Attic Air Quality

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BEFORE THE ARCHITECT – HOME DESIGNING BACKGROUND – UNIQUE HOME DESIGNING ARTICLES

 HOME BUILDING PROBLEMS - HOME ATTIC AIR FLOW, ATTIC VENTING
By Before The Architect  Copyright 2003-2007 Before The Architect



 

Down the years, a few things get clear even to us even in the bad times.  Yep, even in the bad times.  What's gotten clear in the bad times?  Answer:  Some few folks are just not worth working with and vice versa.  Simply, some people will be better served by others. Before The Architect

hOME BUILDING PROBLEMS - HOME ATTIC AIR QUALITY - ATTIC AIR FLOW AND VENTING

ATTIC AIR VENTS

INTRODUCTION
bulletThis home designer is tough on home attic air quality and attic venting.  In this home designer’s opinion, venting for home attic air quality and air flow shall not be less than 1 square foot "net free" vent area to 150 square feet of vented surface area, regardless of the area location or physically proximate form of vapor or moisture barrier

Comment:  Passive air vent is a tougher and tougher issue with which to deal.  This home designer has not succumbed to regard attic space as interior to conditioned space.  If a client insists on engaging an unvented attic space, or ventless air, the relevant annotation in the home plan from this home designing shop will include ‘done by others.’  This dog's too old to abide by ventless air in an attic.

bulletAn underlying theme to this section of standards has been that while you cannot have too much ventilation to abate temperature and moisture, you can have too little.
 
bulletWhere there are high exhaust  attic vents and low intake attic vents, net free area shall apply separately as follows before adjustments
bulletCalculate square feet of total surface area to ventilate
bulletDivide that total by 150
bulletApply that quotient to low ventilation
bulletApply half that quotient to high ventilation

Comment: Note that these calculations are often satisfied by ridge vents, preferably baffled, high with 1 linear inch gaps to sheathing either side of ridge board and doubled vent strips at the soffits. See vent manufacturers’ net free vent area statements about their products and discount those statements by not less than 15%. Apply only top of the line vent products, e.g., Headrick, Certain Teed, and GAF. See http://headrick.net/product_compare_alum.htm for further, albeit conveniently self-serving of the authors. 

bulletWhere there are only high attic vents, net free area shall apply as follows before adjustments
bulletCalculate square feet of total surface area to ventilate
bulletDivide that total by 150

Comment: It shall be understood that soffit or eave vents may be doubled or even tripled to effect adequate intake. 

bulletNet free vent area shall be the area of a vent opening adjusted by the following factors
bullet1/4 linear inch-1/8 linear inch screen alone or louvers alone= 1.25
bullet1/16 linear inches screen=2.00
bulletLouvers and 1/4 linear inch-1/8 linear inches screen=2.25
bulletLouvers and 1/16 linear inch screen=3.00
 
bulletAll reasonable effort shall be made to cross-ventilate each vented area
 
bulletPassive vents shall not be closable, blockable, closed, or blocked
 
bulletContinuous soffit strips
bulletShall be preferred over any other form low-intake ventilation
bulletShall be preferably slotted and not perforated

Comment: Some house designs leave precious little soffit length to vent intake respecting this monograph’s prescripts.  The AG has found so far that when soffit lines are too short to accept a strip intake that either multiple strips or strips and cornice or frieze board vents, e.g. as from Cor-A-Vent, can satisfy metrics. 

bulletVented drip edge shall be a prohibited material on roofs with 4:12 pitch or less
 
bulletVent strips at eaves shall not be sited
bulletWithin 3 linear feet of a window or door in the level immediately below or
bulletWhere wall or attic insulation will block ventilation
 
bulletDrip edge shall be applied to all eaves and
bulletWhere application involves pitches of 4:12 or less,
bulletThen Grace Ice & Water Shield   or equivalent shall be applied both below and above the drip edge,
bulletWhere application involves potential ice dam hazard,
bulletThen Grace Ice & Water Shield or equivalent shall be applied below the drip edge

Comment:  Hip roof-intense home designs can rapidly deteriorate ridge venting metrics hereunder.  The option of venting hip ridges depreciates quickly past, some say, about ¼ down their lengths, thereafter permitting both intake and exhaust in the same limited locale.  In such instances of truly insufficient passive venting opportunities, mechanical venting has been a viable alternative for this home designer or, perhaps, unvented attic done by others.   

bulletFor mechanical vent exhaust
bulletApply no passive ridge or gable-end vents
bulletFan(s) 
bulletShall not be visible from front of house
bulletShall be distributed evenly
bulletFan controls
bulletShall be by thermostat
bulletShall be by humidistat
bulletShall be wired in parallel
bulletPreferably, shall be by automatic heat sensor for high-heat cut-off
bulletPreferably, shall be by 2-speed switching internal to each fan’s wiring (unless you’re electrical engineer buddy will put up a variable speed control)
bulletShall have intakes only at soffit vents or equivalent
bulletShall be clearly identified at interior by 1-120V single-pole pilot light switch locked closed
bulletShall be the quieter the better
bulletRivet heads and sheet-metal laps shall be silicone-sealed

Comment: This point about “quieter the better” is a very tough slog to clarify with roof-mounted exhaust fan manufacturers.  Roof vent fan manufacturers are not cooperative in this regard, including Nutone/Broan. 

Comment: Don’t be put off by this silicone-sealing.  The best of ‘em can leak at these penetrations and joints; the silicone-sealing effort and materials are minimal. 

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