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thailen 04-10-2011 01:20 PM

Radiant Barrier Dilemma
I've been researching radiant barriers for my attic, which has 6" of floor
fiberglass insulation and noticed that, on surfing the net there's a choice between single, double bubble barriers and layers of aluminum foil and other materials, with the cost the highest for the bubbles. Is there a significant difference in heat reflection, given the insulation already in place? Which should I buy?

joecaption 04-11-2011 06:01 AM

What style roof do you have? And why are you thinking of using the radiant barrier.
Do you have vented soffits and a ridge vent now?
Recommended Levels of Insulation : ENERGY STAR

Perry525 04-11-2011 06:52 AM

Aluminum has been banned, as it is a fire risk!
So don't use it!
Most of the heat that gets into a home arrives via conduction.
The heat of the sun, heats the roof and sides of the home and this heat is conducted into the home via the frame of the building.
The insulation between the frame does little to prevent the heat from entering the home.
Their are a number of ways to stop the heat.
One is to take the roof off and cover the rafters with a SIPS product, this is a sandwich of oriented strand board with at least five inches of polystyrene in the middle. This is fixed over the rafters and a new roof is added on top.
This isolates the heat of the sun from the frame of the building and keeps it cool. The thicker the polystyrene the better the protection.
Another way is to paint the roof silver or white, this helps reflect the heat.
You can also fix sheets of polystyrene to the bottom of the ceiling joists to stop the heat entering the room. (this is the second best choice)

thailen 04-11-2011 08:15 AM

Thanks for your Suggestions
First, my roof, installed about 5 years ago, is three-tab architectural shinglles(30-year), which are very tightly overlapped.
Where can I buy polystyrene sheets?

joecaption 04-12-2011 08:44 PM

Heres what going to happen, not only are the shingles going to be heated from above but if the heat also is trapped under them the shingles will last far, far less time.
Example every single time I come across a house with no soffit vents and some dummy installed insulation to the rafter bays instead of the ceiling without any foam baffles the shingle are shot.
An attic should be the same tempature as the outside air. If not condinsation will form. The insulation needs to be over the ceiling not on the roof or just under it.
3 tabs have the shortest life of any style shingle, by adding the barrier I'd bet there shot in 10 to 15 years not the 25 they should have lasted.

Perry525 04-13-2011 12:10 PM

The laws of thermodynamics.
Heat always moves to cold.
Water vapor always moves to cold.

The water vapor in your home and/or attic, is created in your home by cooking, washing, breathing and sweating.
Active children and animals create most, old inactive people less.

This water vapor is held in the air until it comes into contact with a cold surface, or colder air.
When this happens the water vapor drops out as condensation, this you see on windows during the winter, but more dangerous it happens inside walls and the attic, where it condenses onto/into the cold wood leading to mold and wood rot.

The old idea that ventilation stops condensation has been discredited, as ventilation relies on the passing wind to create an area of low pressure to the lee of a building, this low pressure is supposed to pull the humid air from the attic and release it into the cold outside.

In fact what happens is the low pressure pulls the warm wet air from the home into the attic, then the wind stops and the wet air condenses.

More than that, the low pressure pulls your expensive warm air out and pulls in cold air that pushes up your heating costs.

Modern building, see Passive house standard. Enclose the living area in a plastic insulated air tight box, with at least 20mm of polystyrene insulation against the cold side of the plastic to stop condensation by keeping the plastic warm.

You can stop condensation, by opening windows in the winter to let the water vapor out, by keeping the heating at a steady temperature (not turning it down or off) or using a de humidifier, or by painting the ceilings with gloss paint or latex or lining them with plastic to stop the water vapor entering the attic.

Condensation never forms on a warm surface.

Rumi 11-12-2011 09:46 PM

Wow! Joe and Perry, great information!
Very helpful to myself as well. :cool:

picolin 03-07-2012 03:30 PM

Perry, so can I assume that you recommend painting the roof white or silver? Is there something else that I should know about that process before I go out and buy a gallon of roof paint?

Perry525 03-08-2012 09:15 AM


Originally Posted by picolin (Post 68984)
Perry, so can I assume that you recommend painting the roof white or silver? Is there something else that I should know about that process before I go out and buy a gallon of roof paint?

You are lucky in living in such a warm and sunny place.
Keep in mind that all dark colors absorb heat and all light colors reflect
Any pastel shades will work, its just that silver or white work best.
Unfortunately, you will need more than a gallon.
Keep in mind health and safety, don't fall off!

Painting your roof white will save about 5 to 7 degrees F at night.
And about 23 degrees F during a hot sunny day, with temps around 40F.
downside: paint will go off after about three years and will loose some of its effect.

picolin 03-13-2012 09:37 AM

OK, got it. I went to Home Depo on Sunday and found this: Henry 555 Premium Aluminum Roof Coating 4.75 Gal. However, upon closer inspection, I read on the bucket that I should never use it on regular asphalt shingles, as it may curl up the edges of older shingles.

My shingles are one year old, but I don't want to run the risk of ruining them. About half of the roof is mineral surface roll so I will be able to use it on those sections. Am I stuck using white paint on the asphalt shingles, or is there any in silver (I've only found white)?

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