Can a radiant barrier in the attic be effective?

[Grinkle]

TL;DR Summary

Is there an installation approach that will result in a worthwhile radiant barrier?

I am considering installing a radiant barrier in my attic to reduce the thermal load on the A/C in the summer.

The radiant barrier will only help me if it is at a lower temperature than the roof, once its at the same temp at the roof, it just becomes an equivalent radiator into the interior of my attic (equivalent to the inside surface of the roof, I mean).

So, I would need to install the barrier material such that there is as much thermal resistance with respect to conduction as possible. As the roof heats up from the radiation absorbed from the sun, I want the inside barrier to remain at the ambient temp of the attic (ambient outdoor temp at least in the early morning) and not heat up above ambient along with the roof, which is heating up above ambient and becoming a radiator.

The best approach I can think of is to attach barrier material to the roof rafters, leaving an 8" air gap (the depth of the roof rafters) between the radiant barrier and the inside of the roof. This limits heat conduction to the 2" x length surface area of the rafters where the barrier would be in contact with it. I can make it better still if I attach it with screws and put a fiberglass washer between the rafter the barrier at each screw. This is as thermally isolated as I can think to make it and still keep it a reasonable DIY project. Even that is pretty labor intensive compared to using a staple gun.

I suppose I could also just buy some mylar and lay it down on the floor of the attic, but that seems very non-robust and sometimes I do need to access the ceiling from the attic side for some kind of maintenance or project.

Roughly speaking, from June to September my attic temp will cycle between 140F and 95F every 24 hours - basically it will stay pretty hot up there. Does anyone have an instinct for whether this is any point to installing a radiant barrier as I have described, or will the barrier material just quickly heat up to the temp of the roof no matter what I do and it won't be helping me?

I feel confident in my understanding of the principles involved, but I've never done any thermal modelling and I am not confident I could get a useful result if tried to model the surface area of the rafters against barrier material and the energy from the sun and the thermal conductivity of the roof etc. I'm hoping someone has some real world experience that they can share to give me guidance here. Is this worth the bother?

 

[jrmichler]

A radiant barrier will help if there is no or little insulation. While the radiant barrier will reach the same temperature as the air, it will radiate less heat down to the attic floor than a plywood roof deck.

My previous 1300 ft$^2$ house had "only" R60 insulation in the attic. That house was located in an open area with no shade. We set our thermostat to heat the house to 70 deg F, and air condition to 78 deg F. When we had a sudden heat wave to over 100 deg F, the air conditioner did not turn on until the second day. I set up a temperature data logger. The one ton AC only ran 58% of the time when it was over 100 deg F outside.

We did even better in our current house, which is the same size. The attic insulation is about R96. It usually takes two or three days of temperature in the high 80's before the AC turns on. This house is partly shaded in the summer.

We lived for a while in an older house that had only 2" of fiberglass insulation in the attic. That house was miserable to live in. It was common for the furnace to run at night, and the AC to run in the afternoon and evening. The kitchen area was always at a different temperature from the rest of the house.

A well insulated house is comfortable, with even temperature throughout. You do not notice when the furnace or AC turn on. Adding insulation to the attic floor will do much more good than a radiant barrier.

 

[Grinkle]

Adding insulation to the attic floor will do much more good than a radiant barrier.

I am just finishing that up. R60 seems like a LOT of insulation to me! My attic has / had 30'ish year old blown in insulation that was really flattened down and patchy. I went row by row, moving the blown-in stuff aside, laying down R30 bats (R30 is readily available because that is code for new construction where I live) and moving the now fluffed-up old blown in stuff back over the newly installed batting. I didn't dispose of the old insulation because I think it can only help things and I don't know how to legally dispose of it in any case. I'm sure there is a way, but I can't take it to the landfill. So I'm not sure what I have at the end of all that, but I'd guess about R40.

Now that I have that part done, I'm pondering the radiant barrier.

 

[BillTre]

I am not sure how this might apply to your situation, but I will tell you what I did with my garage several years ago. I put in several layers of heat control because it got very hot in the summer and I wanted to grow fish in there. Some parts may apply to your situation.

1. The garage was painted battleship grey. When we got around to repainting things it was painted a light colored (pastelish) yellow, more heat reflective.
2. Same with the shingles. When we replaced them they went from a dark to a light color. There are now special heat repellant roof covering options that I am not very aware of.
3. Insulation (batts) directly under the under the roof with a plastic vapor barrier on the inside of the rafters.
4. If you have roof vents there should be a space between the insulation and the wood roof sheets. Combined with vents at the bottom of each space between the rafters, heating the roof will drive air circulation up from the bottom vents to the roof vents near the top of the roof. This will remove a bunch of heat before it gets further into the house.
5. I had previously installed plywood on the bottom of the roof support triangles as a floor for storing things. I put insulation and another vapor barrier under that (since I was making a humidity producing fish room) and then drywall.
6. Between the roof and the attic floor I put a ventilation fan which removed additional heat from the attic space contained between the two layers of insulation. The fan (~$40) can with a thermostatic control. I could turn it on or off from a switch in the main garage.

This worked great because the garage floor was a cement slab on the ground and usually acted as an AC.
I put a little AC unit in there but really didn't need it. I eventually replaced the with a timer control that fits in to the switch's space in the wall so I can have the fan turn on and off as I program it.

 

[Grinkle]

@BillTre Insulating the inside of the roof directly with batts is an interesting idea - thanks for the response!

I don't have roof vents, and I have to say I don't know how the air flow is intended to happen. I don't have soffit vents, either. The house was built in the 80's and has never had moisture problems and I live in Dallas with lots of humidity, so there has to be sufficient circulation somehow to prevent moisture build up, but I haven't been able to figure it out.

Maybe the roof just has enough gaps in it along the top of the house frame to provide enough circulation to prevent moisture issues.