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 switch 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.

 

[BillTre]

There are two kinds of upper roof vents I know of:

At the bottom edge of the roof there is usually a board going across which may seal the bottom. You can drill holes in this to let cooler air in from the outside. Cover the holes with screen or something to keep animals out.
The air under the roof will heat up and rise to go out the upper vents and thereby removing some of the heat.
There also some flat foam plastic that are formed to maintain a space between the insulation and the roof above it for better air flow.

from my search for"air flow under roof above insulation":

 

[Grinkle]

@BillTre I agree that if in fact I have little or no air flow, that's likely a bigger priority than a radiant barrier. I'll figure that out, and if I don't have any deliberate air flow, figure out how to best address that.

I am a bit nervous about DIY'ing a hole in my roof, I'll admit.

 

[Dullard]

You don't say where your house is. Contrary to some of what's been said above, a radiant barrier is a 'no-brainer' for new construction where I live (FL). Location, location, location.

https://energyresearch.ucf.edu/research/buildings-research/roof-assembly/radiant-barrier/

 

[Flyboy]

@BillTre I agree that if in fact I have little or no air flow, that's likely a bigger priority than a radiant barrier. I'll figure that out, and if I don't have any deliberate air flow, figure out how to best address that.

I am a bit nervous about DIY'ing a hole in my roof, I'll admit.

The other option for ventilation is to put a fan on the shady end of the house, and force airflow through the attic once you reach a certain temperature. My dad did that with a thermostat-activated attic fan when we lived in a 1950s house in inland California. It made a world of difference to pull that attic air out and draw in fresh outside air that was noticeably cooler than the stagnant air in the attic. We would also run it into the night, as it routinely got down to the high 60s(F) at night versus the low to mid 100s during the day.

 

[Grinkle]

You don't say where your house is.

Post 5, Dallas. If your home has a radiant barrier, do you know how it was installed? Stapled to the roof rafters, or something else?

put a fan on the shady end of the house

I like the idea, but I'm concerned I'd need a really big fan to make a difference. My home is single story and including the attached garage the sq footage of the attic is about 3500. That's a lot of air space in what is essentially a single 3:1 aspect ratio rectangular room. Not sure how to size a fan for that, I'll Google.

 

[Dullard]

I've seen a number of different radiant barrier installations. The best are obviously done as part of new construction - the back side of the roof deck is insulated with aluminized board. I have also seen aluminized film 'draped' over the trusses before deck application. I have seen after-market installations stapled to the rafters - it's a real PITA to do. One note: where radiant barrier is worth doing, it will result in significantly higher roof-deck temperatures.

 

[Grinkle]

the back side of the roof deck is insulated with aluminized board

Thanks, you have given me something to think on. My instinct is that this approach is much less beneficial than installing a barrier with an air gap to the roof. The OSB foil will always be at the same temp as the roof and so it will be radiating more into the attic than a barrier that has a standoff from the roof and is presumably at a lower temp than the roof itself. It does have a lower emmisitvity than bare wood, so I can understand it does help some, but I'm not convinced its the best approach with regards to overall cooling benefit. It does strike me as the most economical approach.

Both approaches are subject to the foil getting dirty, and with a barrier stapled to the rafters its impossible to ever clean the inside of it, should it get dusty. That is one benefit I can see to the foil-OSB approach, although I wonder how many homeowners actually clean the inside of their roof.

Do you think I'm missing something here?

 

[Grinkle]

@Flyboy After some reading, I'm going to stick with passive venting approaches. Seems there are ways to shoot below the waterline with attic fans - in my leaky house if I have enough airflow to make a difference I'm likely to draw cool air out of my living space and into the attic. Thanks for the suggestion, though.

 

[Baluncore]

Both approaches are subject to the foil getting dirty, and with a barrier stapled to the rafters its impossible to ever clean the inside of it, should it get dusty. That is one benefit I can see to the foil-OSB approach, although I wonder how many homeowners actually clean the inside of their roof.

Do you think I'm missing something here?

Yes. You are forgetting that dirt is also an insulator.
The foil is still present below the dirt, and will continue to reflect IR.

I do not understand why you would remove old insulation that had settled. It still had most of its insulation value.

 

[Grinkle]

The foil is still present below the dirt, and will continue to reflect IR.

I think if there is a layer of dirt on top of the foil, the dirt will increase the emmisivity of the entire material stack, lowering its effectiveness. Is this not the case? Heat will conduct all the way to outermost layer of material, and the emmisivity of that outermost layer will dominate the overall emmisivity of the stack. No?

 

[Grinkle]

It still had most of its insulation value.

Maybe so - I don't know if I improved its insulation value by re-working it or not. Most of the air was missing, it was pretty much solid fiber. Here's an image. On the left you can see un-worked flattened insulation, on the right is the same stuff re-worked on top of the installed batting - its much fluffier. I was also mindful that the batts I am using are specifically designed to fit with my attic floor layout, width and height to match the ceiling rafters, and wanted to take advantage of that fit.

 

[Baluncore]

Heat will conduct all the way to outermost layer of material, and the emmisivity of that outermost layer will dominate the overall emmisivity of the stack. No?

