radiant barriers

[Uso]

Was looking at a attic build out and the homeowner was asking about using a radiant barrier https://mechanicguides.com/best-radiant-barrier/ . The kind that comes on a roll and has some sort of aluminum foil on both sides and some sort of filler between. I have not seen the stuff. I thought the stuff had been shown to not be that effective.

I am asking if I am not hearing and remembering things correctly? Maybe there is some types that do work? It works in certain areas of the house?

I seem to remember it being used a good deal in the south, maybe New Orleans Bill Robinson can chime in.

[NewBostonConst]

They have to be installed correctly with an air gap. The air gap is best on both side. No gap and the aluminum just conducts heat like a frying pan. The R-values on the stuff is unrealistic. Can learn more by googling the sister site to this one. "GreenBuildingTalk" .com The search engine on that site sucks so when you google add greenbuildingtalk to your search phrase.

[SunEagle]

I had a barrier installed on my Clearwater Florida home back in 2010. It is hard to say how much it saved my on A/C coolig but I will say that the attic was no where near as hot as it had been before the barrier was installed.

[NewBostonConst]

I also did my attic 10 years ago but have no way to quantify. I didn't use the bubble type though. You can get the reinforced thin foil version for a lot cheaper. Like 14 cents per sqft.

[J.P.M.]

Radiant barriers can be very effective but as NewBostonConstruction notes, gaps between layers are an important part of how they work, but air in the gap is not necessary. Actually, a vacuum is better but usually impractical for larger scale applications than a thermos bottle. But the thermos bottle example is good to show just how effective radiant barriers can be at reducing heat transfer.

Flexible thermal barriers for cryogenic applications that have dozens or even hundreds of layers of reflective foil with small gaps between them can be very effective. Something similar but with fewer foil layers is probably what the homeowner heard about. Such things can be effective, but the efficiency is very dependent on where they are used and equally importantly how they are used, and most importantly the care of the install. Just like sealing gaps, the devil is in the details. Most folks not only get the details wrong, but don't as much know the details exist.

For most residential applications, an effective vapor barrier and fiberglass batts are as or more effective than most radiant barrier systems sold (or I'd suggest oversold and overhyped) for home use. What I've seen in radiant barriers (and what I've designed in the past for a few cryogenic applications) can be effective (and expensive), or junk sold to homeowners that's no more than packing bubble wrap with foil on two sides.

For reducing heat gain or loss, it's first important to seal the structure to reduce air in/exfiltration to the greatest extent practical. Using or expecting a radiant barrier to double as a vapor barrier gets tricky and usually isn't very effective.

When installing in cold(er) climates, unless special circumstance apply, the vapor barrier goes in/on the warm side of the insulation or radiant barrier.

A trick/side bar: In cold climates make sure the cold side of the structure or wall being insulation is a bit "looser" than the warm side - but still tight enough to keep bugs out. A little cold air circulating through the insulation will help remove any moisture that does leak from the structure interior (for a residence, and as a practical matter, a perfect vapor barrier, while a good goal - to a point - is not possible using current building techniques). That outer "looseness" may seem counterintuitive and counter productive, but it will keep the insulation dryer. That advice gets tricky in warm, moist climates, particularly where the design dew points are close to or above interior daily common set point temps.

All that needs to be tempered for places like FL, LA or tropical climates where cooling loads predominate, particularly with high latent A/C loads, each location/application can be a special case and general rules of thumb can cause more problems than usual. In such places, all bets on vapor barriers are off. A vapor barrier might even cause more problems than they solve. Be careful, get educated and know what you're doing.

Also: a corollary to the above: never use more than one vapor barrier on or in a wall or barrier. You'll be building a moisture trap - the last thing you want in a building exterior surface.

The foil on foil faced insulation is a form of radiant barrier but is usually not considered a vapor barrier unless the insulation is continuous and sealed - and I mean REALLY sealed to the structure. Thinking it is a vapor barrier without making sure it has NO gaps/seams/tears/sloppy workmanship will lead to damp insulation and moisture damage to the structure in short order. Think of an effective vapor barrier as being one that's tight enough and continuously unbroken so as to be able hold back a fluid - which it actually is doing in the form of holding back air (a fluid) which contains water vapor that will condense out if the air gets cold enough. Less tight will be less than sufficient at some point or location to prevent moisture condensation and the resulting, and often hidden, water induced damage over time.

