This is an offshoot from Hal's thread on the need for better classification of radiant barriers: http://homeenergypros.lbl.gov/forum/topics/different-classification...
I didn't want to drift on his, so I have started new.
Using the sciences to our advantage.
I have a camp in the planning stages (similar to David Meiland's question on the cost of rigid foam insulation), on posts and enclosed on the bottom. Here's David's http://www.linkedin.com/groupItem?view=&srchtype=discussedNews&...
Mine is a heating dominant climate as well, in Maine and close to water, thus the posts will provide a better view and some protection from seasonal high water on rare occasions.
Having a living space over a cold space is a frequent problem in cold country and often results in cold floors. Overhangs, porches, a bedroom over a garage and yes, my proposed camp are all examples. But all of these have one design consideration in common, the warm is stacked over the cold, a configuration that does not support convection. IMO, warm over cold needs some careful consideration.
Of the three modes of heat transfer, radiant, convection, and conduction, any time we can shut one or more down, we have reduced some major paths for heat transfer. Now, picture the floor of my camp design, foil under the flooring, I-joists, and air sealed cavities with no insulation. That's no insulation. There will be some conduction via the bridging and some radiant transfer from the upper portion of the I-joist (which could be detailed with some foil as well), but no conduction through the cavity, virtually no radiant transfer, and because the warm is stacked over the cold (almost) no convection. Add to this that any heat that does migrate down and warm the cavity air, the resulting convection would move the warmer air back to the top of the cavity. It's ironic, but filling these cavities with fiberglass could increase my heat transfer as air is a very poor conductor of heat.
The benefits of warm over cold are not new as our cooling climates benefit from it when the ac is running. Warm attic air does not migrate down to displace the cool air buried under our insulation. In a heating climate, that cold air we vent into our attics goes directly to the lowest point it can access and pushes any warm air it finds up and away.
Instinct says the floors will be freezing, but I can't identify the heat loss path. And if there is any benefit to adding a radiant barrier to the underside of these floors, even with just a small air gap and lots of insulation, I'd like to be able to calculate it.
All comments are welcome, I think.
Hi Bud! Thanks for starting this discussion, it is much needed (for me anyways). My last response to a post on Hals thread was made with the knowledge I've gained on how radiant barriers work and the physics associated with them - especially in hot, sunny climates in the south where I think they are more advantageous. I actually live in Des Moines, Iowa and even though we are in zone 5, I still think radiant barriers can be useful in this climate (especially after this historic drought and the unusual heat wave we've just encountered) but can even help in winter, which I will discuss some more below and hopefully address your point more specifically.
Here's my thinking - obviously during hot summer months in central Iowa, we are going to have maybe 5 good months where radiant heat gain is a problem, not 7 or 8 months like Phoenix will. Nevertheless, a properly installed radiant barrier will reflect a good majority of that back to its source. My approach to the different installation methods have followed this train of thought: For the hot, sunny climates in the south, it makes more sense to install radiant barrier on the underside of the roof rafters (leaving a void / air space in the stud bay as to allow reflection to take place). This becomes the first line of defense and can actually cool down attics by as much as 30 degrees.
In fact, I just installed 4300 SF of radiant barrier in this manner in an attic of a home in Phoenix in which their "pre-installation" attic temp ranged from 146 to 152 degrees F (I used my Flir infrared camera on a number of surfaces - joists, underside of roof sheathing, gable end walls, etc) and this was during the hottest stretch of August where the daily outside temp was averaging 112 degrees F. After the installation, we waited until the hottest part of the day (between 3 - 6pm) just like we did when gauging the initial temps and the inside attic temp was holding steady at 108 degrees! That is a huge difference and probably not the norm, but it illustrates the point. By the way, we air sealed all can lights and penetrations that were leaky....and the homeowners, like many folks throughout the country have their air handlers and a lot of ductwork up there - so cooling off the attic was a no brainer. Especially because he used a good portion of his attic for storage (laid plywood down) and could not add anymore cellulose unless he abandoned this idea...
