Radiant barrier vs add'l insulation
Last Post 04 Jun 2009 08:28 AM by Dana1. 11 Replies.
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sastexanUser is Offline
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22 May 2009 01:51 PM
My wife and I started getting bids to add insulation to the attic of our 1951 rambler.  The current blown insulation in the attic is between R-12 and R-20, not enough for the Washington DC area.  One of our neighbors with a similar house recommended a contractor who put a radiant barrier on the floor of their attic on top of the existing insulation - similar setup to ours.  The neighbor swears their heating and A/C bills dropped, less dust in the house, much more comfortable.  I'm dubious though - with that little insulation this would be effective.  Plus, one contractor told me that radiant barriers are less effective when they get dust on them - which it would, being on the floor of the open attic.

In addition, I put in 8 soffit vents to add to the 2 gable vents last year, plus a powered attic fan on a thermostat, which seemed to help slightly on days below 90 but over 90 our upstairs level was just blazing hot, unable to get cooled by the A/C (which is a separate issue - old compressor and crummy ventilation).

Does anyone on here have experience with radiant barriers on the attic floor?  Or knowledge if this solution is less effective than just adding more blown in insulation?  The cost is similar to bring the attic up to R-30 blown in.

Thanks!
wildblueUser is Offline
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29 May 2009 01:58 AM
If you only have R20 you NEED more insulation. Do both, the radiant barrier and the insulation but do the RB yourself, it's easy.

Here is what I'm doing.

I am also going to get additional insulation blown into my attic because of heat transfer to the conditioned space from it. However I am doing all the prep work myself, then I will hire the actual blowing of the insulation out. Man hours are a bigger expense than the materials so all my contractor will have to do is show up, blow in the material and leave. I would say three hours tops.

I am going arround and sealing up every potential air leak from the conditioned space. Then I will install all the foam air ducts to keep the insulation from touching the roof deck and dams to prevent the soffit vents from being closed up by insulation. Then I am stapling up radiant barrier to the roof rafters and the gable walls leaving proper gaps for ventilation. The jury is still out on the dust issue but it doesn't matter if you staple the RB to the rafters.

Also in your climate that powered fan is probably doing more harm than good.

If you can only do one, do the insulation.
sastexanUser is Offline
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29 May 2009 08:24 AM
Why would the attic fan be doing more harm than good? It should be pulling only the superheated air in the attic out (I have the thermostat set to somewhere between 120 and 130 degrees), and the source is from the area of least resistance - the soffits and the gables. Sure, there are probably some ways for air to get from the conditioned space into the attic, but all the light fixtures are air tight (new) and I have sealed up the attic access panel with foam board and insulation.

I know I need more insulation - but the question is, would the radiant barrier take the place of additional insulation since it also would help seal, and is the dust really an issue?
Dana1User is Offline
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29 May 2009 11:21 AM
Posted By sastexan on 05/29/2009 8:24 AM
Why would the attic fan be doing more harm than good? It should be pulling only the superheated air in the attic out (I have the thermostat set to somewhere between 120 and 130 degrees), and the source is from the area of least resistance - the soffits and the gables. Sure, there are probably some ways for air to get from the conditioned space into the attic, but all the light fixtures are air tight (new) and I have sealed up the attic access panel with foam board and insulation.

I know I need more insulation - but the question is, would the radiant barrier take the place of additional insulation since it also would help seal, and is the dust really an issue?

Numerous studies FL in TX have shown that powered attic fans draw conditioned air into the attic in the vast majority of cases, defeating the energy savings.  Unless you've foam sealed the entire attic floor including all plumbing & electrical penetrations it's likely to be net-negative from a total energy use point of view.  Even in the idealized cases you won't reduce your cooling energy use by more than ~5%. Powered active ventilation is highly over-sold. See:

http://www.fsec.ucf.edu/en/publications/html/FSEC-GP-171-00/

"...the powered ventilation does not typically result in a net energy savings for powered vent fans unless the attic is uninsulated. Under this scenario, other means of controlling attic heat gain are preferable and more cost effective than forced ventilation."

Radiant barrier WILL cut the radiated heat hitting the upper layer of insulation or any exposed joists.  In a 120F attic the conducted heat through the insulation is less of an issue than radiated heat from the 150F roof deck.  Installing the RB on the rafters reduced the dust-accumulation factor, but that factor is much exaggerated. Even dirty RB retains better than 50% of it's' effectiveness. Putting it on the rafters takes more material, but it'll retain it's full effectiveness longer.

If laid on the floor it will also form a secondary air-barrier, which may be useful.  But that air barrier is more cheaply achieved using permeable housewrap. (Under no conditions should you put non-permeable types of radiant barrier down on the attic floor- it will cause winter-time condensation issues in the insulation in your climate zone. Only if it's installed on the conditioned-space side should bubblepack or un-perforated foil types of RB be used.)