[NewBostonConst]

Yes dew point control via a vapor barrier is critical to keeping everything dry and mold free. In my new house we just finished we used vapor barrier paint that to me is the current best way. This way you stop the moisture from getting into the wall assembly right at the paint.

Most people put the foil in the attic so air is free to move on both sides and the vapor control is still the old fashion fiber glass bats way.

[J.P.M.]

Yes dew point control via a vapor barrier is critical to keeping everything dry and mold free. In my new house we just finished we used vapor barrier paint that to me is the current best way. This way you stop the moisture from getting into the wall assembly right at the paint.

Most people put the foil in the attic so air is free to move on both sides and the vapor control is still the old fashion fiber glass bats way.

I used vapor barrier paint from Glidden for my first and 2d retrofit/rebuilds in Buffalo many years ago. That product seemed effective and necessary as I used blown cellulose for insulation retrofit. Tightening up and extra care to close gaps where surfaces ought to but don't quite meet is a necessary and essential adjunct to a vapor barrier - window frames to walls, electrical outlet details, rim joists to sill plates, etc. I'd also be less than honest if I left out that I also loosened up the exterior surface a measured amount and I believe that made things dryer. For such tasks I've found that attention to detail is essential.

I'm a big fan of fiberglass batts, but I never considered or saw them referred to, in and of themselves, as a moisture barrier. Tight (rolled/overlapped and caulked and stapled at the insulation flanges) fitting insulation with a foil vapor barrier is suitable, but that can be accomplished without the insulation. To be clear, the insulation, in and of itself is not a vapor barrier.

But I do agree that vapor barrier paint can be a very useful and effective tool - just not at the expense of or thinking that it by itself is sufficient. It is not.

[NewBostonConst]

Agree.....and also should add things can very greatly on where your house is....I live in Michigan.

[J.P.M.]

U.P.

Agree.....and also should add things can very greatly on where your house is....I live in Michigan.

My experience is that weather in lots of MI isn't that much different than western/upstate NY around Buffalo except maybe for in the U.P. around Houghton/environs where the snow totals are close to unholy @ ~ 300+ inches/yr.

[peakbagger]

Not sure on radiant barriers for AC but there is lot of BS about R values with radiant barrier (AKA bubble wrap)

https://www.jlconline.com/how-to/insulation/are-bubble-wrap-r-value-claims-accurate_o

Note this is discussing using it under a slab but also references other applications

Here is another one

https://tedsenergytips.com/2011/04/01/the-perfect-insulation/

[Sunking]

Actually, a vacuum is better but usually impractical for larger scale applications than a thermos bottle. But the thermos bottle example is good to show just how effective radiant barriers can be at reducing heat transfer.

Funny the ole thermos bottle has reinvented itself and now calls itself Yetti. Almost impossible for wall construction, the air pressure would crush the wall panels. Perhaps argon or nitrogen pressurized might work.

Thi si sout of my area of expertise, bu tare in my partners tool bag and has helped me design a few homes that seem to work very well long term. Home in TX to my surprise used 2 x 4 construction, and skinned with 2-inch foam vapor barrier which seems to work really good at soaking up moisture, and a good insect barrier. The 2 x 4 act as a thermal bridge, but with the foam skin really insulates quite well.

Now in Panama 10-inch ICF works great, and it is tropical. So far no mold problems or insect problems. Termites down here are the national bird as they can carve up a house and fly away in a day. Two things you learn to hare real quick down here are Termites and %#@#^&*^% Monkeys, Got a hunting air gun for those %$^&*@ Monkeys. Birds are not much better. Imagine being in a Parrot Bird shop all day. Fist light and they scream..

[J.P.M.]