And the reason I just mentioned this story is because I must respectfully disagree with you when you say that "warm attic air does not migrate down to displace the cool air buried under our insulation" because according to the second law of thermodynamics, heat always travels from warm to cold. Doesn't matter if the cold air is above or below. Since both of us live in a colder climate, I know we are familiar with the stack effect where through convection, warm air rises. But it will also move down through leaky gaps and cracks in certain conditions (like hot attics in summer) and displace the cooler air below. When we went through our initial energy inspection - we could feel a hot draft infiltrating in through many can lights and solar tubes skylights that were not sealed properly. That is why I never install radiant barrier without first testing and fixing air leakage and sufficient traditional insulation levels first, if applicable.
It gets a little trickier in a "mixed" climate zone like central Iowa because you would almost need to install the radiant barrier on both the underside of the roof rafters to combat heat gain in the summer, and lay it down on the attic floor in the winter to reflect the warm air back down into the living space where it belongs. There is a problem with dust, insulation and other particulates accumulating on the laid down radiant barrier and negating it's effectiveness, but part of my maintenance plan is to educate the homeowner to check it at least twice a year and use a leaf blower on it if they can. Otherwise, we also provide this service. Well, the other problem with installing it both ways in a single attic is the added cost that most people (here in the Midwest at least) are not willing to spend even when they understand the benefits of doing so. Maybe it's because of the economy. Maybe its due to the uncertainty of an election year. Maybe it's because we are all going crazy thinking it's the end of the world. Who knows, lol.
But to get back on track, I wholeheartedly agree with you about addressing all (3) methods of heat transfer! It's the Trifecta that will really make a difference. Consequently, there are (3) main ways to deal with those methods of heat transfer (even though there is some overlap and versatility in what each of them can do) but they are as we all know:
Radiant Barrier, Bulk Insulation and Air Sealing..
To help answer your last question - It is obviously not ideal to have a conditioned living space sitting directly over an non-conditioned space, but we all know it happens, whether due to budget constraints or simple design decisions. So, in your specific situation - I think a radiant barrier installed to the underside of the floor joists (on the cold side) could help some as long as all penetrations and air leaks were sealed and you had as much insulation as possible crammed into the joist bays. I know the studs will be thermal bridge and conduct some cold, but not much because as I'm sure you know - wood is a lousy conductor, hence the reason we've never seen wood wiring (Lol, Joe Lstiburek) but you could maybe apply a 1" thick piece of rigid foam insulation either under the subfloor or under the finished flooring? Radiant tubing may be out of the budget, but that would also be a solution to consider.
Speaking of Joe Lstiburek, here is a nice Q & A article on building science from Inhabitat:
Thanks again for starting the thread Bud and good luck!
Hi Josh, you are welcome for the thread, I just hope you are still talking to me after I have made some corrections.
In business and in life we should all try to identify our strengths and weaknesses. One of my weaknesses is trying to be tactful, as I tend to be blunt and put others on the defensive. So let me apologize in advance as I TRY to comment.
I believe you when you say "we could feel a hot draft infiltrating in through many can lights", but it was not the second law of thermodynamics which deals with heat not hot air, more likely a gentle breeze. If warm lighter air was moving downward to invade an area of heavier cooler air, then something was pushing it. In the absence of a breeze or a fan, cold air will push warm air up. We are all more accustom to the phrase "warm air rises" but in fact it is simply lighter and the cold surrounding air pushes it up, it's called buoyancy. There have been several discussion on "hot air rising" and I can dig them out if you would like.
So the warm over cold still stands as a static condition with the warm staying up top and the cold staying down below.
As for winter stack effect, it is the colder outside air pushing into the lower portions of our homes and forcing the warmer inside air up and out the top. There are equations with which to calculate the pressures involved and I have some diagrams to explain how the neutral pressure plane sets up to divide that pressure between a positive upper pressure and a negative lower pressure. Developing this understanding has been a real eye opener for myself and several others. Getting many to grasp it has been difficult.