If your attic floor insulation doen't cover the tops of the joists you will get  significant benefits both winter & summer by blowing 6" of cellulose over the tops to provide a thermal break against the thermal short-circuit of the joist.  Blown fiberglass will also help but it's translucent to radiated heat, and allows more convection-losses during the winter, losing R-value the colder the attic gets.  Cellulose has fewer convective losses (even gains R-value slightly with higher temperature differences), and is opaque to radiated heat.  If your blown insulation is fiberglass and it covers the tops of the joists, as little as 3" of cellulose overblow can fix the bulk of the convective-loss & radiative-gain issues with the fiberglass making it more effective, both winter & summer.

This is a lot to think about for sure, the thicker the insulation, the less net-benefit you get out of radiant barriers.  If it's over R35 and something other than fiberglass the benefit of radiant barriers are pretty limited.  (But even R38 fiberglass will still perform better with RB though.)
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29 May 2009 01:52 PM
Posted By sastexan on 05/29/2009 8:24 AM
Why would the attic fan be doing more harm than good? It should be pulling only the superheated air in the attic out (I have the thermostat set to somewhere between 120 and 130 degrees), and the source is from the area of least resistance - the soffits and the gables. Sure, there are probably some ways for air to get from the conditioned space into the attic, but all the light fixtures are air tight (new) and I have sealed up the attic access panel with foam board and insulation.

I know I need more insulation - but the question is, would the radiant barrier take the place of additional insulation since it also would help seal, and is the dust really an issue?
If you have AC ducts in the attic, then a power vent is not good. However, if your HVAC system supplies are from underneath , like many northern homes , then the attic fan is OK.
in your climate I would opt for additional blown insulation, if you use a radiant barrier attach to the bottom side of the roof. You neighbors probably laid it on the floor because that is the easiest.


Chris Kavala<br>[email protected]<br>1-877-321-SIPS<br />
Dana1User is Offline
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29 May 2009 05:17 PM
According the the body of published study, even with the air handlers & ducts located elsewhere, the power used by the fan to reduce the cooling load is greater than the power saved whenever the COP of the AC system is 2.5 or better. Money spent on purchasing, installing & operating attic fans is better spent on more insulation in any climate zone, or insulation + radiant barriers in mixed-humid or hot climate zones.

Attic fans/whole-house fans used for night-ventilation in lieu of AC can be worthwhile in mixed-dry climates (portions of California, etc.) with low latent loads and good nighttime radiational cooling (during the day it adds to the power load even if the compressor runs a lower duty cycle.) In humid regions it often just raises the latent load, even if the sensible load may/may-not be reduced (depending on actual conditions du jour). I doubt it's worthwhile in DC though.

This subject has been studied to death- read some of the studies referred to in this article:

http://www.fsec.ucf.edu/en/publications/html/FSEC-GP-171-00/

This type of system could be great in low latent-load climates though:

http://www.davisenergy.com/technologies/nightbreeze.php

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29 May 2009 06:49 PM
Don't mix the entirely different concepts of whole house fans and attic fans.

The former allows you to turn off the air conditioning when it is mildly hot whether there is sufficient natural breeze or not.  In the humid and sometimes hot Michigan, mine allows me to almost never use AC.  Plus I find flowing outside air far more pleasant.

People with allergies to pollens, noisy neighbors and burglars might want a more complex system.

People in dry climates can combine whole house fans with misters for even more low cost cooling.









Dana1User is Offline
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01 Jun 2009 12:28 PM
There is a now-classic 1985 study that is something of a trifecta of what I'd posted previously (with way too much verbiage, showing the negligible energy benefits of active attic ventilation, the positive cooling benefits of radiant barriers in attics, as well as cooling season performance differences between cellulose & fiberglass in attic applications. It can be found online here:

http://txspace.tamu.edu/bitstream/handle/1969.1/6841/ESL-HH-85-09-33.pdf?sequence=3

Bottom line- if it's a choice between bumping up the insulation to R30 using CELLULOSE (not fiberglass or rock wool) and installing a radiant barrier for the same money, go with the cellulose- it'll give you year-round benefit and achieve the same or better cooling season performance of adding radiant barrier.

And unplug the attic fan- if it was doing anything at all positive for you before, it will be doing even less after you've overblown 3-6" of cellulose.

