Radiant Barrier (Dallas, Addison, Santa Fe: credit, home, transfer to)

[DPeter]

NRGpro, you have provided a lot of useful information and seem very informed on the subject of hot attics. I will be building a new home in the near future and have been researching attic insulation, among other energy conservation techniques. But now, I am thoroughly confused.

You state, "Radiant barriers placed up against your roof deck is the worst possible thing you can do aside from adding more insulation (sweaters in Dallas), in your attic." However, you later indicate you installed a radiant barrier (among other things) on a Dallas area house. You further indicate you have been building homes in Phoenix "without any insulation". I have read your reasons (twice) but this seems entirely contradictory to everything else I have read. Can you point to any studys that support your views? I could definitely save money by not adding the cost of insulation into my new home; but, again, it doesn't seem logical.

One other thing, you state, "Which way does hot air go? Up or down? Down." This is incorrect; hot air rises; ie, hot air balloons.

[loves2read]

it is NOT logical to build a home in this area w/o insulation...you are right about that...
and also about the physical attributes of hot air...

frankly I believe it would be smart to have radiant barrier decking or sheathing around all walls of a home as well as roof decking---
most of our HVAC costs come from using AC to cool interiors and since it seems that hot weather is going to be problem in future being efficient with initial construction seems prudent...
there is a new vapor barrier being used that has a covering of reflective material that acts to repel heat intrusion and conversely keep interior temp from migrating to external...
see if you can find something on -line...don't know the name but have seen it on some homes/commercial bldgs...
putting better R factor insulation into the walls helps with COLD weather
using radiant barrier helps with HOT weather...
getting good air ventilation in attic helps with preventing moisture buildup/condensation and venting hot air...
make sure you get wind turbines and their shafts are the largest--maybe 14" in dia. vs the smaller ones--much more efficient at moving air--and remember that hot air rises so the hot air IN the attic space would rise to the top and be drawn to outside--making the turbines move even when there is not lot of air to turn them from the outside...
try to get roof/eves vents fully open and like every foot on eaves--that helps intake cooler outside air--
having wider eaves (usually not part of the original design) provides shading for windows and cooler air to draw into attic...

[lh_newbie]

That post, when it got to "no insulation in Phoenix" cracked me up. Either it's a person that's just baiting people and having fun or someone that is completely full of themselves.

I don't think spray attic barriers are any good - I've seen data to indicate that it's better to just add insulation in the attic. There are commercials for places for spray on barriers - and they include 5" of attic insulation - do you think they're doing that because they're nice? Or because the barrier is a waste of money and to actually see a benefit, you have to add the insulation?

We added 12" of insulation to our attic. I've gathered heating/cooling bills before/after this - and while my sampling size is just one house over a couple years, after normalizing the data for heating-degree-days and cooling-degree-days, I can say that just the insulation has decreased our cooling bills by 20% and our heating bills by 30%.

Based on my experience, research of DOE reports and talking with folks in the industry, there are two methods that work best in Dallas and they generally fit into two categories:

Retrofitting a ventilated attic
Most attic space in DFW is ventilated - meaning the insulation sits on top of your ceiling drywall and that the furnace/condenser are sitting in unconditioned attic space, above said insulation. There are several items that must be addressed here when retrofitting a home for better efficiency: (1) increasing attic insulation - DOE recommends in our region about 15", (2) proper attic insulation - this includes adequate soffit vents (low vents to bring in fresh, cooler air) and ridge venting (vents at the top of the roof to allow hot air to escape because hot air RISES) and (3) sealed and insulated ductwork. Radiant barriers in retrofit situations are a waste of money IMO. Take that money and put it into an account to be used to buy a higher efficiency air conditioner when your unit is replaced - the money is better allocated there.

New Construction - SEALED attic space
If I were to build a home today, I would put the insulation directly under the roof decking and then run the wall insulation all the way up to the roof decking, creating a fully sealed attic space. This totally and completely eliminates the issue with duct leaking (even the best sealed ducts leak some 5% or so), as it will just leak into an insulated space. It also ensures your duct work isn't running through a space that is 20-30 degrees (or more) hotter than the outside temp (even a well ventilated attic is hot) will eliminate the energy loss through the walls of the duct work itself. It also eliminates the energy loss through attic stairs if you have them inside your home, since the attic is sealed. A non-energy benefit is that your attic (aka storage space) has a nice clean floor, free of insluation, and is semi-conditioned.

Incidentally, if I were to build a home today, I would site the home properly, ensuring a bulk of my windows faced North of South with an overhand; the home would be built with a sealed attic design, a metal roof and the walls would be ICF (insulated concrete forms). With this construction style, you can comfortably condition a 2500 sq ft home with only 2 to 2.5 tons of AC. Talk about a massive energy savings!