Funny the ole thermos bottle has reinvented itself and now calls itself Yetti. Almost impossible for wall construction, the air pressure would crush the wall panels. Perhaps argon or nitrogen pressurized might work.

Thi si sout of my area of expertise, bu tare in my partners tool bag and has helped me design a few homes that seem to work very well long term. Home in TX to my surprise used 2 x 4 construction, and skinned with 2-inch foam vapor barrier which seems to work really good at soaking up moisture, and a good insect barrier. The 2 x 4 act as a thermal bridge, but with the foam skin really insulates quite well.

Now in Panama 10-inch ICF works great, and it is tropical. So far no mold problems or insect problems. Termites down here are the national bird as they can carve up a house and fly away in a day. Two things you learn to hare real quick down here are Termites and %#@#^&*^% Monkeys, Got a hunting air gun for those %$^&*@ Monkeys. Birds are not much better. Imagine being in a Parrot Bird shop all day. Fist light and they scream..

I believe you are on the mark for difficulty of fabrication with the devil again being in the details, in this case mostly as the size of the container increases.

External (atmospheric) pressure on a flat pressure boundary (~ 15 PSI at sea level) is difficult to design for in a pressure vessel situation for anything more than 0.1 m^2 or so, and is thus rarely done for a surface the size of a door without lots of structural considerations (read $$). For example, a 24" manway made of steel on a pressure vessel designed for full vacuum service might be 1/2 - 5/8" thick. For wall sections for buildings where the flat surface is much larger, the materials are less strong, and where weight, cost and ease/speed of fabrication are concerns, vacuum chambers are still possible can quickly get expensive.

But the (perhaps) bigger (and not talked about dirty little secret) problem with constructing walls for vacuum service, with similar dimensions to windows BTW, is maintaining the vacuum over time, and doing so becomes problematic using current techniques.

As for using an inert gas as sort of a magic bullet - just like for widows - that's mostly a scam. First, for inert gasses such as argon, which is marginally the best for most applications, the transport properties of the gas - the properties used in estimating/determining convective heat transfer coefficients and thus heat loss - are not that much different from air. Most gasses at atmos. pressure have relatively low thermal conductivity. Bottom line: If you can keep a gas still, most of them make pretty good insulators, including air. Using/trapping some other gas is marginal at best for improving thermal performance, and at this time unjustifiable from a cost standpoint for most applications.

Keeping the gas from movement that's mostly caused by gravity induced convection is how most insulating materials work. Closed cell foam is the best of such insulators for convection suppression, but the urethane foams, while about the best insulators when new, degrade somewhat over time as the cells become less closed, break down some, and exchange the containing gas for air and also lose a fair amount of their marginally better convection suppression properties and insulate about as well as fiberglass after a few years or less - still not bad, but not whiz-bang great for very long. Some of them also have the lovely property of emitting cyanide gas when burned although most of those have been reformulated/removed from the market.

The ICF's have promise and are a real improvement, but I'm not convinced they've been optimized yet. I've heard a few horror stories and saw a couple out in the desert.

A concrete building with a heavily insulated exterior envelope completely encased by high insulation levels and perimeter insulation, all under a heavily insulated truss roof is one good way I know of to reduce building conduction losses. The magnum opus desert home I designed would have had 4" of rigid foam on the outside and 6" concrete walls and floors. The time constant for the dwelling would be about 2 weeks vs. about 24-48 hrs. for most conventional construction. I would have used mini-splits for required HVAC, but the plan was to never need to operate them and the design would have probably made that a day/day reality.

[Sunking]

External (atmospheric) pressure on a flat pressure boundary (~ 15 PSI at sea level) is difficult to design for in a pressure vessel situation for anything more than 0.1 m^2 or so, and is thus rarely done for a surface the size of a door without lots of structural considerations (read $$).

RUA being a dolphin spending a year 800 feet under seems I recall the difficulties . Quite and decent food though. Well except crossing the equator can get a bit noisy and greasy.

As for using an inert gas as sort of a magic bullet - just like for widows - that's mostly a scam.

Perhaps but better than air (drier) since a vacuum is much harder to maintain than a lower positive pressure across a flat surface.

