Opinions on Radiant Barriers

[MarkF]

JMB,
Insulation is important for us too but just like humans, houses (and attics/roofs) have to breathe.  If you don't allow for it you end up with very hot attics in the summer and very wet attics in the winter.  Physics are the same everywhere.

 

[jmbfestool]

JMB,
Insulation is important for us too but just like humans, houses (and attics/roofs) have to breathe.  If you don't allow for it you end up with very hot attics in the summer and very wet attics in the winter.  Physics are the same everywhere.

I wasn't debating the fact the roof needs venting but just mentioning how we try and keep houses warm you try and keep it cold thats all I was saying.

We need to do the same we need to keep the roof vented even though we want to keep it as warm as possible but if we don't vent we will get damp and dry rot.

JMB

 

[Silverado2312]

HowardH,

We live in Weatherford, TX and built our home in 2002 the builder installed radiant barrier TechShield. as our roof decking.

We have been very pleased with it.  Our electric bills in the summer are lower then friends that have much smaller homes then we do that were built at the same time or are newer.
When the builder was building the house he commented more then once how much hotter our roof was then other houses he had built.  The theory is that it reflects a certain type of sun ray that produces heat. 

We have friends from other parts of the county that have no idea what it is......I think it is only used in the lower southern states.  As I type it is a sweltering 103 on my back patio.
Hope this helps!!
Goos luck with your project.

 

[Dane]

Just to add some perspective to this ventilation conversation-  I am an energy auditor and have spent quite a bit of time reading research surrounding ventilation, insulation, etc....There are some old myths about ventilation that still persist and do a lot of harm to a home's energy usage. 

The first is the idea that you need airflow up the surface of the roof deck to provide cooling to the underside of the shingles.  There have been studies that have shown that the temperature difference at the backside of the shingles between a roof with soffit venting and an un-vented roof assembly is 5 degrees.  Now, a roofing manufacturer will tell you that you will void the warranty if you install most shingles without venting.  They do that, not because the lack of venting causes shingle failure, but because roof venting is non-functional 90 percent of the time due to blocked soffit vents and then they have something to blame the shingle failure on and refuse the claim.  There is no evidence that a lack of venting causes shingle failure and there are a number of companies that are taking that out of their warranty restrictions because of being called out on the science.

So, you might ask, it's still 5 degrees and still cooler, what's the harm in having those soffit vents?  Well, the problem with the soffit vent is that it is improperly installed almost every time.  If that vent is not perfectly installed with hermetically sealed baffles that block all airflow into the insulation, then you are doing more harm than good.  Standard fiberglass or loose pack cellulose insulation has no resistance to airflow.  Basic building science tells us that most heat transfer through an assembly occurs via airflow, not conduction. So, if you allow even the smallest amount of airflow through that insulation you are reducing it's effectiveness by as much as 80 percent.

Airflow is the most mis-understood aspect of heating and cooling problems in a house.  Before you install radiant barriers, extra insulation, extra venting- you must make sure that you have airsealed the attic floor so that no exchange of air is happening between the attic and the living space.  If you aren't airsealing any tiny leak- at a light fixture, a top-plate, a chimney chase, or(worst of all) recessed cans- you are basically spending your money to heat and cool your attic as much as you are your living space.  That airflow carries with is huge amounts of heat and it seeps right through that insulation and into or out of your house.

After you have done the airsealing, then a properly installed radiant barrier can do some good in blocking the heatload radiating from the roof.  But that radiant load is far from the biggest problem- it's that air exchange.

Now, as to venting- a properly sized gable vent is a perfectly acceptable solution most of the time.  There is a simple calculation that will tell you what that size should be, anything over that is not doing you much good in evacuating heat and can cause problems with the stack effect in the home and increase that air exchange between the house and the attic.

I could go on and on- but a couple of good books you might want to take a look at are: Residential Energy by Krigger and Insulate and Weatherize by the Taunton Press.  The Building Science Corporation also has a great website with loads of information on all aspects of building science.  Very good references for how to most efficiently address your home heating an cooling issues. 

I hate to see folks spend time and money on things that are not the most efficient way of attacking a problem.  Hope that helps.

 

[PaulMarcel]

Another product I have used is radiant heat reflecting (insulating) paints, primarily from a company which makes and markets them under the brand name Nansulate.   See www.nansulate.com which also contains testimonials and data.  I have used these special paints on outside walls under the finish color coat and on ceiling areas that do not have heat piping and on some aluminum window trim (which did stop sweating in winter due to condensation of humidity in the house).  As Warner Construction has stated, in our northern climate, underside of roof radiant barriers are not recommended.  The opposite is common practice in your area. 

Dave R.

Dave,

I've read about nansulate in the past and it looks like a great idea; basically reflect the long wave radiation away from the house before it even gets to it.  Naturally statistics and testimonials can be made to sell anything so I'd like your perspective on the product.  Do you think it really worked?  Worked well enough to merit the cost?  If I remember correctly, you need a certified installer to paint your house with it as they treat certain areas like windows differently and they prime the surface specially (etched?)

I'm in Arizona and my garage shop is at 99 right now.  It'll hit 103 before sundown.  [eek]

 

[PaulMarcel]

Just to add some perspective to this ventilation conversation-  I am an energy auditor and have spent quite a bit of time reading research surrounding ventilation, insulation, etc....There are some old myths about ventilation that still persist and do a lot of harm to a home's energy usage. 

-snip!-

Nice post, Dane; thanks!

 

[WarnerConstCo.]

Radiant barrier's are really the best, like mentioned earlier, installed under the shingles or steel roof.

thanks for the good explanation dane.

guys can always go to building science and check it out there.