As for the whole house fan- issue yes, they are another animal- more like a mutant attic fan on steroids(?). :-)

But they are similar in concept & technology- you're removing sensible heat from an enclosed space using air as the working fluid. In the attic case it's intent is to remove superheated air from unconditioned attic space displacing it with cooler outdoor air. With a whole house fan you're removing heat from the conditioned space with cooler outside air a condition that occurs much fewer hours out of a typical day than the attic space situation, yet with lower delta-Ts to limit it's efficiency further. From a typical as-used efficiency POV they're remarkably more similar than different. In the attic case, the high delta-T makes the attic cooling more efficient, but the net effect on conditioned space cooling load is blunted by good insulation, and reversed if there is much air-infiltration. In the whole-house fan case the delta-Ts are quite low making net-efficiency marginal (even negative in humid areas, since latent loads aren't controlled.)

While it's possible to run whole house fans in the best/most efficient manner and beat central-AC efficiency, it's pretty clear that many/most people don't. Lots of people run them even when it's hotter outside than inside, raising the indoor air temps for the skin-cooling effect of the breeze, only to later turn on the AC once it becomes unbearable. The same people leave the windows open during the day in the mis-perception that it's keeping the house cooler for similar reasons. Sophisticated controls with both interior & exterior humidity & temperature sensing could probably make that more foolproof, but it may take an educated user to use it. (I'm not sure how much they've simplified the NightBreeze system to keep it simple for the user.) Ceiling fans (open windows or closed, depending on conditions) in occupied rooms can give you the same moving-air aesthetic & higher-temp comfort levels for a fraction of the power use of a whole house fan in low cooling load conditions.

There's a pretty-good discussion of what's realisically achievable here:

http://www.fsec.ucf.edu/en/publications/html/fsec-pf-273-92/index.htm

In heating dominated climates (~4500HDD or above) the wintertime heat loss through poorly designed or poorly installed whole house fans is huge. If you're going that route, insulating it and testing it for air leaks every few years is advisable. Even the best of louvered systems are leaky. The best whole house fans have insulated doors on the exterior side with good weatherstripping (eg. Tamarack Technologies). Many louvered systems even when closed represent a leakage equivalant of a 6" diameter hole in the ceiling with a 1' insulation clearance around it- hardly a net "energy efficiency" enhancement measured on an annual basis, even if operated perfectly in the cooling season. If you're going this route go with the smallest, best insulated version you can, and be sure to check the air tightness, don't assume it.

Whole house fans are getting better- ~500W used to be pretty typical, now you can get 'em with multi-speed high efficiency motors in the sub-300W range. Still, even modest-efficiency air conditioners achieve double-digit COPs when operating at the negative-delta-T hours where a whole house fan can actually cool the house- the net savings are far lower than the nameplate ratings might imply. Whether and where a whole house fan is actually more efficient is highly site & system dependent. The current rule-of-thumb is that when the average daily temps are ~85F or more whole house fan energy savings disappear completely. But that's based on 30 year old data- houses are better insulated, tighter, and both AC and fan efficiencies have improved- it's a LOUSY rule of thumb at best.

If you have a gas/propane/oil fired hot water/heating systems, it's important to test that a whole house fan can't induce backdrafting (sucking the exhaust into the house) if the fan running with all of the windows closed. (Sucking in pollen & humidity is one thing, but carbon monoxide is quite another.)

Misting or evaporative cooling "swamp coolers", AZ style are definitely more efficient, but can raise the latent loads to uncomfortable/unhealthy levels in all but the driest of climates.
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03 Jun 2009 01:24 PM
The long of the short of it is this. Yes, put more blown insulation in your home, keeping in mind that after R19 (Which slows 85% of conductive heat transfer) is going to be nearly effective as R30 which is only fractionally better ( a few percentage points at best). Radiant barrier will be of some help during the winter months by reflecting the radiant heat from your homes interior back towards your home. The pay off in radiant barrier is during the summer months. Foil radiant barrier will reflect 97% of radiant heat, allowing only 3% to enter your home (Spray / Paint types are inferior only blocking 70% of radiant heat). By installing radiant barrier foil over your existing insulation, you insulation will heat up to ambient temperature (whatever your outside temperature is) as opposed to the 140-150 degree attic temperature.
It's the equivalent of placing your entire house under a shade tree. EPA states that Radiant barrier will save homeowners (nationally) up to 17% on their heating/cooling bills. The better radiant barrier products are all EnergyStar rated. (For good reason - they save money).
I hope that helps.
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03 Jun 2009 05:15 PM

Posted By RadiantBarrier.com on 06/03/2009 1:24 PM
The long of the short of it is this. Yes, put more blown insulation in your home, keeping in mind that after R19 (Which slows 85% of conductive heat transfer) is going to be nearly effective as R30 which is only fractionally better ( a few percentage points at best). Radiant barrier will be of some help during the winter months by reflecting the radiant heat from your homes interior back towards your home. The pay off in radiant barrier is during the summer months. Foil radiant barrier will reflect 97% of radiant heat, allowing only 3% to enter your home (Spray / Paint types are inferior only blocking 70% of radiant heat). By installing radiant barrier foil over your existing insulation, you insulation will heat up to ambient temperature (whatever your outside temperature is) as opposed to the 140-150 degree attic temperature.
It's the equivalent of placing your entire house under a shade tree. EPA states that Radiant barrier will save homeowners (nationally) up to 17% on their heating/cooling bills. The better radiant barrier products are all EnergyStar rated. (For good reason - they save money).
I hope that helps.

