Sealed vs vented attic on remodel in Central Florida

[David Jensen]

Greetings. I am remodeling a 50 year old house in warm and humid central Florida. The house needs additional insulation and I'm trying to figure out the best thing to do. I feel that I have two practial choices. I could just blow fiberglass on the floor of the attic like everyone else does or I could insulate the top chord of the 2 X 8 rafters with fiberglass batts. I don't see a spray foam option as very viable on the top chord since it is difficult for them to get their gun down near the eaves in existing construction, plus it is far more expensive than fiberglass for the same R-value. If I choose the batts between the top chord rafters, I am wondering how important it is that I leave air space above the batts for ventilation from soffet to ridge. I will be installing a natural color (mill finish color) galvalume metal roof on top which absorbs much less heat than medium to dark asphalt shingles do.

My choice would be to put an R-25 batt in the top chord rafter and seal the attic with no ventilation above the insulation. The reason for this is:

I have some old poorly insulated duct work that would be difficult to replace in the attic and I have storage in the attic. I would like all of that to be in the semi-conditioned temperatures. There are several ceiling drops which means that I would have to cover over them to have a flat attic floor surface for max efficiency of blown insulation. I suspect that there is a fair amount of air leakage from wall cavity to attic. I have some attic areas that have a floor down for storage that would be difficult to insulate underneath. Finally, I am putting up new soffet material and would prefer the look of various non-vented products.

So what is the scoop? Is it acceptable to seal the attic using fiberglass on the top chord and to not vent it? Or, does one need to reduce to R-19 which will allow circulation above the insulation with vented soffet and ridge. I also have the option of putting a foil barrier on after the insulation to further reduce heat gain into the attic. With either of these options, is it better to use insulation with a kraft paper vapor barrier or does that depend on whether I put a foil barrier on underneath it? Or, is it better to forget all of that and just blow in insulation on the floor?

Thanks so much for your thoughts.

[tbasc]

Check the Building Science Corporation web site for suggestions based on research. A sealed / unvented roof will, as you say, help with losses from the duct work. I would not put a foil barrier up. I would use R 19 batt insulation at the underside of the roof deck. TB

[David Jensen]

When you say you wouldn't put up a foil barrier, were you meaning you wouldn't do that instead of R-19 insulation or were you meaning that you wouldn't add it in addition to insulation. Most of the reading I've done suggests that a radiant barrier really does work.

Thanks.

David

[Jonny]

Maybe some folks may call this oversimplification, but I have a simple rule of thumb. If you live in a hot climate, you need to ventilate the attic. There's no need to seal the attic, hold in the heat, and melt the snow on the roof. As you won't have any snow. Also beneficial is using light colored roofing as close to white as you can get to reflect the sunlight. Color coordinating roof color is not functional. A metal roof is best bet against hail. You want to insulate the area between the attic and the rooms below in order to separate the cool air below and the warmer air in the attic. You want to ventilate the attic to keep the heat down to a minimum. A simple ridge vent and soffit vent system can do this well without aid of fans. They make ducting, if you will, that will go in the soffit vent locations where the rafters meet the top plate. This is useful both with batten type insulation and blown-in insulation.

Humidity is another factor as well. In hot climate areas, you want to keep the air moving. High humidity aids in molds. Sealing the attic can make a mold chamber.

[tbasc]

David, I am taking my info from "Builder's Guide to Hot Humid Climates". The book suggests that radiant barrier below the deck increases deck & shingle temperature more than is good for the shingles. TB

[PPS]

I agree, insulate the floor of the attic and "over-ventilate". Ridge vent, soffit vents -and- some additional round or a power vent. Humid climates can actually lower the R-value of the insulation in a closed or near closed attic. It's even more so detrimental in cold climates where the humid air of the living space acclimates in the insulation.

However in Florida (or any other hot climate) I would go a step further and use radiant foil on the south and west roof lines. Studies have shown that the infra-red rays actually lower the cooling costs (dramatically in Az and states like that.)