Replies

1. Expert Member
   MALCOLM TAYLOR | Jul 16, 2015 11:51pm | #1

   Erik,
   What does your building code say? Ours mandates the amount and location of the roof ventilation.

2. 
   user-1105327 | Jul 17, 2015 12:12am | #2

   Malcolm,
   You're in BC, right? I don't have the code handy but if my memory is fleek, its 1/150 and "balanced". I would prefer not to have ridge or roof vents as spindrift is a problem here. Also don't want to poke a bunch of holes in the metal roof because an outdated building code mandates it

3. GBA Editor
   Martin Holladay | Jul 17, 2015 05:11am | #3

   Erik,
   If this is a cathedral ceiling rather than a vented attic, the 1/150 rule of thumb doesn't apply.

   An insulated cathedral ceiling can be either vented or unvented, depending on what type of insulation you are installing. If the roof assembly is vented, you definitely need both soffit vents and ridge vents if you want your roof assembly to stay out of trouble.

   For more information on the many ways to build a cathedral ceiling, see How to Build an Insulated Cathedral Ceiling.

4. GBA Editor
   Martin Holladay | Jul 17, 2015 07:08am | #4

   Erik,
   Here is a GBA detail that meets some, but not all, of your criteria. GBA Prime subscribers have access to hundreds of architectural details in our detail library.

   .

   
5. Expert Member
   MALCOLM TAYLOR | Jul 17, 2015 09:56am | #5

   Unfortunately the 1:150 isn't a rule of thumb, it's in the code for cathedral roofs in BC. Vented attics are 1:300. The distribution calls for not less than 25% at the top of the roof.
   Setting aside the code (and good luck with that one!), while better air sealing may have made roof ventilation less important, it hasn't changed the basic physics of the situation. You still need an exit for the moisture at the ridge.
   For decades roofs here have relied on well sealed poly as their air barrier. The code takes this into account. There is no reason to assume that new building practices have reduced the rate of leakage, or the need for the ventilation.
   Assuming you are using snap-lock panels, the installation of the roof is almost identical whether you vent or not. You put vented, rather than unvented, closure strips under the ridge flashing. No cutting holes, no real added work.

6. 
   user-1105327 | Jul 29, 2015 03:27pm | #6

   I have read about some thick wall designers connecting the rain screen vent system to the roof vent system. Would this be considered best practice? (regardless of various code interpretations)

7. GBA Editor
   Martin Holladay | Jul 29, 2015 03:59pm | #7

   Erik,
   Here's what I wrote about the topic in my article titled All About Rainscreens:

   "Is it permissible to vent the top of the rainscreen gaps into the soffit? Opinions differ. Joe Lstiburek says it’s OK, and so do I. The amount of moisture carried by this air isn't enough to cause any problems in your attic. However, builders in British Columbia should note that while the building code in that province does not require rainscreen gaps to have openings at the top, the code stipulates that if there are openings at the top, they can’t be connected to a soffit or attic."

   While I stand by that advice for homes with attics, I would modify the advice for homes with cathedral ceilings. I don't think that it is a good idea to connect the walls' ventilation channels with the ventilation channels that are part of a cathedral ceiling assembly, because cathedral ceilings are much more susceptible to moisture problems than vented attics.

8. 
   user-1105327 | Jul 29, 2015 05:07pm | #8

   Martin, you said

   I don't think that it is a good idea to connect the walls' ventilation channels with the ventilation channels that are part of a cathedral ceiling assembly, because cathedral ceilings are much more susceptible to moisture problems than vented attics.

   Why? Wouldn't connecting the two create more air movement? Or do you worry about introducing moisture that exists behind the rain screen?

9. GBA Editor
   Martin Holladay | Jul 30, 2015 06:18am | #9

   Erik,
   I would be worried about introducing moisture from the walls into the cathedral ceiling assembly. I don't have any data to back up my worry, but I don't think the risk is worth taking.