I live outside of Atlanta, in the 30043 zip code – Southern zone.
I have just finished installing radiant barrier material in my attic rafters. My attic is composed of two main sections connected by a transitional section over the garage which connects the bonus room and about the next ten feet of living space to the rest of the home. The bonus room sits above a large portion of the garage. That portion of the attic has a very low roof peak of four feet from the attic joists to the bottom of the ridge. It has two static roof vents on the sunny, Southside, two on the Northside, and a gable vent on the East end. The roof angle is quite steep – roughly 45 degrees. The portion of the rafters that one can see above the bonus room (the horizontal span) is about ten feet wide, which equals the horizontal ceiling width of the bonus room. However, the bonus room is 14 feet wide with the ceiling sloping at 45 degrees, running two feet horizontally to meet the walls of the room. Basically, only about half of the line length of the rafters can be seen in the portion of the attic directly over the bonus room. There is no way to install a radiant barrier over this concealed portion without replacing the roof decking.
At the point where the rafters meet the attic joists above the bonus room, there is a fiberglass batt tucked in just below a hard, cardboard piece, that creates an air gap between the roof decking and the fiber glass. What just kills me is that the builders only installed one soffit vent on the front of the garage and one on the back of the garage, that directly feed the portion of the roof directly over the bonus room. (The other two soffit vents service that transitional space.) The bonus room is 12 feet long. The portion of the roof rafters that are not visible cannot be getting much air flow. There is a lot of airflow directly above the bonus room due to the gable vent.
OK, for the portion of the attic directly above the bonus room, as well as the portion over the other small portion of the house (transition area) that sits above the garage, I used the “side of the rafter method,” using the Reflectix bubble insulation radiant barrier product and left about two inches of air space above the barrier and a two inch gap on either side of the roof ridge. The other end of the bubble product (where the rafters meet the joists) meets the fiber glass batt and continues the air space created by the stiff cardboard at that point.
For the main portion of the attic (ten foot from floor to the ridge), I used the Reflective/woven fabric radiant barrier. This product is perforated. I used the “staple to the deck method.” Reflectix recommends this method for the Southern zone. (The main portion of the attic has adequate soffit vents, plus one additional gable vent and a powered roof vent taking the air to the outside.
The results: The main part of the attic is quite a bit cooler than is the portion that sits above the garage. I have not taken a temperature reading, but it feels like ten to fifteen degrees warmer above the garage and more heat is definitely radiating downward as the areas above the garage are warmer.
So, I am wondering if I should remove the bubble product that installed above the garage and replace it (using the staple to the deck method) with the perforated reflective fabric? If I had a ridge vent above the garage and more soffit vents, I think the heat would be less. I can put in a couple of more soffit vents, but for now I cannot do a ridge vent. Another idea might be to install an additional radiant barrier over the insulation in the floor of the attic.
The blown in insulation job was terribly done. It settled and there are plenty of areas insufficiently covered. In some portions of the attic, I am removing the blown-in and replacing it with Roxul Batts and then covering the batts with the blown-in material that I removed.
About the “Staple to the Deck Method” – obviously, there is no gap, but that is the method that Reflectix suggests for the Southern zone. Other sites state that without a gap, a radiant barrier cannot function properly. However, it seems to be working fine.
What you are describing is a common building practice known as a story and half over the garage. What you did is fine. I would not change anything. I believe the reason for the temperature difference is due to less ventilation. Anything you can do to improve ventilation would be helpful, except a roof fan. They are generally a waste of money and in your case probably will not work.
Pulling out the bubble pack and replacing it with radiant barrier won’t help.
Any roof area that isn’t covered with a radiant barrier, you should cover the floor under it with a 2-sided perforated radiant barrier. If you can’t reach to the perimeter of the floor, fold an edge about 1” and use a yard stick or whatever you need to make the distance and push the radiant barrier to the edge.
I compliment you on the job you have done and hope my comments will be of additional help.