Interpretation #7656

Perhaps there should not be a difference. Our opinion, hence a commentary, is Section R806.2 seems to more accurately describe the intent. Since we are in a code development cycle, a coordination of these two sections appears to be in order. The following is offered as an explanation of the intent of these two sections. Both sections contemplate ventilation at the eaves alone with a net free ventilation area of 1/150 (square feet) or a combination of eave and ridge ventilation where the net free ventilation area may be reduced to 1/300 (sq ft). Proper attic ventilation must ensure balanced venting. Balance is achieved when no more than 50 percent of the ventilation installed is provided in the upper portion of the attic and the remainder of ventilation is provided at eaves or cornices. The key to balanced ventilation is to make sure air moves into the attic at the eaves and out through the ridge vents or other vents in the upper portion of the attic. The object is to make certain air enters the attic at the eaves and exits through the ridge vents. However, if more ventilation is provided at the ridge or upper portion of the attic, the flow of ventilation will be reversed. This causes a lack of efficiency as air entering at the ridge encounters rising hot air interfering with the removal of hot humid air from the attic. Additionally, the reversal of attic air flow can be expected to draw in precipitation at the ridge. Section 1203.2 indicates as much as not less than 50 and no more than 80 percent of the �required ventilation� may be located in the upper portion of the attic. However, the commentary for this section stresses that the ventilation at the eave and upper portion of the attic must be balanced. Having said that, in some townhome buildings the interior units with only front or rear eaves are limited with the eave ventilation. Therefore, it may be difficult to meet the max of 50% in the upper portion of the roof. The Building Official may allow an alternate method like a power ventilator to meet the intent of the code.