Attic Air Quality

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

HOME BUILDING PROBLEMS - HOME ATTIC AIR QUALITY
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 QUALITY

INTRODUCTION

In this home designer’s opinion, venting for home attic air quality 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 venting 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, the relevant annotation in the home plan from this home designing shop will include ‘done by others.’

An 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.

Where there are high exhaust vents and low intake vents, net free area shall apply separately as follows before adjustments

	Calculate square feet of total surface area to ventilate
	Divide that total by 150
	Apply that quotient to low ventilation
	Apply 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.

Where there are only high vents, net free area shall apply as follows before adjustments

	Calculate square feet of total surface area to ventilate
	Divide that total by 150

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

Net free vent area shall be the area of a vent opening adjusted by the following factors

	1/4 linear inch-1/8 linear inch screen alone or louvers alone= 1.25
	1/16 linear inches screen=2.00
	Louvers and 1/4 linear inch-1/8 linear inches screen=2.25
	Louvers and 1/16 linear inch screen=3.00

All reasonable effort shall be made to cross-ventilate each vented area

Passive vents shall not be closable, blockable, closed, or blocked

Continuous soffit strips

	Shall be preferred over any other form low-intake ventilation
	Shall 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.

Vented drip edge shall be a prohibited material on roofs with 4:12 pitch or less

Vent strips at eaves shall not be sited

	Within 3 linear feet of a window or door in the level immediately below or
	Where wall or attic insulation will block ventilation

Drip edge shall be applied to all eaves and

Where application involves pitches of 4:12 or less,

Then Grace Ice & Water Shield or equivalent shall be applied both below and above the drip edge,

Where application involves potential ice dam hazard,

Then 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.

For mechanical vent exhaust

Apply no passive ridge or gable-end vents

Fan(s)

	Shall not be visible from front of house
	Shall be distributed evenly

Fan controls

	Shall be by thermostat
	Shall be by humidistat
	Shall be wired in parallel
	Preferably, shall be by automatic heat sensor for high-heat cut-off
	Preferably, 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)
	Shall have intakes only at soffit vents or equivalent
	Shall be clearly identified at interior by 1-120V single-pole pilot light switch locked closed
	Shall be the quieter the better
	Rivet 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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