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Question DetailsAsked on 6/27/2016

I have a ridge vent but the attic still has a lot of heat. What can I add to assist in moving the hot air out?

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Check the previous questions with responses in the Home > Roofing and somewhat in the Home > Insulation links in Browse Projects, at lower left.


Here are a couple of diagrams illustrating what I will be discussing below, for reference:


http://www.tr-construction.com/roofin...


http://www.constructionjargon.com/ima...


http://inspectapedia.com/ventilation/...


http://www.energyauditingblog.com/how...


Depends on your type of construction of course, but with a typical peaked/ridged roof, you should have ridge vent "open air space" (shown as a square inches per lineal foot number on manufacturer spec sheet) equal to at least 1/300 the square footage of the roof surface, and 1/200 or even 1/100 makes a dramatic improvement over that. So - say you have a 30x50 roof (in plan view) at 4:12 pitch: the actual roof surface area at 4:12 pitch is 5% more than the plan area, so you would have 1575 SF of roof area - so 1/300 vent area would be 5.25 SF of ridge venting, 1/100 would be 15.75 SF, etc. Divide that by the 50' length of the roof and convert to square inches from square foot and you need about 15 sq inches/LF for 1/300, or 45 sq in/lf for 1/100. Normal ridge vent runs from about 9-30 sq in/lf of open area - so a mid-range open area product would give you the "standard" 1/300 open space, and one near the top end of normal products would give 1/200 - to get 1/100 you generally need a specialty product or use ventilation cupolas with a continuous ridge soffit or such - not normally done except for the hottest roofs, though does show upon some metal panel roofing as a significantly raised celestory vent at the ridge - standing up several inches as opposed to just an inch or two.


Of course, the ridge slot (cut in the roof sheathing when the vent is installed) needs to have at least equal open area per lineal foot so it does not restrict the airflow, but the normal 1-1/2 to 3 inch wide cutout of the sheathing usually meets that requirement fine. Obviously, like for the example case, about 1-1/2" wide slot gives about 1/300 open area (after taking into the account the area blocked by rafters - wider by ridge pole width if you have continous ridge pole like in photo above), 3" (about the widest that will fit under normal ridge venting) gives about 1/200, and for 1/100 free area you need about 4-1/2" wide slot - wider than most ridge vents will cover properly.


Now - at the other end of the airflow (which occurs mostly in the space between the rafters) - you need at least as much open intake area at the eaves, including through any soffit vents you have. Actually, if you have screened eave vents AND soffit vents (unless soffits have greatly greater open area in them or are screened soffit area only) you need about 50% greater open area in the soffits to prevent the cumulative resistance to airflow of the soffits plus the eave vents from inhibiting airflow. Usually, unless they just have the little round screened holes in the soffits, louvered or screened strip openings in soffits typically have significantly more than the 15-45 sq in/lf (taking both sides of house into account) so generally are not a problem - but there are some inadequate open area ones out there with jusat dinky little 3-4" vent holes every few feet.


The eaves - to get about 15-45 sq in/lf open area, with 24 inch rafter or joist spacing, that works out to about 42" open width between each pair of rafters (both sides of house included), so to get 15-45 sq in/lf of roof you need about only a (taking into effect you have intake air coming from both sides of the roof) 15/42 to 45/42 inch gap above the blocking, or equivalent open screened area as holes through the blocking that goes between the rafters/trusses. That is about 3/8" gap between blocking and roof sheathing for 1/300 open area, or about 1 inch gap for 1/100 - generally easy to obtain.


And you need baffles or air chutes from the eaves a couple to a few feet up into the attic to maintain a clear airflow passageway above the insulation near the eave area - see comments on air chutes in the referred to links above. You may also need this over insulation surrounding any enclosed attic rooms which have batt or blown-in insulation on/around them.

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If you do not have the ability to get eave venting, then you are stuck with gable end vents (in the end walls of the attic) feeding to ridge vents, or with very low slope or essentially flat roofs where ridge vents may leak (especially in snow country) you may have to go to either powered ventilation, or a number of raised roof vents through the roof, each of which forms a potential future leakage point.


Note that even with venting, an attic will commonly run into the 100+ range even in 70-80 degree outside temps, and might reach 120-140 in very hot sunny conditions, though over about 130-140 really promotes wood deterioration over the long run.

Answered 2 years ago by LCD




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