Always wondered about a clear liquid insulator like Aerogel for windows. Too bad ceramic cannot be made crystal clear. Not sure how to convince you ICF is the ticket other than my 3 years living in an ICF home in the Tropics, so far has been very positive. No mold or mildew issues, rarely ever have to use AC and quite as a mouse which is important with dang MF'ing monkeys and parrots loose everywhere. Really would like to have shotgun down here.

[Mike90250]

.... Too bad ceramic cannot be made crystal clear......

The glass on our heater door is pretty darn close, have to hold a sheet of white paper up to see any difference. But it does not have good thermal R value and as expensive as all get out.

https://www.fireplacedoorsonline.com/air-tight-masonry-fireplace-doors.html

[J.P.M.]

RUA being a dolphin spending a year 800 feet under seems I recall the difficulties . Quite and decent food though. Well except crossing the equator can get a bit noisy and greasy.

Perhaps but better than air (drier) since a vacuum is much harder to maintain than a lower positive pressure across a flat surface.

Always wondered about a clear liquid insulator like Aerogel for windows. Too bad ceramic cannot be made crystal clear. Not sure how to convince you ICF is the ticket other than my 3 years living in an ICF home in the Tropics, so far has been very positive. No mold or mildew issues, rarely ever have to use AC and quite as a mouse which is important with dang MF'ing monkeys and parrots loose everywhere. Really would like to have shotgun down here.

My point was that inert gas in windows doesn't perform that much better than air, which, BTW for well made windows has been dried down to a - 40 deg. dewpoint for any injected air or any of the inerts. Besides, it's actually not so much the thermal conductivity of the gas in the gap, but rather the convection suppression as f(gap width). Ideally, the best gap is roughly 1 cm. for max. convection suppression. Wider and convection currents will start and any inert gas advantage from lower thermal conductivity will be wasted. Narrower and thermal conductivity starts to negate any benefit of any gas.

As for vacuums, a near perfect vacuum is needed to suppress convection. Believe it or not, but several of the open literature sources show that even a vacuum of EE - 5 bar - pretty high - will (re)establish normal levels of conduction across a gap. IF a FULL vacuum can be established and maintained (a real trick to maintain for a residential fenestration system as a practical matter), one trick is to make the gap ~~0.5 mm (since no convection will take place, the gap width doesn't matter any more) and support the lites with small and mostly invisible glass beads. The devil is again maintaining the FULL vac. with no leakage.


As for aerogel, which is ~ 4% silica foam and 96 % air, the foam cells trap air, suppressing convection and radiation heat transfer as well. Great idea that's been around since the late '70's, but my understanding at this time is that it creates a haze due to light scattering that's noticeable, but I haven't seen the product in use so that's hearsay from me until I find out more.


As for ICF's, I didn't claim ICF's are not the ticket. I did write that ICF's have promise and are a real improvement. No bugs, a lot less infiltration and quiet as a tomb. I'm a fan. I get it and have for a long time, but I'd be more convinced of their structural longevity and acceptance as a general building concept for residential if I hadn't seen the 4 demo houses in Borrego Springs that developed cracks, settling and some other problems that looked associated with builder unfamiliarity. The unfamiliarity and acceptance by builders and the public will be addressed as time goes on, but I'm still puzzled by some of the ICF system details and configurations available - like sections with insulation embedded/sandwiched between concrete sections. Also, 5 years ago my experience was that the cost estimates for a 3,200 ft.^2 desert home using ICF's was a bit more than poured concrete and exterior insulation, and I had more thermal mass and better insulation levels with the poured concrete. There were logistic concerns with the ICF's from the somewhat isolated environment and builder learning curves there, as well as CA title 24 part 6 issues with ICF's at the time, so that may not have been a fair comparison.

So, while not a complete naysayer, call me part of the friendly and trust but verify crowd on ICF's.

As for convincing me that ICF's are the ticket: Fill one up with melon breasted women who know a lot about creating vacuums and invite me to the party and I might have a change of heart.

Respectfully,