Pros and cons of silver foil on rafters in attic
mcStHUSXoK
| Posted in General Questions on
What are the benefits and also the concerns for having this type of insulating done? Does it affect the life of the shingles? How about if you have any small leaks, say around your vent pipes. How would you know? There is a company here in VA that says they can do it for $1.50 sq. ft. Doesn’t sound like a bad price, but what potential problems could there be? Thanks for any advice.
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Replies
Randy,
Don't waste your money an an attic radiant barrier. The same amount of money would do much more good if you invested instead in air sealing work and additional attic insulation. Read more on the topic here:
Radiant Barriers: A Solution in Search of a Problem.
Martin,
Thanks for the article.
Randy,
The value or lack thereof of a radiant roof barrier depends on your climate zone, amount of sunshine, the color of your roofing and whether your attic is ventilated.
Radiant roof barriers can be an inexpensive way to reduce summer radiant heat gain into the house, but light colored roofing and attic ventilation and increasing the attic floor insulation are more effective.
A radiant barrier under the rafters will keep more heat in the roofing and hence somewhat reduce the lifespan of the shingles, and it will limit drying to the interior should there be a roof leak that saturates the sheathing (unless the rafters are vented from bottom to top).
If you have a dark roof and high AC bills, then a radiant barrier will help, but there are other ways to solve that problem more effectively.
Of course the primary downside of "silver foil" is the exorbitant price, which is why I recommend aluminum foil.
But, on the serious side, the proprietary bubble-foils work no better than plain old builder's foil, which is a kraft paper backed aluminum foil and is very inexpensive. The shiny side has to be facing down.
This can typically be purchased for around 20 cents a square foot - $1.50 to install is not a very good price. Typically a better value as a DIY project.
Randy,
Radiant barriers in general are pretty laughable, particularly given how they actually get installed in the field vs. a "perfect install" They are a very high-profit margin item for unscrupulous insulation companies, and as Robert says, there are much better (and better documented) uses for your money.
If you're in northern Virginia, then I suspect that you may be asking about Long Attic Mirror which is getting tremendously heavy ad rotation right now courtesy of their massive marketing budget. If that's the case, all I can tell you is "run, Forrest, run!" Both from the product and the provider...
Find a local home energy contractor or auditor (not a sales and marketing company) who actually researches their recommendations and can back them up with third party testing and you will be much better off in the end.
"Laughable" isn't a very quantifiable term.
In fact, a Florida Solar Energy Center study which compared different roofing materials with various venting ratios and with or without radiant barrier systems (RBS) found that a black shingled roof vented at 1:300 (vent area to ceiling area) with an RBS had a 6.1° reduction in attic temperature at design maximum summer conditions and a 26.4° reduction in maximum ceiling heat flux (insulation surface temp vs drywall ceiling temp) with R-19 insulation.
Given the ASHRAE determination that hot ceilings make occupants far more uncomfortable than other warm surfaces, that's a significant difference and not one to be laughed off.
Clarification: those temperature differences are between the black, vented roof with RBS and the identical roof without RBS.
Laughable is building a home in Florida with a black shingled roof
and only R-19 ceiling insulation
Thanks for all the comments. Our attic has a roof fan on a thermostat and blown in insulation. The rafters are also vented. We do have darker shingles. Our upstairs often stays hotter especially since the sun goes across the front of the house all day long. The rooms on the front of the house heat up much more than the ones on the back. I didn't know if this type of insulation would help with the problem. And I'm sure I would do it myself. Just didn't know if it worth it or not.
Randy,
you may have multiple problems (opportunities)
*the roof fan may be working against you
*poor airtightness
*unshaded windows...yada,yada
An Energy Audit is usually a good idea
John,
Could you explain your first two observations. How can the fan work against you? I can hear it running when the attic gets hot. On the second one, are you talking about the whole house, not just the attice?
We have bought nice double comb shades for two of the rooms on the front of the house. It does seem to be helping.
Thanks!
Randy,
here is a link warning about attic fans
https://www.greenbuildingadvisor.com/blogs/dept/building-science/are-solar-powered-attic-ventilators-green
Martin made a good comment about air tightness and more insulation early on.
Improving airtightness and adding insulation will help you during the cooling season and heating season
An Energy Auditor can diagnose and make recommendations for both.
if you have ductwork in the attic it needs to be tight and well insulated as well.
Concerning the interior window shades they do help a little... but stopping the radiation BEFORE it goes thru the glass is far better. .. sometimes this can be done with awnings, exterior shutters or trees
or new windows
Robert R
What type of insulation was used. R 19 is pretty weak insulation and fiberglass would be even worse. I have looked at the FSEC site before. Some of the things I noticed is that much of the data is based on homes with low levels of insulation and fiberglass insulation. I think the study's have dealt with homes built to code in years past and reflect a good portion of existing homes in the area.
When you focus just on the radiant barrier in extreme cooling climate and in homes that are under insulated, then a radiant barrier looks like a great solution. Now get rid of the fiberglass and beef up the insulation and a radiant barrier has a minimal effect.
If your ceiling/attic barrier is not sealed then that should get your first dollar. Next is adding insulation to at least the max for your location. In the deep south it is R30-49. If you have air sealed and are at the max insulation level then benefit of a barrier is not worth $1.50 SF. The farther north you go the value is even less.
At this point is probably makes more sense to spend that $1.50/SF on a different project.
Robert H,
I never suggested that a radiant barrier was worth $1.50 SF. I've recommended only builder's foil (kraft paper-backed aluminum foil) that costs less than $0.05 (a nickel) per square foot. Any homeowner with a staple gun can install this.
While it's not a substitute for air sealing and code-minimum insulation (there is no "max R-value" and the IECC standard for zones 2 and 3 is R-30 ceiling), it can be an effective part of a summer heat reduction strategy.
Robert R
Foil radiant barrier material can be had for $0.15/SF and your kraft faced is cheaper. As a homeowner project it might be a good deal.
Homeowners need to understand the best place to spend their money. I don't need to tell you but it is those with limited knowledge coming to this site need to understand what is the best course of action.
Several years ago when I started looking in to radiant barriers I found it hard to find good data on barriers. It was visiting many sites, reading reports, etc that I piece together my thoughts on barriers.
First if you are paying someone to install a barrier you are wasting your money, A barrier does nothing to stop convection or conduction. These are things that are costing you more money. If you have not taken the time to seal the attic floor then that is the place to start you project. Then top it off with lots of cellulose. After you have done that the benefit of a barrier is minuscule. At a $1.50 /sf that will go a long ways to pay someone for sealing and insulation.
To boil it down barriers work best when a property is not insulated. As the insulation goes up you don't need it. If you were going out in the cold would you want a regular jacket or a space blanket. With a good jacket you wouldn't feel the effect of the space blanket. Without a jacket the space blanket might be a lifesaver.