Lower utility bills and underlayment


#1

I have Hurricane Gustav to thank for the new roof I will have on my own house. Which underlayment would be best for hot, South Louisiana to help lower utility bills? I’ve gotten a lot of conflicting info.


#2

From what i know there is not an underlayment that will lower your heating and cooling bills.

Insulation and ventilation of the roof will do that, not underlayment.


#3

I thought there might be a radiant barrier/reflective underlayment that would help reduce heat gain. I know radiant barriers are usually installed on the underneath side of the decking, but didn’t know if there was one that worked well on top of the decking.


#4

Well Julie, the idea of reflective barrier assumes it is exposed to the sun and reflects the sun light. Underlayment by it’s nature should not be exposed. Best thing is ti install a light color metal roof. You will get your home to be much cooler. If you cannot afford metal roof, go with a light color shingle… but bear in mind that it won’t make your home that much cooler.

Other than that, gtp is 100% on the money… get insulation and ventilation in order.


#5

How would a reflective barrier help after it is covered up with shingles? You can however get different types of insulation to go over the decking which would help but venting the attic space seems to be the proper method or you could do both. The hot air in the attic needs to vent, I would work on that.

A friend of mine does heating and air and couldn’t figure out why his customers air was working fine but just wouldn’t cool properly, I installed powervents in the homes that didn’t have ventilation and it dropped the temperature in some 7 degrees.Just a little ventilation made his customers happy.


#6

[quote=“LAMetalRoofs”]Well Julie, the idea of reflective barrier assumes it is exposed to the sun and reflects the sun light. Underlayment by it’s nature should not be exposed. Best thing is ti install a light color metal roof. You will get your home to be much cooler. If you cannot afford metal roof, go with a light color shingle… but bear in mind that it won’t make your home that much cooler.

Other than that, gtp is 100% on the money… get insulation and ventilation in order.[/quote]

You need to read up on Radiant Barrier Systems please. The line I bolded in your statement is completely wrong.

Ed


#7

Here are some links I have saved.

Ed

ornl.gov/sci/roofs+walls/radiant/index.html

toolbase.org/Technology-Inve … t-barriers

ainsworth.ca/pdf/products/LuminOX%20Main.pdf

airtightinsulation.com/UserF … rticle.pdf

airtightinsulation.com/UserF … rticle.pdf

afs-foil.com/

savenrg.com/1faqpage.htm

savenrg.com/1rbschip.htm

greenbuilder.com/sourcebook/ … rrier.html


#8

Ed, i was not talking about radiant barrier, but ** reflective** barrier.

I was referring to a 'bubble wrap" type insulation with foil on one side. The reflective foil is useless when it does not reflect sun. Other insulating properties were not discussed.


#9

Sorry,

I misread due to the topical theme. I should have and did know that you were more well versed than that, and that is what threw me for a loop, so that is why I supplied all of those reference links.

Ed


#10

Ed, thanks for all of the links!

In terms of reflective/radiant barriers, I’ve seen new, modular houses that have the foil face facing the attic so the sun definitely does not hit it. I asked this building scientist who advises me if this way correct. He said yes that is was the proper installation. Seems completely counter to logic in that one would think that the sun needs to hit it for it to be effective, but not so.

Also, I’ve done quite a bit of research about venting of attics. Vented attics work well in our hot-humid climate as long as there is no HVAC equipment in the attic. If there is, then an un-vented attic is definitely the way to go.


#11

If there is HVAC equipment in your attic,You should definitely vent your attic.This will result in less stress on your cooling system.


#12

That may be true in some climates, but in hot-humid there is a bit of a myth that venting decreases the temp in an attic. The air brought into the attic is hot and humid, it does not dry out or cool down an attic.

An un-vented attic puts the thermal barrier at the rafters, not at the ceiling. Then, the attic becomes “semi-conditioned” and is considered part of the conditioned space. Sort of like a closet is. An un-vented attic can be anywhere from 7-20 degrees of the temp of the house. That is significant when you think of it. If I keep my house at 75 degrees, then my attic might be 85 degrees. Compare that to an un-vented attic that might be 125 degrees.

An HVAC system trying to operate in that level of heat creates all sorts of problems: increased loads and moisture developing from condensation.


#13

okay i misunderstood you.
you are referring to having an unvented with a thermal barrier.
I am very familiar with humidity and attics being here in Florida.
An unvented attic here can be upwards of 30-45 degress above ambient. A roof with proper soffit venting and ridge venting will be within 15-20.
Also keep in mind what type of roofing you are putting on. Keeping that much heat at deck level can fry out shingles prematurely.


#14

Chris, I don’t think that you have your temps correct. Of course, it depends upon the roof covering, etc. I did hijack this thread into talking about un-vented attics. Un-vented attics, by definition, have the thermal barrier at the rafters and are cooler–by far–than vented attics. That’s pretty much the way they are in hot-humid climates. Florida would certainly apply!!


#15

Let me correct the above. A properly installed un-vented attic which means spray foam at the rafters. (although, there may be other materials that work for insulation).

I guess there could be a situation where a house has an un-vented attic with no insulation at the rafters. THAT would be an incredibly hot attic!


#16

a rare misread from etr.
LA is quite the adviser indeed.

and it looks like miss julie should be answering instead of asking.


#17

Julie,

May I offer you a position on this forum to answer Building Science questions. You are very good.

The only consideration of note that you have not thrown in, is leakage from the duct-work in the attic.

Did you read up on www,BuildingScience.org and especially the papers by Joe Lstirbuk per chance? It sure sounds like you have.

Ed


#18

I’ve read them. I consider myself a smart woman and reading building scientists is like reading a legal brief or doctor’s paper…very hard and boring and 1/2 the time I barely understand what they try to say!

I have begun to write some articles. I’d like to take the information from the building science world, which where we (builders) get our direction and translate it in a way that builders/homeowners can understand.

That sounds interesting. Why don’t you send me an email about it.