Slate roof and ventilation channel: Does the roof need ridge vents?
I have an old slate roof, want to insulate between the rafters in the conditioned attic and provide a ventilation channel of about 1.5 inches depth. My roof does not currently have any ridge vent or other ventilation outlets at the top of the roof. Does that mean that the ventilation chute is useless for venting away moist air? Or is it still acting its purpose, because the roof is made of slate ( on full board ) which is air permeable enough that the moist air will not condensate on the underside of the wood during the winter, but essentially finds its way out through any small gaps between the wooden board and the slate shingles? I am trying to determine whether installing ridge vents or similar ways for the air to escape from the ventilation channel is a must with this kind of roof or an unnecessary expense given that the air can permeate between slate shingles. Thanks in advance for anyone offering their views ! (I asked a similar question here (https://www.greenbuildingadvisor.com/community/forum/general-questions/97421/attic-insulation-and-slate-roof), but did not hear a response, so I apologize for posting again.
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Describe the wood decking in greater detail.
You say it is a conditioned attic. How is it currently insulated and tied into your hvac system? Or did you intend to say that the attic is unconditioned?
Matthias,
In my judgment, you can skip the ridge vent. As you noted, slate roofs tend to allow air movement through the slates. Moreover, slates are vapor-permeable, so moisture problems at the sheathing layer are highly unlikely.
If a local code inspector is involved, however, the code official may not agree with me -- so keep that in mind.
For more information on this issue, see these three articles:
Site-Built Ventilation Baffles for Roofs
How to Build an Insulated Cathedral Ceiling
All About Attic Venting
@Dana: thanks. I attach a picture. The wooden board is of irregular width, probably ten inches on average. There are noticeable gaps between boards, but the width of the gaps are also irregular.
@ Steve: thanks. There used to air handler , condensor and ductwork in poorly insulated knee wall space. I took that out and put a minisplit system into the house, leaving the attic without a condensor, aka conditioning it from lower levels of the house, focusing on getting things airtight and well insulated in the attic that we do no use much. Current insulation is fiberbatts, then drywall, simple styrofoam baffles .... The fiberbatts are doing a poor job at insulating, because of substantial airflow ...
@ Martin: Thanks ! That is what I was hoping for
@Dan: thanks. Creating a ridge vent is expensive in my high cost area ( D.C.) so if I can do without a a copper cap that would help my budget. Thanks for the reminder about the underlayment.
Just need to make sure that future roofers know not to put down bituthane underlayment or something else vapor-impermeable.
Alternately, it's not that hard to make a copper cap that integrates ventilation.