thoughts on radiant barrier in attic

[BobRip]

In order to get moisture to build up, you need a surface cooler than the surrounding air in order to condense the moisture out of the air. I don't think you are going to find this typically on the inside of a roof, particularly in summer, as just about any daylight at all is going to make the roof warmer than it's surroundings. IF the attic space is cool, there is not much airflow thru it to draw in much moist air.

The moisture Bob was refering to being trapped, comes from inside the living spaces(showers, cooking, breathing ect...). There are some different ways of dealing with moisture, but it is generally excepted that it is best to have the insulation exposed to the air outside so it stays at least at the ambient relative humidity. Just like heat always flows to cold, so does moist flow to dry. Any moisture that finds it's way into the insulation from the inside will eventually evaporate into the air it is exposed to. Remove that ability to evaporate, and you will have rot and mold issues inside the walls. I think it is generally accepted as best to do all the sealing and trapping of moisture inside the living space(vapor barrier right behind drywall/plaster/paneling). The insulation keeps the inner wall warm and helps keep the moisture from condensing on the walls. The moisture content inside the home is controlled with ventilation typically...

Thanks for the elaboration of my too short explanation. Good job.

 

[dex3361]

Good demonstration with the IR camera Dex3361. That is exactly what the IR barrier helps to prevent. Radiant heat from the inside of the roof sheething to the other absorbtive surfaces in the attic. All that surface area absorb the heat from the sheething and in turn heats the attic air thru conduction/contact.

Ronmar, Thanks, I wanted to illustrate the benifit of the low emissivity of the foil. In this type of application the bubble wrap is of litle value. The benifit is in how you are changing the emissivity of the under side of the roofexposed to the living area. it would be proper to the rafters so dudt does not collect it it gets dirty it will not do its job. Infrared emissivity is the ability of a surface to emit infrared light/heat /radiation or said another way radiant heat. the emissivity of electrical tape(Scotch 31) is .97(good emitter) the emissivity of polished aluminum is around .05 (bad emitter), that is why the black tape shows more heat. By the way the emissivity of wood is about .8 (good emitter)wood. The closer the number is to 1 the better the emitteris in transmitting infrared heat. So when you put the barrier on the under side you effectively block the transmittance of most of the radiant heat. The space left above allows the heat to rise to the top of the roof and out the ridge vent (assuming there is a ridge vent).

 

[RonMar]

I work with heat issues all the time and people talk about funky IR thermometer readings. The first thing I always ask them is if they prepared the areas they were measuring so they have the same emissivity

 

[dex3361]

I work with heat issues all the time and people talk about funky IR thermometer readings. The first thing I always ask them is if they prepared the areas they were measuring so they have the same emissivity

Same here, one of the things I do is industrial thermography so you could say that I see heat, with the IR camera. There are a few things to do to help get good temps and understand where you will not get an accurate temp reading. Its all in the emissivity.

 

[Patriotic Stabilist]

All I know is we can tell a huge difference. Where our attic used to be like an oven, even on the hottest days it isn't now.

Hubby put in a ridge vent all the way across and plenty of undereave vents. But there is also venting where the panels are covered by the top vents on the metal too on the corners of the hip roof. I think for Texas this is the way to go. We also put the Tech Shield in the rent house we just redid, making the bills there very affordable too.

And to the fellow that said he couldn't stand it 78 to 80. I work outside a lot and for me it gets to cold turned down low. Now if I'm canning or working hard on something in the house I will turn it to 75 or so, and perhaps my thermometer on the thermostat could be off too.

And you know my doc told me to cold temps in the house isn't good for allergies and sinus issues. My DD has been taking the shots for allergies and they have really really helped her, she has for almost a year and is down to just taking a Zyrtec, not sure she needs it at times.

 

[Alan L.]

All I know is we can tell a huge difference. Where our attic used to be like an oven, even on the hottest days it isn't now.

Hubby put in a ridge vent all the way across and plenty of undereave vents. But there is also venting where the panels are covered by the top vents on the metal too on the corners of the hip roof. I think for Texas this is the way to go. We also put the Tech Shield in the rent house we just redid, making the bills there very affordable too.

And to the fellow that said he couldn't stand it 78 to 80. I work outside a lot and for me it gets to cold turned down low. Now if I'm canning or working hard on something in the house I will turn it to 75 or so, and perhaps my thermometer on the thermostat could be off too.

And you know my doc told me to cold temps in the house isn't good for allergies and sinus issues. My DD has been taking the shots for allergies and they have really really helped her, she has for almost a year and is down to just taking a Zyrtec, not sure she needs it at times.

I guess it would depend on what you are allergic to. I am more allergic to molds than anything else and they thrive in warm humid conditions. Allergic to pollens and all the other stuff too, but even when its 50 outside we close the windows and run the air unless the humidity is way down.

I have worked in the heat and still work out in it on weekends, but I have had a desk job for 35 years and so would probably not be as tolerant of the heat as one who stays in it all the time. I don't mind sweating in the heat when I am working in it, but I don't like to sweat when I am trying to relax, and especially if I am trying to sleep, and if its warm and muggy my nostrils get plugged up in a hurry.

 

[dmccarty]

Should a building material such plastic sheeting be used to keep moisture inside the living space where it was generated? Is so, where should the plastic be placed? What works in one part of the country can cause problems in another area of the country.

Lstiburek has literally written books with details on home construction to cover the different conditions across the country.

The concern I have with radiant barriers was decreasing the life of my shingles. The discussions I read years ago on the subject caused me to question the cost/benefit of radiant barriers. Having to replace my roof early would eat up any energy savings. The point of radiant barriers and insulation is maintain a target temperature in the living spaces of a structure. If I add insulation I can maintain that temperature with no risk to the roof.

If one has a metal roof then I don't think the use of radiant would matter.

Later,
Dan

 

[piller]

Original poster has a roof with black shingles and says that the attic is too hot and is therefore thinking about installing a radiant barrier inside the attic to help lower attic temp/reduce cooling costs. Radiant barrier inside attic would result in reflecting more heat through roof, increasing the shingle temperatures, possibly decreasing life of black shingles.

If it was me I would get rid of the black shingles and increase attic ventilation. Lot's of older houses without AC used tin metal roofs, good attic/house ventilation, and white exteriors.

 

[texasjohn]

A couple of years ago I built a metal work shed.... installed the radiant bubble barrier under the metal roof and sides and on top of the purlins.

This year I am disappointed to say that the white plastic underside of the barrier has become extremely brittle and is flaking off in large chunks. I don't know the brand used, but it has a silver side that goes up, plastic enclosing bubbles and a white plastic sheet on the bottom side.

This is happening all over the building, roof and sides, so it isn't just one spot.

I don't know what the long term outcome will be, but fear that the entire barrier will become so brittle that it will simply fall down...

Anybody know what the lifetime of this plastic barrier is supposed to be?

 

[thatguy]

A couple of years ago I built a metal work shed.... installed the radiant bubble barrier under the metal roof and sides and on top of the purlins.

This year I am disappointed to say that the white plastic underside of the barrier has become extremely brittle and is flaking off in large chunks. I don't know the brand used, but it has a silver side that goes up, plastic enclosing bubbles and a white plastic sheet on the bottom side.

This is happening all over the building, roof and sides, so it isn't just one spot.

I don't know what the long term outcome will be, but fear that the entire barrier will become so brittle that it will simply fall down...

Anybody know what the lifetime of this plastic barrier is supposed to be?

The radiant barrier stuff I have seen advertised for the under roof application was NOT the bubble type it seems. It was just a fabric type stuff.. I wonder if the bubble was not the best material for that use.


Brian