Comments on: Attic Ventilation – Don’t Mess It Up

By: Rick

Rick — Thu, 08 Jul 2010 21:48:39 +0000

I prefer large passive domes to whirlybirds or slantbacks. Whirlybirds have no screening against critters and wasps, and slantbacks are too small for effective ventilation even though they are very weather resistant.
My next project is radiant barrier up there.

By: Ed Fritz

Ed Fritz — Fri, 18 Jun 2010 03:00:04 +0000

John,

With vinyl siding, you can just take a circular saw and cut out a bunch of rectangles in the soffit before you install the vinyl siding. You are correct, the vinyl siding that has holes is a little lacking in open area, so you want to install a lot of it. Another option is to replace the existing soffit with a hardy board type soffit that has continuous vent holes. Finally, you can cut a hole in a porch and install an air conditioning vent grill and paint to match. It will act as several soffit vents. It’s easy and rain won’t be able to get up and inside.

By: Josh

Josh — Tue, 15 Jun 2010 14:36:58 +0000

Thanks again Ed for such valuable information. I see another project to add to my growing list!

By: John Scott

John Scott — Tue, 15 Jun 2010 13:13:56 +0000

I have about 2,600 square feet in my home. I currently have 2 power vents near the top of my roof line. I have 30 (8 X 16) soffit vents around the house. Yes, after two repaintings I am sure that those vents are somewhat restricted with paint, dirt, etc. I have bought 30 new vent covers to replace those old covers. We do need to repaint and/or install vinyl siding on those soffits. I will install the new vent covers if we repaint. However, if we go with the vinyl on those soffits I have noticed that the vinyl covering for the vents are only 12 inches wide with fairly small vent holes. Those would be placed over the current holes cut for the soffit vents. QUESTION: Would I still get adequate venting with the vinyl coverings over those soffit holes with those two power vent fans? I am probably over-powered with the power vents and I do not want to have too little venting from those soffit vents so as to cause the air conditioning to be pulled from inside my home into the hot attic. Your advise PLEASE!

By: Ed Fritz

Ed Fritz — Tue, 15 Jun 2010 02:55:50 +0000

The general rule is about 1 square foot of NFA (net free area) or “hole” for every 150-300 sq. ft of attic space. If you have 1700 ft, then you would need about 5 and 11 total square feet of NFA. This is generally split between topside exhaust vents and lower intake vents either 50/50 or 40/60 with more on the lower side. If you go in the middle with 8 square ft. NFA then you would need about 4 sq. ft of of NFA for the soffit vents. At 144 sq. inches per sq. ft. this is 576 sq. inches.

An 8″ x 16″ vent provides 56 sq. inches of NFA per vent. http://www.airvent.com/homeown.....eave.shtml Note: NFA is different from the vent size. You would think that an 8×16 vent would provide 128 sq inches, but it’s only 56 of OPEN space.

Given all this, you are pretty close with 8 or 10 vents. 8″x16″ soffit vents will do the trick. If you arer using smaller vents 4″x16″ or 6″x16″ you will need more.

By: Josh

Josh — Fri, 11 Jun 2010 14:16:26 +0000

Ed,

Thanks for the response. It confirms what I was thinking. I need to add soffit vents. How many would you recommend based on the size of my home of 1700 sq ft? I was thinking maybe 8 total. 4 on the front and 4 on the back with a large enough whole in each one to have plenty of ventilation. Of course I’d make sure to not cover the holes with insulation.

Thanks!

By: Ed Fritz

Ed Fritz — Thu, 10 Jun 2010 22:29:35 +0000

Bobby,

Wow, I think you are WAY over-thinking this. Often, there is too much emphasis on attic ventilation when installing a radiant barrier. Let me give you an example: If you park you car in the shade, does it really matter if you crack a window? Here is why. The radiant barrier rejects so much heat that usually whatever attic ventilation you’ve got is usually pretty adequate. I always recommend installing the radiant barrier FIRST. Then, put a thermometer inside the attic below the foil. If the air below the foil gets more than 15-20 degrees hotter then the outside temperature THEN increase the attic ventilation. You do not want to “seal” the AtticFoil, this is not the intention. You want air to flow freely from the bottom of the attic to the top and you want to install the radiant barrier in a way that will not impede, restrict or bottleneck the airflow. Remember, the BEST CASE lowest air temperature inside the attic is going to be the outside air temperature.

By: Ed Fritz

Ed Fritz — Thu, 10 Jun 2010 22:15:03 +0000

Josh,
Do you have soffit vents? Your home may have originally had a wood roof with no soffit vents. If you don’t have soffit vents, the the ideal system would be to install soffit vents to act as intake vents.

Radiant barrier will still be beneficial even with limited ventilation. I would be sure to leave a gap at the bottom near the attic insulation. Air will naturally be drawn in the bottom and head towards the top as it heats up. Here are some install instructions: http://www.atticfoil.com/radia.....uction.htm

By: Josh

Josh — Thu, 10 Jun 2010 16:50:22 +0000

I have a house built in the late 70’s I am looking to add a radiant barrier to. The ventilation in the attic consists of 2 whirly birds and 2 end gable vents roughly 2×3’s? Is this enough ventilation to support a radiant barrier or do I need more? (House is rought 1700 sq ft)

Thanks.

By: Bobby

Bobby — Thu, 10 Jun 2010 01:28:52 +0000

I have looked over and read most of the information on your site. The “hybrid” method seems to be the fit for what I have in mind. I am thinking of using two separate inlet/outlet systems for the attic. I would install the radiant barrier, as per the hybrid method, at the eave, and incorporate the flat top method with static ridge vents. This would cause the heat to rise with a vertical vortex. Secondly, after covering the gable ends with radiant barrier to completely close in the attic, I would use the gable vents for the other system. I would probably add unfaced batt insulation on top of the blown-in fiberglass. One end open and the other has a thermostatically controlled fan for the exhaust. Please comment on this arrangement. I am planning to install the radiant barrier next week. Thank you.