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Question DetailsAsked on 12/5/2015

Should there be an attic space on a roof over a vaulted ceiling?

Our house is vaulted ceilings (no enclosed sheet rocked attic space) on the main floor. Some roofers suggest we have a 1 inch attic space when installing composition roofing above our vaulted ceilings, whereas others say this will cause mold to grow in there and there should be no vacant space. Which is best for the Pacific Northwest and why?

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2 Answers


Do yourself a favor. Don't trust the local roofers on ceiling and ventilation design work. Sometimes the local home remodeler can take on much more than they are qualified for. So please be careful. Trust such important decisions to Architects and Structural engineers. A wrong decision could literally ruin your home. So don't go super frugal. Get the plan designed from someone in your area that is used to designing for your climate and your specific situation.

Hope this helps.

Exterior Upgrader is owner of

Euro-Tech, Inc.

Servicing much of Illinois and Wisconsin


Answered 4 years ago by ExteriorUpgrader


Controlversial issue, so you may get emphatic recommendations both ways. My take:

As ExteriorUpgrader says, proper DESIGN is necessary for roof/attic systems - and the factors determining which system you should have include roofing type, roof framing type, whether attic/crawlspace penetrations have been blocked off, form or existince of existing "attic" ventilation (eave and ridge vents, gable vents, roof vents, none), type of insulation, what the finish is on the vaulted/cathedral ceiling (butted, lap or T&G wood; drywalled interior, wood panelled, or whatever - and what lies between it and the trusses/rafters (vapor barrier, insulation, craft paper, tar paper, etc.

And of course - the interior and exterior environment - and all these are the type of factors most roofers and insulation contractors generally have a concept of (and sometimes grossly mistaken in their beliefs) but an Architect is generally the correct expert for this type decision though your situation in a damp environment is one of the tougher to make the right call on, so you will definitely get opinions both ways.

In the PNW, because of the high outside humidity and generally cool temperatures, if a limited "attic" space is ventilated, generally it has to be quite well ventilated - extensive eave and ridge venting or energy-inefficient power venting, to prevent buildup of moisture in that area - and should generally NOT have heavy insuylation below it, so there is warming from the underside to promote convection and to warm the incoming air a bit to drop its humidioty below the dew point. This is directly opposite common aittinc insulation and energy efficiency standards, but sometimes it is a tradeoff.

Commonly, this moist outside air situation is avoided by filling the area with closed-cell foam so there is essentially NO air access. Closed cell only, so it cannot absorb moisture - so a lot more expensive than normal insulation types. Good and bad to that - it blocks moisture coming in to the "attic" from either outside or the house, but also prevents evaporation of any moisture that does get into the vaulted ceiling materials or into the roof sheathing in the event of a roof leak (so can promote decay/mold), prevents any roof leaks from being seen and when they do pop up it is commonly a long ways from the actual leakage point in the roof, and prevents inspection so generally when you do discover a problem it is well advanced - usually during a shingle replacement or mold/fungus appearance on the ceiling is the first time you see it. So comes down to a system that might be better technically, but is not as good when you DO have a leak, and prevents inspection.

My preference - unless there is about a foot of "attic" space to allow insulation AND good ventilation (bare minimum 3 inch air space) then I recommend the foam-in-place rigid foam, with vapor barrier under it both to stop airflow from the house into any skips, but also to prevent foam staining/extrusion form the vaulted cailing.

One thing I do NOT recommend in ANY case, though some insulation contractors are pushing this as the supposed "state of the art" - is tight-packed cellulose insulation. BAAAD idea - first it does not totally stop airflow so does not achieve that stated objective, absorbs water heavily in damp environments (I have seen it so damp you can maske "snowballs" of it even without any "leaks" nearby), will (even if treated with borax) support mildew and mold at least after it has aged some, and still settles so you still get minimal (and inadequate) airflow right under the roof sheathing - but not enough for proper ventilation, so condensation and frost can form easily.

Lots more discussions on this matter in the Home > Roofing and Home > Insulation links in Browse Projects, at lower left. Lots of articles and videos (some good, some dead wrong) on the web too - This Old House, Hometimes, Inspectopedia, Homebuilder magazine website, etc.

Answered 4 years ago by LCD

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