'm concerned about.

Coming back to those membranes, i think the major reason to use them, besides "you have to have one, what do you mean you don't want it ?" mantra, is that you can be less careful to the details (which is how the building industry works unfortunately) and in case something happens to the roof you can just say, heck, the membrane will direct that water to the gutter anyway so i don't care.

BUT, and there's a big BUT.
As you said, what about moisture entrapment ?
I visited a large shop and they had for this purpose some pretty expensive semi-permeable membranes.
My question was, why use them if they're not needed and can cause problems ?
I did not expect an answer.

So, back to the story.
Only reason i would use this membrane is for "backup", in case rain / snow gets thru the ridge vent.
But i get to keep all pieces in case something breaks due to moisture and i don't want that.
I prefer to use a proper solution which is : a good ridge vent.

And this is where i get into trouble big time.
Available are only the 2 types i showed before, both from reputable manufacturers but at different "performance" levels.
These may not be enough and i might need to make a ridge vent myself.
Argh, this is what happens when you care and want to make things right (unlike most contractors, architects, etc.)

Hi everybody.

Well, i'm a roofer by trade. In France tho.

I would say, use a membrane, something like tar paper may be . Tar felt. These are the most unprocessed membranes i know off, besides waxed wood wool. Normally, a roof of this kind is vented outside and inside, bellow the tiles and through battens and counter battens. And if the rafters are visible, i don't see any issue with the membrane. I don't do rooves like this. Space is at à premium here. So it's high end vapour permeable barriers, like delta foxx or permo forte rs for me.  But if i had to do one. I would put a membrane, because it's a second protection, belt and braces. Plus it doesn't give any trouble. So i would use it.

Hi Ionel,

Some things to consider:

1. Wind howling through your attic may indicate excessive air changes.  Too many air changes will likely result in excessive air-borne moisture migration into the space.  To strike a balance between adequate ventilation and minimal moisture migration, use a 1/200 ratio (free vent/surface area of attic floor) in zone 5. (different zones, exposures, and roofing systems require different ratios.) The roof vent shown is manufactured for tile roofs; a metal roof will likely get too hot and cause the butyl tape at the edge of the exterior membrane apron to fail.  The most reliable ridge vents for metal roofs are mechanically installed without relying on sealants, adhesives or tapes.

2. Air leakage through building components is a major contributor to residential moisture migration, energy loss and HVAC equipment failure.  Attic spaces are the major contributor to residential building air leakage. Over/under venting attic spaces have substantial effects on building health and occupant comfort and health due to the above.  I have tested more than 100 residential ridge vent installations on metal, asphalt shingle and tile roofs, and moisture leakage/migration via wind blown rain or snow through a ridge vent that has been installed per the manufacturer's instructions has never been an issue.  The issue is almost always over/under ventilating an unoccupied attic space.

3. Draped membrane installed over rafters under battens in an attempt to create a continuous waterproof membrane results in no effect on testing I have performed on tile roofs.  This is likely because it provides no benefit or detriment; a membrane that has been punctured hundreds of times by the fasteners securing the battens is no longer a continuous membrane in any case.  I have never seen it installed under a metal roof so have no data on that.  

Brett

Brett,

1. The house is not built yet, i'm at the foundation stage atm.
I can design the system for 1/200 ratio.
Can you give a link that explains what "zones, exposures, and roofing systems require different ratios" means ?
I figured i would have to have a custom built metal ridge vent since what's available readily doesn't fit the purpose.

2. Got it. If you have any info on how to build a proper ridge vent for a metal roof please put it here.

3. Well, the membrane here has no other purpose except to act as a "backup" roof.
If something happens to the metal roof (punctured, etc), water/snow will get in.
If there's a membrane in place it will stop here and drain outside.

So the whole thing with any membrane in this scenario is that it's a backup roof.
I can think of pros and cons for having one.

Hi Ionel,

Zone refers to the average climate data available for the area in which one builds or lives.  For example, climate data would include average seasonal temperatures, humidity, wind and rain/snow moisture as well as elevation.  Exposure is specific to a building in this case, and refers to the amount sun, wind, and moisture the building will be exposed to within its microclimate.  Building system refers to the physical components of a building and how they work together to provide the occupant(s) a sustainable, durable, comfortable, and delightful shelter.

For example, a house built on the Baltic coast will be subject to a very different climate than one built in La Mancha because they are in different climactic zones; one house built on the Baltic coast in the mountains will have a far different exposure or microclimate than one built in a sheltered valley on the same coast.  A straw bale house built in the mountains on the Baltic coast will require the use of a different building system than a house built in La Mancha.

I recommend the SMACNA sheet metal guide as a reference for all sheet metal work.

Thanks Brett for the info.

Maybe it's my cultural heritage or way of thinking but i can't gen anything out of the SMACNA website ...
Google also did not help ...