Ask The Guru: Attic Ventilation
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Ed - I live in the Dallas area and am getting ready to install a new roof. I plan to have the decking replaced with radiant barrier decking. The house faces north with the main section running east/west. There is a south facing 2 car garage and a south facing extension of the master bedroom. Both of these have roof extensions coming from the main east/west roof. There is currently about a 1600cfm electric fan on the main roof and another of approximately 1200 cfm on the garage roof. There are large gable vents on the east and west ends of the main roof and a large gable at the south end of the garage roof. The master bedroom is just an extension with no gable vent. My question is what kind of ventilation do I need? If I leave all the roof vents off and add additional soffit vents, will the large gable vents exhaust the heat adequately? The gable vents are probably 10 - 20 ft. long by about 3 ft. high and extend to the peak of the roof.
Ideally, you would have plenty of soffit vents for air to enter and then exit though the fans. Most likely, you have air entering through the gable vents and exiting through the fans. This creates good airflow through the top of the attic but you don't get the "in the bottom out the top" which is preferred. I'm not big on electric fans and they probably will be overkill after installing the radiant barrier decking which will greatly reduce the heat in the attic anyway.
The best ventilation method would be to have plenty of soffit vents and then block the gable vents and either go with several 14" wind turbines or a high-profile baffled ridge vent. This will FORCE the air to come in the bottom and out the top. You've got pretty big gable vents, but I'd probably lean towards the ridge vents while a new roof is being installed. -
I have corner areas in the attic which are so remote that ventilation is nearly impossible. What to you suggest in the installation method for those areas: lay over insulation or staple to rafters? Will the lack of ventilation effect the foil's performance?
You have a couple of choices. Lack of ventilation does not effect the foil's performance, it effects the attic as a whole in the ability to help move heat outside. So if you don't have great ventilation in a limited area, I would not worry about it. You can either lay it out or you could staple it up but let it "droop." This would allow a little more airflow over/under the radiant barrier.
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FINISH GARAGE BEDROOM ABOVE GARAGE THE HALF THAT IS OVER THE GARAGE IS COLD YOU CAN FEEL THE DIFFERENCE JUST WALKING ACROSS THE ROOM COULD I JUST STAPLE THE FOIL RIGHT ON THE CEILING AND WALLS
If you have access to the inside of the walls (from the attic space), then yes, you can staple the foil directly over the insulation, across the rafters.
If you haven't put the sheetrock/drywall on yet, then I recommend you watch the 3rd video down on this page, called "How to Install Radiant Barrier in a Cathedral Ceiling" and install it that way. Go with foam strips like in the video and afterward you can put in R-19 insulation. Then, if you REALLY want to insulate, put 3/4" foam board on the bottom of the rafters BETWEEN the sheetrock and the rafters. This will significantly reduce "thermal bridging."
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I installed a gable fan and am also wanting to install a radiant barrier. The insructions for the fan say to cover the vents closest to the fan so the fan can pull the air fron the other side of the attic. When installing the radiant barrier do I still need to plug the vent holes closest to the fan?
Yes, you want to allow the airflow to circulate as if the foil isn't even there. For more information, check out one of my newest videos about installing AtticFoil radiant barrier and the recommended air gaps around the foil.
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Ed: I have a large attic with soffits all along the 36' length of the house, front and back, and a ridge vent all along the roof. With that type of ventilation, would reflective paint or foil help much?
Your attic is just like your car and ventilation (which you have) is like rolling down the windows of your car. It helps, but if the car is in the sun it STILL gets hot due to radiant heat gain. Installing AtticFoil® Radiant Barrier Foil is the most effective method to stop heat gain; just like parking your car in the shade would help with the heat gain, radiant barrier it is like putting the roof of the house in the SHADE from the HEAT.
How much will it help? If the roof gets hot, it will help a lot. If the roof is in the shade and does not get hot, then it will have limited benefit. You can call us at 800-595-8772 with any more questions.
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Do I need an attic fan if I have proper attic holes: Low and high? I am installing a new roof with comp, I will spec an radiant barrier, we already have attic insulation.
Most customers opt for a passive ventilation system which is just a combination of working (ie. clean, unobstructed) soffit vents and either a ridge vent or a pair of gable vents; oftentimes nothing more is necessary. Once a radiant barrier is installed, your attic air temperature should be within 10 degrees of outside air temperature. If it's not, then more ventilation is probably the next step.
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I am going to do a vinyl siding job soon. I have read a number of articles about radiant barrier products. Questions: Our typical exterior remodel consists of old siding tear off--apply 1 inch Dow brand foam board with taped seams--apply a house wrap--after the wrap is that when I should apply the radiant barrier or should I put that on first before the foam board goes on? In the attic should I do a layover on top of the attic insulation or should I staple to the underside of the rafters instead?
You should apply the radiant barrier OVER (after) the housewrap - you want it to be the LAST layer before the battens and the siding. Take a look at the second to last row of photos on this page: http://www.atticfoil.com/radiant-barrier-customer-install-pictures.htm
In the attic it's hard to say without some more details from you; installation is not based solely on where you are located, there are several other factors that determine the best installation method for your home. Every home is different, so while there are general recommendations for certain climates, sometimes one method is best for a particular home. To determine which installation method would be best for your home, I'd recommend you read this article on our website: http://www.atticfoil.com/best-install-method.htm
Read the article and consider your situation and then decide. Either way you go, the first layer of foil is going to offer you the benefit of blocking 97% of the radiant heat gain that you are currently allowing all 100% of it to enter the home. The difference between 100% heat gain and 3% heat gain is tremendous, not only for comfort but for energy savings as well.
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I am preparing to reroof with metal. My house has no overhang for soffit vents. I have one gable vent at the front of the house. I have no ridge vents, but currently have a power attic fan. I want to eliminate the attic fan and use ridge vents. My question: Is one front gable sufficient of air intake? If not, is there an edge vent system that works with metal? Thank you, Sandy Bourdon
Most customers opt for a passive ventilation system which is just a combination of soffit vents and either a ridge vent or gable vents; oftentimes nothing more is necessary. Once a radiant barrier is installed, your attic air temperature should be within 10 degrees of outside air temperature. If it's not, then more ventilation is typically the first place to start.
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This question is in regards to new construction. Part of the project is for a detached 30' x 54'garage that will have manufactured attic trusses for a habitable room over the garage. The roof design is a simple 8/12 pitch gable. The insulation sub recommended using closed cell foam and blown cellulose at the top portion of the trusses to get the mandated R-38. This is great for R value, but now I can no longer use a soffit vent/ridge vent combination because the airflow to the ridge is now blocked by the closed cell foam. Making the entire roof a "hot" one would be the easiest but would add about $7000 to the bid. Yikes. We know proper ventilation for the cold section of the garage roof is essential. What are some realistic options? Thanks for your time. Don
Foam is a great insulator, but it only works to slow conductive heat flow, not radiant heat flow. Is this garage going to be conditioned? Where are you located? Send me an email at support(at)atticfoil.com so we can get some more details from you and help give you the best advice for this situation.