One more side note... talking about radiant barriers in a retrofit situation... we replaced our AC unit 2 years ago. We paid $6600. Our cooling bills were literally cut in half because we got a 16-SEER two-speed unit (has low and a high mode, roughly 3 tons in low; 5 tons in high) that runs in low mode 80-90% of the time. So when I say spend the money on energy efficienct HVAC instead of radiant barrier. If we would have split that same money between a less efficient HVAC (let's say 13 SEER) and a radiant barrier, I would be willing to bet our bills would be higher than they are today.

[DaveG99]

Radiant barreriers liek the foil type are way over rated. Im an estimator for a roofing company by the way.

[DPeter]

Ih_newbie - Thanks for the good advice. It does seem to make sense to install attic insulation on the underside of the roof decking. Keep the heat out of the attic in the first place. I wonder why that isn't typically (or ever) done. Guess you could still have insulation on the attic floor also.

[lh_newbie]

Originally Posted by DPeter

Ih_newbie - Thanks for the good advice. It does seem to make sense to install attic insulation on the underside of the roof decking. Keep the heat out of the attic in the first place. I wonder why that isn't typically (or ever) done. Guess you could still have insulation on the attic floor also.

Last summer I was helping with a Habitat for Humanity build. They are now doing sealed attics and using spray foam for the entire envelope (walls + underside of roof). These homes are, in fact, so well sealed, that the cold air return (where your HVAC system "sucks" air from the house) has a 4" line running outside, to pull in a small percentage of fresh air. Sealing a home too well can create what's called "sick home syndrome" - basically, stale air inside the home that decreases air quality over time as polutants and CO2 build up in your home. If you retrofit a home to do this sealed attic design, please keep that in mind.

For what it's worth, I don't have statistics on it, but I do think that sealed attic designs are starting to be used by some builders. As with any new technique, it takes a long time from proving that something works to actually getting builders to change their habits as this requires training.

Since our home was built in 1976, we just increased our attic insulation, sealed/insulated our ducts and increased attic ventilation (house already had two whirlybirds and two powered vents, but the soffit vents were clogged... we ended up replacing the soffits completely as some were damaged - we put in hardi-soffit so we have continuous venting around the entire perimeter). If you go into our attic on a warm, sunny day... you can actually feel the breeze of fresh air coming from the soffits. When the time comes to replace the roof, we'll install a metal roof which is a VERY effective radiant barrier... but probably more important... almost impervious to hail. Hopefully that will drive down our insurance rates.

Brian

[DPeter]

Yeah, I'm thinking of making the roof on my new home a metal roof, but the cost is nearly 3x that of a composition shingle roof. I guess the idea of adding the insulation to the underside of the roof is still applicable with a metal roof.

On another note, it seems like builders these days always want to put the HVAC unit(s) up in the attic but it seems like it would be more efficient if they were in conditioned space (hall closet). But, maybe not that much more since the ducting still goes through the attic. Then there is the noise factor, less if in the attic, and the maintenance factor, more trouble if they are in the attic. Decisions, decisions.

[lh_newbie]

Originally Posted by DPeter

On another note, it seems like builders these days always want to put the HVAC unit(s) up in the attic but it seems like it would be more efficient if they were in conditioned space (hall closet). But, maybe not that much more since the ducting still goes through the attic. Then there is the noise factor, less if in the attic, and the maintenance factor, more trouble if they are in the attic. Decisions, decisions.

Most of the efficiency loss is in the duct work being in the unconditioned space, less so for the HVAC unit itself. And regarding noise-vs-maint factor... Noise = I have to deal with every day; Maint factor = someone else has to deal with (excluding filter changes). Since I don't have to deal with maintance and it's not like they're being replaced all the time, that doesn't even factor into the equation to me.

[DaveG99]

In Florida they put the A/C units in the garage.

[Lakewooder]

I got this from a roofer (I'm replacing a couple of roofs on the rentals):

"In short, it works and is not expensive.

Radiant barrier decking will block around 95% of the radiant energy a house will soak up from the sun. Anywhere the aluminum underside is in contact with the frame, the energy will transfer and not reflect back out.

As a roofer, we will sometimes remove an old cedar shake roof under a layer or two of asphalt composition shingles. When we do this we will have a lath board that the cedar was nailed to. We remove ever other lath board to make the radiant barrier more effective, this method results in about 75% effectiveness which is what the spray on product claim.

I am a little skeptical of the spray on products claims.

The wholesale cost of the radiant barrier decking is generally around three dollars a sheet more. Our company charges around $20.00 per square more to upgrade to radiant barrier decking (but that would include a little more labor as well) New construction would have no additional labor."