No.
This is a case of resistance in series, where every impedance mismatch reflects energy at the boundary. A metal foil will have an oxide layer, then some air and some dirt. Many layers of mismatched refractive index, make the best screen. That can be a mat of many crossed fibres with some air.

 

[Grinkle]

No.

I can't get my head around that. If there is no dirt on the foil, it is trying to radiate into the attic space, and it is not a very efficient emitter. If there is dirt on the foil, it is instead conducting energy into the dirt, and the dirt is an efficient emitter, compared to bare foil. The foil/air resistance (dominated by radiation) is different than the foil dirt resistance (dominated by conduction), which is different than the dirt/air resistance (dominated by radiation and lower resistance than foil/air).

If I wrap a potato in foil, it should take a long time to cool down relative to an unwrapped potato. If I take a third potato and cover that foil wrapping in something with high emissivity, I predict that third potato will cool down faster than if I didn't do that. I think you are saying I am wrong, and it wouldn't shock me, but I am not able to get what you are telling me.

 

[Baluncore]

Foil insulation usually has a paper substrate on one side, with a reflective metallic film on the other. It does not matter which way around you hang it, there is still the same R value. IR radiation that passes through the resistive substrate will be reflected from the back of the conductive foil. The only difference between outside and in, is the environmental degradation over time.

 

[jrmichler]

AC ducts in unconditioned space are very bad. They gain heat from the hot space AND leak cool dry air out of conditioned space, which sucks hot humid air into the conditioned space.

I see wiring in the attic. Every penetration through the attic floor should be sealed against air leaks. Every wire, every plumbing vent, every AC duct connection. This can be as important as the insulation. Every cubic foot of conditioned air that leaks out is a cubic foot of hot humid air that is sucked in.

An excellent resource on hydrothermal design of buildings is BuildingScience.com. Their site has a lot of good information. Search terms "radiant barrier" finds a number of good hits. This paper shows simulation results: https://buildingscience.com/sites/default/files/document/rr-0981_vented_sealed_attics.pdf. And this paper has measured results: https://buildingscience.com/sites/default/files/document/rr-0917_attics_las_vegas.pdf. They have much more.

I added turtle vents similar to the photo in Post #6 to the roof of the house I owned in the late 1980's. While hot air flowed out of them, I was unable to see any evidence that they did anything to reduce attic temperature. This agrees with the BuildingScience paper linked above.

If you add enough insulation, your AC unit will be oversized. The first sign will be that temperature fluctuations when the AC turns on and off will be more noticeable. The next sign is that interior humidity will increase, especially on warm, but not hot days. Many people react by turning the temperature down to get the AC to run to reduce humidity, which leads to a vicious circle that ends in a cold house. Much better is to run a dehumidifier when needed.

 

[BillTre]

I added turtle vents similar to the photo in Post #6 to the roof of the house I owned in the late 1980's. While hot air flowed out of them, I was unable to see any evidence that they did anything to reduce attic temperature.

They are not being used right if they are drawing air from the sub-roof space and just from the space between the rook and the insulation or some other separation between the roof adjacent area and larger sub-roof space.

 

[Grinkle]

AC ducts in unconditioned space are very bad.

I get it. No option for me, though, unless I want to install a third split system to cool the attic.

Every penetration through the attic floor should be sealed against air leaks.

Thanks for flagging this - I have a LOT of leaks, most of them are canister light fixtures. I'll have to consider what I can do about them. My entire home is lit with them and they are not at all sealed. Not an easy solve I expect. The canister fixtures are as old as the house (80's vintage). Unless I have a approach for fixing those, I'm not sure its worth addressing wire holes for which I'd use expanding spray foam, I think.

Much better is to run a dehumidifier when needed.

Thanks, I'll keep that in mind if I am seeing humidity issues inside the house.

 

[jrmichler]

I have a LOT of leaks, most of them are canister light fixtures.

Those fixtures are real energy hogs - incandescent lights that must be ventilated. They cannot be sealed. You could consider replacing with LED fixtures. Search Amazon for led ceiling flush mount to find a large selection of fixtures, many of which are dimmable. One image from that search:

They are also available in other colors, including white. These fixtures can be sealed, and the LEDs last almost forever. The LED lights use a small fraction of the electricity, which means less heat in the house.

Expanding spray foam is the right stuff for sealing wire holes.

Check the AC ducts for air leaks. There is a special tape for sealing AC ducts. Search mastic tape HVAC to find it. Then bury the ducts with insulation.

 

[Grinkle]

These fixtures can be sealed

How does one seal them?

I'd end up spending more on lights than I did on the insulation, not sure I'm going to go there, but its good to know in any case.

 

[Dullard]

Maybe worth remembering:

Radiant-mode heating is a fundamentally different mechanism than the 'conducted' heating for which wool insulation is intended. I agree that after-market solutions for radiant heating are all a PITA. The Florida Solar Energy Center (FSEC) has done a significant amount of research in this area - they are an excellent source for questions like this.

Latitude matters. The best (net) solution in Dallas isn't the same as in Minneapolis. I see many examples (mostly North to South) of people transporting geographically-specific conventional wisdom to places where it isn't wise.

 

[Grinkle]

@Dullard Thanks for the references and discussion - very helpful.