That's a real knee-slapper! 

There's no way in heaven or hell that the radiant barrier can somehow intuit what the outdoor ambient temperature is.  It only reacts to the radiant environment of the attic which is NOT a direct function of the outdoor ambient.  With or without the radiant barrier the top of the insulation WILL heat up to the attic-ambient, but  not the roof-deck temp (which is the radiating source.) 

But if the insulation is translucent to infra-red (like fiberglass is), radiated heat from the roof deck penetrates to significant depths, reducing the apparent R-value. Cellulose is IR-opaque- the cellulose alone will block the radiated heat effectively even without a radiant barrier.

The classic study demonstrates this thesis:

http://txspace.tamu.edu/bitstream/handle/1969.1/6841/ESL-HH-85-09-33.pdf?sequence=3

Yes, add insulation- preferably cellulose. With fiberglass the net effect of a radiant barrier is large, but with even as little as 2" of cellulose the bulk of radiated heat is already blocked, reducing the net benefit of an additional radiant-barrier layer to a whopping R3.

R3 is not nothing, if you only have R5 of blown insulation up there, but at R30 with 3"+ of cellulose as the top layer the benefit of the RB will be somewhat academic.  The only way you'll ever see a 17% improvement with a single layer of RB is in a completely un-insulated attic.  (That's the joy of using "...up to..." performance numbers, eh? :-) )  With blown R30 cellulose an RB would be reducing peak radiant gain by at most an academic 9%, with low single-digit effects on the actual cooling bills, best case.

And it's in no way the equivalent of (or effective as) exterior shading.

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04 Jun 2009 07:21 AM
"...keeping in mind that after R19 (Which slows 85% of conductive heat transfer) is going to be nearly effective as R30 which is only fractionally better ( a few percentage points at best)."

This statement is much like the one often used by spray foam vendors to argue that only a few inches of foam is enough. Never does one see an explanation of "percent of how much." In the above statement, if R19 "slows" 85% of conductive heat transfer (which I interpret to mean "reduces conductive heat transfer to 15% of what it was relative to some reference point"), then the reference point would be R=2.85. To say that R30 is only fractionally better than R19 is grossly misleading. Specifically, the conductive heat transfer through R30 will be 37% less than it will be through R19.

One has to convert to absolute heat transferred to judge if the additional insulation is cost-effective. For a mild climate, going from R19 to R30 might not be justified. In a cold climate, it certainly would be.
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04 Jun 2009 08:28 AM
In the D.C. area the ORNL zip-code based cost effective insulation level analyzer recommends bumping existing R19 attics to R38 in wood framed homes with gas-fired 80% AFUE type heating systems:

http://www.ornl.gov/~roofs/Zip/tmp/results28596.html

Forget R30- bump it up to at LEAST R38.

You can always insert your own zip code and come up with levels here (requires Java):

http://www.ornl.gov/~roofs/Zip/ZipHome.html

Some foam vendors/installers and some radiant-barrier vendors distort & exaggerate the data- sometimes they may even BELIEVE their own hype. But I tend to take the word of folks who actually measure stuff. There's a lot of data out there easily accessible via web-searches, and the US DOE has done a lot of actual measurement over the past 30 years- the models are getting better, the results more predictable, yet industry BS and installer arm-waving arguments persist...

DOE recommendations of insulation levels like that on the ORNL site are quite conservative from the cost/benefit point of view. If you plan to continue live there and pay the bills for another 10 years, going 50% over the DOErecommended levels is still likely to do better than break-even (depends a lot on what you expect utility & fuel prices to do over the next decade.) In this case it's probably worth finding out what it costs to bump it up to R50 instead of only to R30 (adding another 5-6" of cellulose over-blow). The added material cost just isn't that much, and the addtional labor isn't much either. Twice the R-value of blown insulation isn't anything like twice the price. A large part of the cost is just getting the crew there, prepping & breaking down tying up the equipment & crew for the afternoon or day. It doesn't take any longer to prep for or smooth off the top of R50 overblow vs. R30, but does add some blowing time and additional material. Bumping up attic insulation tends to be far cheaper/easier than walls, crawlspaces, or basements, and therefore more cost-effective.
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