Radiant barrier on rafters or directly on deck?
 

I'm going to be installing a radiant barrier this weekend, Location lower Central Florida so my only goal is keeping the heat out.
From everything I've read radiant barriers need an air pocket to be effective, but the manufacturer of the radiant barrier suggest you can staple it directly to the deck if in a southern climate. I assume they suggest that method in the south to keep heat out.
Page 3 here http://pdf.lowes.com/productdocument...hRQXZEX0J3RQ.. advises a direct to deck staple method.


Should I hang it between the rafters creating an air channel, or should i staple directly to the deck?

I'm not in the south but I put it across the rafters so that space is a chimney to the vents, so heat flows from eaves to the peak and vents higher up on the roof. Then I added fiberglass on the deck.

I'm not in the south either, but last summer we had several days in a row with temperatures over 90 deg F. The AC finally started on the fourth day because it took that long for the house to heat up. We do not have any radiant barriers, but do have 32 inches of insulation in the attic.

A radiant barrier is better than nothing, and plenty of insulation is much better than any radiant barrier.

Why not both? Radiant barrier will be about $100, then I'm gonna add blow in insulation, not 32" but ~R40 which is adding 12" to the current insulation of about 6-8", should get me close to R60

Radiant energy is transmitted from a hot body (surface) to a colder body (surface) through a vacuum or air (not solid or liquid materials) under specific thermodynamic rules and conditions. If the colder body is a reflective surface to that wave length of radiation, a portion (the efficiency of the reflective surface) of the transmitted radiant energy will be reflected back to the hotter emitting surface.

Hence, the reflective surface material must be facing the heat source without intermediate material interference. Thus even dust accumulation on the reflective surface will reduce its reflective efficiency over time.

Radiant barriers can be anywhere in any position as long as one side of them is not contacting any surface as in it has to be in free air on one side. Hence radiant if it's touching both sides all the heat is conducted through conduction and radiant heat isn't in the equation if they are touching.

You can attach it to the underside of the roof rafters and it will work fine. In fact it's best if the shiny side is facing down as the build up of dirt on the radiant material will reduce it's effectiveness.

What Bernie said. I've not seen any real science that says radiant barrier is particularly effective as an insulation on planet earth. If you're doing an attic top up already maybe just save the time and money and go straight to the insulation.

Not quite correct!
The reflective surface of a "radiant barrier" must be facing the source of the radiation, in a vacuum or air. If the reflective surface is facing the radiant sink (away from the source) it becomes a possible emitter of radiant energy dependent upon the temperature of the "barrier material".

I must disagree with berniebenz. Any material that is a good reflector of thermal radiation is automatically a very poor emitter of it as well. A barrier that has the reflective side facing away from the heat source will, of course, heat up but it will emit little from the other side, hence acting as a barrier just as well as if it was mounted the other way around.

Regardless of the theory however, I have to agree with creepy that such barriers are almost never worth doing. Dust and condensation can ruin both the reflective and emissive effects and in an attic you can never guarantee to avoid them. Same goes for most places where people place such barriers.

If you must fit a barrier, be very careful to ensure that you do not trap water, whether it be condensation or possible driven rain, where it can cause structural or toxic mould problems.

Radiant barriers are not insulation. They limit the heat radiated. The science is well proven in every field that uses hot and cold stuff. Cars use radiant barriers in underhood/exhaust applications. Aircraft use them in many places. Everything that has anything to do with space uses radiant barriers. Fire protection blankets are basically lie those cheap thin aluminum coated mylar ones.

It reduces radiated heat where as insulation reduces conducted heat. As for attics it does make a noticeable difference in attics usually attached to the roof decking.

There are three forms of heat transfer. Conduction convection and radiation.
Conduction is slowed by insulation.
Convection is somewhat slowed by air sealing and having areas too small for convection to happen.
Radiation is slowed by reflective materials like aluminum or better yet gold which is what some hyper cars use to fight radiated heat.

It works in houses but it's best to do it when you have the roof replaced than to go through the misery of stapling up rolls of mylar in the attic. Galvalum roofing also helps.