Replies

1. 
   tedd_weyman | Jun 11, 2000 10:48pm | #1

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   You may or may not be confusing RRB's (radiant reflective barriers) with "radiant barriers." Everything is a radiant barrier. But some things reflect more and absorb less than others. This is a controversial topic and at the risk of being dissed, I will venture a reply.

   Heat travels by convection, conduction and radiation. In the matter of roofs, the objective is to have a reflective coating to reduce absorption of the sun's rays. You cannot stop the conduction of heat into and through a roof occuring when the air touching it warms it or when the inside framing members pass heat to it from inside. You can reduce absorption or radiant heat from the sun (the greatest source of unwanted heat in a roof) by using a reflective coating.

   You can find the reflective/absorptive ratings of different coatings and colors on the web. Be aware, that its not just a color question, its a surface texture issue. You cannot assume a material is a good reflector by simply looking at its color. Metal roofing manufacturers usually have specs on reflectivity and absorption. Check them on the web as well. Also, remember, that high conductivity and absorption can be an advantage in a steel roof if you do the other things necessary to enable it to exhaust heat without sending it into the building. A steel roofing or siding may be of a surface and color texture that absorbs heat very fast - that's why they crack and moan when the sun hits them or when the shade of a cloud or cool breeze blows over. The steel is fast absorber but it is also a fast conductor back to the air.

   Metal roofs are the preferred material (because they can be treated with reflective coatings) for reflective roofs. But more importanty, metal is fast consuctor and transmitter of heat. The question is, is it worth the price to get a highly specialized reflective metal roof or simply to go with off-the-shelf, light colors such as grey, white and silver. Again, check the manufacturers for reflective performance of different colors. My experience is that metal cools very fast without heating up the building is the ventilated air space is provided between it and the sheathing. So, combining an off-the-shelf reflective coating with a ventilated air space type of application (using purlins)is the best combination for keeping the sun from overheating the building. Its the lowest cost solution in materials and the easiest to install as well.

   Now, the controversy starts. I would and do use light colored galvalume roofing for its reflective value BUT, more importantly than this are (1) the ventilation space and convective pathways under the sheeting and (2) the insulation in the roof cavity itself.

   An insulated roofing cavity (between exterior deck and interior ceiling, preferably cellulose), or a polystyrene foam insulation (Types 1, 2 or 3) layed on the exterior deck between the purlins, are much more effective insulators than regularly available, affordable, reflective roofing materials are radiant reflectors. I would bet on the value of the under steel-over the deck, air space, combined with the insulation as providing better all around value and performance than an expensive or special exterior roof, reflective surface.

   The use of purlins or horizontal strapping that not only anchors the steel sheets but also creates a "cold roof" or ventilating space between the top deck and the steel will convect-off the absorbed radiant(sun's) heat and break the thermal connection between the steel and the deck. Leaving the steel edges open to the air and making some vertical (up the roof) breaks in the purlin rows will help convection exhaust the heat absorbed by the steel. It also reduces if not eliminates condensation build up by allowing evaporation and then convection to take moisture away rather than collect under the steel.

   The issue is frought with a number of variables and problems when considering roofing materials laid flat and directly on felt and decking. In conventional attics the issues are very controversial.

   If your putting steel on roof with attic floor insultation, the same rules apply to the installation of the metal roof. RRB's on the underside of trusses and rafters, or atop insulation laid on an attic floor do little other than to conduct heat absorbed from super-heated attic spaces, down into the buidling. Most RRB's are metal so they are great conductors. A late afternoon/early envening super-heating of attic air (conducted from the roof top and west ends) will heat the adjacent framing members and building materials in the attic.

   Remember, a reflective barrier is not a barrier to conduction; in fact, it facilitates it. Keeping the heat out of the roof and attic in the first place is better than having to exhaust it mechanically in the late afternoon and evening (eg; roof, not attic insulation and ventilated spacing under the steel).
   Radiant heat absorbed across an unvented air space under steel, or from steel attached directly to the deck, conducts through the roof sheathing and and along rafters/trusses, and then radiates into the rafter bays and attic cavity.

   Absorbing a lot of heat through the roof and having it radiate from the underside into bays and attic is a late afternoorn and early evening problem. The 4 PM to 10 PM temperature in these spaces will be higher than high noon temperatures. This means that the ceilings and walls will absorb and conduct heat to the inside and radiate that heat into interior spaces at the very time you want the place to be cooling. This makes the case against under-rafter/truss and attic floor radiant reflective barriers. They will work the opposite of intended, by conducting, radiating and reflecting new heat down into and old heat back into the building.

