Question

How effective are the different attic ventilation methods? I read mixed reviews, and many have a commercial bias.

Asked by Alan Speed
Houston, TX

I live in Houston, where it is hot and humid. My house is two stories and the attic gets very hot in the summer. The roof is asphalt shingle. It has continuous soffit vents and a ridge vent. I've been reading online about additional venting methods, and would like some unbiased, scientifically tested reviews of same. Some things I have read but am not sure if they're true: Continuous soffits don't provide enough air flow, better to cut larger holes use a vent cover. Ridge vents don't work because the cap sits too low and blocks the air. Also, in a place like Houston, there is not a sufficient temperature difference from low to high to get natural venting to work. Solar powered vents don't have enough power to create adequate air flow. Electric powered vents don't save more power than they use. Radiant barriers make an attic even hotter if not ventilated properly. So, is there an unbiased, scientific review of these methods that consumers can understand?

Answer

Danny Kelly

Answered by Danny Kelly

Charlotte, NC

Kelly McArdle Construction

March 25, 2010

Soffit and ridge vents are the most common roof venting strategies and typically the most affordable and do a fine job when they meet the code requirements – typically about 1:300 – check your local code.

  • Your attic in Houston will get around 130–150 degrees so the hot air will certainly rise and exit through the ridge vent.
  • If you do not meet the required ventilation requirements, adding turbine vents or vent covers over large holes can be added to increase the amount of ventilation but if you meet the requirements, adding these measures will not get you much better performance.

Attic air barrier

For an energy efficient home, even more important than attic ventilation is being sure you have a proper air barrier between your attic and the conditioned area of your home to limit infiltration and exfiltration from the attic to the conditioned space and vice versa.  

  • Pull back your insulation and caulk/foam around all penetrations and along all top plates.  
  • If your HVAC unit is in your attic – be sure all of your duct work is well sealed – typically on older homes, the joints are not sealed with mastic but simply taped and the tape wears off over time and there are leaks where the flex duct joins the system, plenum or boot.
  • If you have more leaks on your supply side than you do on your return side (typical) then you will have a negative pressure on your home pulling hot attic air into your conditioned envelope.

Electric attic ventilators

I typically advise against electric and solar power attic ventilators.  

  • The companies that install these rarely will run the venting calculations prior to installing them and there usually is not enough intake air to make up for the air the fan is exhausting.
  • Once this happens, there is a negative pressure in your attic and sucks your conditioned air out of your house (this is why your attic is cooler with these PAVs) which will actually increase your energy bills – exactly what you are trying to avoid by venting your attic in the first place.
  • The electric fans also use energy so getting hit twice on your energy bills when using these.

Radiant barriers

Radiant barriers are an excellent choice as an upgraded solution. Typically installed on the bottom side of the rafters, they will not affect the ventilation and should reduce the temperature by 10% - 20%.

  • This is especially helpful if you have your HVAC unit in your attic - will reduce the load on your system saving more energy.
  • A radiant barrier will increase the surface temperature of the shingles but will be minimal and the benefits will outweigh the minor downside.

An unvented attic

Another option is to foam your roof deck and actually have an unvented attic. This works best but is also the most expensive option.
 

For more information:

Here is a link from the Department of Energy:
http://www.energystar.gov/index.cfm?c=diy.diy_attic_ventilation

DOE – radiant barriers:
http://www.energycodes.gov/rc/Vol4_RadiantBarriers.pdf

Tagged In: summer energy, radiant barrier, attic ventilation

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