A radiant barrier below the rafters in a southern zone with mechanicals in the attic has been shown to be a positive step. Placing one on top of the insulation in a heating climate has not. Besides the dust issue, if there has been extensive air sealing and sufficient insulation installed, then there is little left for the RB to save. Its cost vs savings becomes difficult to justify. In a mixed climate where a RB under the rafters can be justified, if it were double sided it would also serve to some degree during the cold season. But again, others will quote some examples where it was shown not to be worthwhile. But remember, you are working in the attic and I'm proposing an application below the house. Apples and oranges.
Your suggestions represent traditional thinking RB on the bottom and insulation in between. I'm still hoping to discuss RB below the sub-floor, air sealed, and no insulation.
Josh, based upon your interest I will post links as mentioned.
Hey Bud, no need to apologize. Sometimes I think we are all here for the constructive criticism just as the much as the education. I've learned something from you, so thank you.
I agree that air sealing and sufficient insulation should always be the first things to address when assessing a homes energy efficiency - (and weatherization) and from a cost perspective, I don't see why a homeowner wouldn't want to start there either. Those are two of the most effective things you can do, regardless of cost or not. And this is kind of an apples to oranges thing (and overall a 'green' thing) but I think solar PV and other renewable forms of energy can be ideal under the right conditions (and one day I hope that they'll be more accessible and not so cost prohibitive) but I knew we might never reach that point unless I got involved, much like yourself - to help push it along and do what simple things we can now and chalk it up to a cautionary tale for future generations. My feeling is that if you can afford to "go green" by buying PV panels and geothermal, etc.....then go for it,but it's still wise to button up your building envelope and make sure your house is a tight ship before throwing $20 -30k worth of equipment on it. Start with the basics. Build tight, ventilate right. Rant, over.
Just want to comment on this though....
"Besides the dust issue, if there has been extensive air sealing and sufficient insulation installed, then there is little left for the RB to save"
I still think RB provides value in that aforementioned scenario. That is why I provide both services, as well as RB. In the scenario you mention above (in reference to my RB on the attic floor method) Correct me if I'm wrong, but if there are three ways for heat to 'transfer, aka move' wouldn't some of the heat from a conditioned space below transfer via radiation, in direct proportion of how much of it transferred via convection (air sealing and insulation) and conduction (insulation). Radiation being the invisible energy in the form of heat that simply moves from a surface of a hotter temp to a surface of a cooler one. I think a RB could help reflect that heat back down and keep it inside the house where the homeowner wants it. It acts as a last line of defense against heat loss by working on radiant heat - while air sealing will work on convective heat loss and traditional insulation will work on both convective and conductive heat loss.
It appears that some just assume its a gimmick right off the bat, without further investigation while I feel it's worth defending..........so that's why I've been a little vocal about it - to stir it up a little.....get people talking about it so we can all learn more about it. But I also realize that there are ALOT of RB dealers out there who do believe its a magik silver bullet, know nothing of building science and are the reason some people are turned off by it and dismiss it without giving it a fair chance. We are also so accustomed to doing things a certain way - some things because they have been tried and true (in which case I say continue on) and others just because we have been told to do it that way, or are lazy, or we just haven't thought outside the box yet. That's the biggest hurdle I'm currently facing, is introducing this technology that is not really all that new but seems like it to many people. And it doesn't sound like you are standing in staunch opposition to RB, so I apologize if I come across as brash or rude - guess that's a weakness of mine.
I appreciate your reply, very informative.
:"If warm lighter air was moving downward to invade an area of heavier cooler air, then something was pushing it. In the absence of a breeze or a fan, cold air will push warm air up."
I agree SOMETHING WAS PUSHING IT ....
The weight of the atmosphere "above" was pushing it.
It is not necessary to have a breeze or a fan.
All it takes are openings in the lower portions of the house.
Openings that allow the cold dense air conditioned air to spill out of the the house.
AFTER the cold dense air spills out ... the density inside the house is reduced ... and the air inside the house can no longer "support" or resist the weight of the air above the house.
Thank you John. You can tell I'm a northern boy and not at all accustom to reverse stack effect.
I know that when the reverse stack effect sets up in a house and a normal stack effect sets up in the attic above, you end up with a negative (wrto) pressure at the attic floor and a negative pressure at the ceiling below. And indeed, the ceiling can be more negative.
maybe we should think of it as "Container Effect" instead of Stack Effect.......
we don't really need to "reverse" effect anything.
