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Question DetailsAsked on 8/7/2015

What is the optimal ventilation strategy for a Hip Roof?

We have a hip roof in need of replacement and there is discrepancy in the ventilation suggestions. One contractor is recommending 2 attic fans and a ridge vent, one is recommending 1 attic fan and a ridge vent, and one is recommending 1 attic fan and 2 box vents. Is there a rule of thumb to follow here?

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Generally speaking (aside from odd "blind" or boxed-in areas of attic) eave vents matched with ridge venting is the most effective method of providing proper attic ventilation for an "unconditioned space" or "cold roof" attic. Basically, the principle is hot air rises and the attic air is almost always considerably warmer than the outside air in the winter so it rises to the ridge or peak - and in the summer the attic air under the roof is hotter than that at the eaves outside or lower down in the attic, so that hot air also rises to the ridge or peak. So - peak exhaust works best - and you need equivalent or greater free air inlet capacity to crearte circulation, because high-point only exhausts do not provide airflow to remove the heat, and also tend to create a partial cacuum in the attic which pulls "conditioned" indoor air in through air leaks into the attic, wasting heat or cooled air.


The primary airflow tendency is eave to ridge in each rafter bay - commonly with little air movement between rafter bays even if the ventilation exhaust point is in only certain bays (as with box vents and turbines and such) so on sloped roofs, the best solution is air intake at each eave and exhaust at each rafter bay at the ridge - so unless you want about 20-30 vents near the top of the roof, eave vents (or other continuous ridge venting mechanism like a raised continuous celestory or such with exhaust vents) and ridge venting is the best solution.


Powered fans are sometimes necessary to provide ventilation in "blind" areas like galleries low down on the roof created by improper construction of attic rooms (like not providing airflow space over the top of the room through the rafter bays), odd shut-off or blind attic areas without eave space, some houses where a braindead architect did a zero-overhang roof with no reasonable way to put eaves in after the fact without major work or losing indoor space, and situations like that - but generally powered fans are to be avoided because they give only spot coverage for a total-attic ventilation need. Plus they waste energy, and usually do not have alarming to tell if they are not working, so many houses go a decade or more with dead ventilation fans - commonly not identified until a home inspector notices it at resale time. The attic fans with solar panels mounted on them are pretty much a joke too - like big or powerful enough to be very effective even ignoring the disadvantages of spot ventilation compared to continuous ridge strip ventilation.


Gable vents (end of house vents under roof peaks) likewise generally do a poor ventilation job - if the wind always blows in an area sometimes they can work OK, but commonly both the coldest days (cold high pressure airmass sitting over an area) and the hottest days (calm hot summer days) are without wind, when you need it most.


Here are links to a few previous similar questions with answers - more can be found in the Home > Roofing and Home > Insulation categories under Browse Projects, at lower left.


https://answers.angieslist.com/ViewQue...


https://answers.angieslist.com/gaf-cob...


https://answers.angieslist.com/Ridge-v...


Answered 2 years ago by LCD




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