   You may or may not know that an RRB relects heat in both directions. Even if only one side of the RRB is the treated reflective surface it reflects both ways provided there is an air space adjacent to eahc side the RRB (eg; RBS on the underside of uninsulated rafters). So an underside of rafter RRB will send heat back towards the ceiling just as effectively as it sends heat back up into the rafter bays. The way to prevent one side of the RRB from reflecting radiant heat (remember, it will not reflect conductive heat) is to eliminate any airspace on the side you don't want it to reflect towards. Also, remember that a horizontally layed RRB will accumulate a layer of dust and simply not work after a while. At least an exterior metal roof gets washed off by the rain and is recommended for annual hand washing as well.

1. 
   Badhabits | Jun 14, 2000 02:33pm | #2

   *Wow!,What a reply. I certainly appreciate your time and help Todd. My roof is already on, standing seam metal (medium grey) applied directly to the 30lb felt/plywood decking. My roofer knew everything about metal roofing and recommended not using purlins or foamboard insulation, like he also recommended not cutting out the felt at the ridge for the vents, (only slitting the felt would allow the heated air to build up pressure in the attic and result in a positive flow of air out of the ridge)Hell yes I'm smarter than this! But I've let him get away with it... So I'm trying to consider all the possibilities. The fellow at savenrg.com assured me that their rbs (radiant barrier system)which is a foil product, properly applied with nothing touching the barrier,in the walls and against the rafters (in conjunction with r11 glass batts)would work wonders reflecting (90% or so) of radiant heat back out of the house. He also recommended downsizing my HVAC units, I think, 20% if following his system. It's a complicated subject, and I don't have time to get a degree in thermal physics before I finish my house. But I'd sure like to learn a few more facts before deciding on an isulation system. Thanks again for your time.Bill

1. 
   tedd_weyman | Jun 14, 2000 10:46pm | #3

   *I did a ton of personal research on RRB's last year and I found the manufacturers claims and theories about performance were contradictory. If you web search this issue and stick to the research results, you will find a paper (I have lost the reference) that shows in the south where RRB's are used more often, that the heat in the attic from 4 PM on is actually increased compared to attics with no RRB's.

1. 
   Rob_Susz_ | Jun 16, 2000 05:33am | #4

   *How well will the RRB work after it gets covered with a layer of dust conveniently supplied by the soffit vents?I'll bet that 90% drops off real quickly.Have you seen the reflected solar collecting systems that have a mirror array that reflects on one focal point of molten sodium. They have them in deserts. They dust the mirrors twice a day. Apparently small amounts of dust cut doen on the reflectivity. Hell, I know for a fact large amounts do.Ask them to see a time vs. reflectivity performance curve.Can you please tell me why a RRB is easier than insulating the attic? Perhaps a decpetive question - but you need merely insulate the attic with cellulose and you no longer need the RRB?-Rob

1. 
   weyman | Jun 16, 2000 07:01am | #5

   *All the issues seem to have a common point. Rather than convoluted and expensive solutions to handling unwanted exterior heat and moisture inside a house, why the heck don't we keep these things from entering in the first place. I agree with Rob, 100%. Insulate either the attic floor and vapro/air barrier the ceiling below it or inslate the rafter bays with air/vapor sealing above it. To keep the beating sun's rays from entering the roof, use a surface color and texture that reflects it away. HIgh performing houses (ok, I said it), have the insulation on the outside, not the inside. And, they are designed to breath (oh boy, now I will get it) to the outside as well. The interior is sealed off and mechnaically (using passive or active ventilation and exchangers) control humidity levels and fesh air quality.These RRB's for interior use are only useful on the interior ceiling and walls to reflect interior made heat back to the living and inanimate objects in the room. I would put one on a workshop ceiling but never in an attic or inside a wall. So who wants a ceiling that looks like a somewone stuck up alluminum foil.By the way, not only is dust a problem -- rendering the RRB in an attic useless in less than a year -- but one has to consider the affects of induatrial eshaust and wood smoke, vehicle exhaust, oil based fertilizers and insecticides in the air; etc. All these things will coat the RRB like cigarette smoke coats the windows in a car. Do you want to take down and scrub your RRB every 6 months.

2. 
   Badhabits | Jun 16, 2000 07:01am | #6

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   What are the effects of radiant barriers and grey or mill finish metal roofing? Is the roof metal not essentially a radiant barrier? What happens when another barrier is put on the bottoms of the roof trusses or rafters in the attic? This could get confusing... I'm still open to the possibilities though.
   Thanks,
   Bill