The Cold Dense Air intiates the action by either spilling In-to or Out-of our enclosures(containers)
The formula for determining the height of the Neutral Pressure Plane is the same for heating season or cooling season.....(temperature is in Kelvin)
LOL, renaming it is probably a good idea, but my track record for changing names is rather poor. Continuing to describe it as "stack effect" is a step back to the good old days (last year) when people thought hot air could rise all by itself. Now that we know better, chimney stacks, houses, and attics all need new wording to highlight that there is a new understanding.
Your response is chock full of misconceptions (I've even more blunt than Bud).
"according to the second law of thermodynamics, heat always travels from warm to cold. Doesn't matter if the cold air is above or below."
Yes, heat (the kinetic energy of molecules) equilibrates from warm to cold, but convection is not the same process. As Bud points out, it is a matter of warmer air being less dense and hence buoyant in colder air - in other words, it floats. Heat does not rise - it's isotropic, moving equally in all directions - but warm fluids (gasses or liquids) rise in a cooler environment.
"it will also move down through leaky gaps and cracks in certain conditions (like hot attics in summer) and displace the cooler air below."
Warm air can't displace cooler air, but any air will move under a pressure differential caused by wind or mechanical pressure (fan) or stack effect differentials, which is what you were feeling.
"in a "mixed" climate zone... you would almost need to install the radiant barrier on both the underside of the roof rafters to combat heat gain in the summer, and lay it down on the attic floor in the winter to reflect the warm air back down into the living space where it belongs."
A radiant barrier doesn't "reflect warm air back down". It may serve as an air barrier, but that's completely independent of its radiant qualities. A single attic radiant barrier will work in both summer and winter, and the only sensible location is under the rafters with the shiny side down to prevent dust accumulation, but it will work poorly in winter because there isn't enough surface temperature difference to make radiant transfer a problem in terms of heat loss upwards.
"I think a radiant barrier installed to the underside of the floor joists (on the cold side) could help some as long as all penetrations and air leaks were sealed and you had as much insulation as possible crammed into the joist bays. I know the studs will be thermal bridge and conduct some cold..."
A radiant barrier under the subfloor, with shiny side down, will noticeably decrease heat loss downward by radiantly decoupling the floor from the ground below. If insulation were "crammed" up against the radiant surface, there will be no radiant decoupling and no effect.
Thermal bridges don't conduct cold, they conduct heat (by the Second Law).
"wood is a lousy conductor, hence the reason we've never seen wood wiring"
Maybe this was meant as a joke, but thermal conduction and electrical conduction are two completely different processes.
"Radiant tubing may be out of the budget, but that would also be a solution to consider."
Putting hydronic tubing in the floor assembly would make the floor warmer but would dramatically increase heat loss downward if there is no insulation below it, as radiant heat moves to the coldest surface proportionately to the fourth power of the temperature difference.
Here's a question to work on. What is one of the most effective and decorative radiant barriers and where can it be used to our benefit?
Bud, my hot/humid climate perspective.....I would say a vented roof with an ample overhang
...because the roof "sees" the most radiation?
except you are from a cold climate... so hmmm..... I give up
A mirror. Although the outside of the home is a really great place to block the sun's energy. There is a company called Moss Inc that started up here in Maine making decorative fabric art. Well half art and half function, but their materials would provide a moderate level of shading and they were very stylish. Their web site now is exotic, but I bet they still have something like their initial products. Point being, when trees are not available for shade, this approach would be stylish and functional.
As for the mirror, when one wants a RB on an interior wall, you can't get much more reflective than a mirror.
Hi Bud, I am attaching a chart from a 70's textbook (Concepts in Thermal Comfort)
according to this chart
R-value of the air space is "better" when heat flow is in the down direction...
and "better" when it includes a reflective surface
but it seems to me that with approx 4" air space ...you are only getting about R-1 per inch
and as the air space gets larger the "R" per inch goes down
my take is that after a very small air space...you would be better off to use Ample Insulation.