Will a radiant barrier reduce heat gain in the home?
GBA Editor
| Posted in Energy Efficiency and Durability on
I live in Oklahoma City and my home has an insulated cellulose attic floor with two whirly-bird vents. We have about 1500 square feet of attic. I was wondering if a radiant barrier would be beneficial to help reduce heat gain in the home?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
In most cases, a good air sealing job at the attic level and increasing the amount of insulation is the best value and should be done first. If your HVAC unit and ductwork is in your attic, a radiant barrier will help some but only after air sealing and insulating. Be sure you install it to the bottom of your rafters and not just lay it across the top of your ceiling joists.
Do you have soffit and ridge vents? They are better for eliminating moisture than the whirly bird vents.
If you have a light-colored roof, a radiant barrier won't make much difference. But with a dark roof, particularly a low-mass roof like asphalt shingles, a radiant barrier can dramatically reduce peak attic temperatures and that will reduce radiant flux and heat conduction into your living space.
But a radiant barrier is most effective when coupled with 1:150 roof venting (1 sf of net free vent area - split bottom and top - for each 150 sf of attic), which requires soffit and ridge vents, not "whirlybirds" or gable vents, because the same barrier that slows daytime heat gain also slows nighttime heat loss to the sky. Thorough and effective ventilation is required, then, to eliminate the stored attic heat.
What Danny said plus......
Don't forget to consider the Heating Season.
OK City is a Mixed Climate
Consider energy use over the WHOLE year.
I think good airtightness combined with extra insulation would serve you well during cooling AND heating seasons.
Todd,
We need to know more.
Have you noticed any problems? Are you having trouble keeping your house cool? Is your air conditioner running all day long?
How deep is your cellulose insulation?
Has anyone ever performed air sealing work at your attic floor? Is your attic hatch weatherstripped? Do you have any can lights in the ceiling under the attic? Are there any utility chases that run into your attic?
Most important: Is there any ductwork in your attic? Are the duct seams sealed with mastic? How thick is your duct insulation?
I read another discussion on this recently. Here's a link that came up:
http://www.ornl.gov/sci/roofs+walls/SteepSlopeCalc/index.htm
The bottom line seems to be that if you have a reasonable level of insulation on the attic floor, the attic temperature will have minimal impact on the temperature in the house.
We just moved into the home two months ago and it seems with the recent high temps 99-107 the AC runs very often, which I understand. My return ducts are in the attic, supplies are in the concrete slab. I have lots of dust which I'm guessing the return ducts are leaking. The duct work is not sealed with mastic and only insulate with R-4 duct wrap. The furnace sits in the unconditioned garage and Plenum leaks, the location prohibits me at this point to do any because it so restricted in space.
The insulation is only about 4" thick and poor done. As far as air sealing I have not done that yet, I've identified many location that need to be sealed.
My water heater closet is about 5' of the ground and has access doors to our utility room(conditioned space), and inside the closet is fully open to the attic. I've sealed the doors to the utility room to help with air leakage.
My home has soffits in the kitchen and built in closets which have lower ceiling than the main home. I can see all this location are not air sealed and not insulated.
Many of the older homes here in OKC don't have soffit vents at all. My has none just the whirly bird vents.
Thanks for all the great advice, I really appreciate it.
Light colored shingles or metal and shade trees would help. The radiant barrier also helps. Few contractors now how to install (shiny down so as not to acummalate dust). Recommend a diy approach. Be sure to have a goog balanced soffit / ridge vent in place. R. Hall TN home inspector
After hearing your description, a radiant barrier will do little good if you do not first tackle all the air sealing and increase insulation. Making sure you have a continuous air barrier is the best bang for your buck and where you are going to get the best payback. Install a radiant barrier only after all the other measures are complete - duct sealing, air sealing, add insulation, soffit and ridge vent in that order. Be sure to do your combustion testing on the water heater you mentioned.
A Florida Solar Energy Center Study that measured roof, attic and ceiling temperatures with different roofings, venting and radiant barriers measured peak attic temperatures under a vented black asphalt roof of 135° on a 92° summer day, whereas the attic under a white tile roof was barely more than ambient temperature.
This represented more than a 40° variation in peak attic temperatures and attic/ceiling delta-T. No matter how much attic floor insulation is present, this difference in delta-T will certainly make a significant difference in heat flux downward. Additionally, there is often greater attic heat transfer through AC ducts than through the ceiling.
Todd,
You have provided a lot of scary details! You need to address a few very basic issues before you even consider installing a radiant barrier.
"My return ducts are in the attic." Get them out of there if possible.
"I have lots of dust which I'm guessing the return ducts are leaking. The duct work is not sealed with mastic." Seal your duct seams with mastic!
"The duct work is ... only insulated with R-4 duct wrap." If you can't move your ducts, at least insulate them better!
"The furnace sits in the unconditioned garage." The furnace should be located within your conditioned space.
"The furnace ... plenum leaks." Seal all leaks with mastic!
"The location prohibits me at this point to do anything because it so restricted in space." If necessary, build a small addition to your house -- a conditioned mechanical room.
"The [attic] insulation is only about 4" thick and poorly done." For heaven's sake, install a decent thick layer of attic insulation.
"As far as air sealing I have not done that yet." First things first! Address your home's air leaks.
"My water heater closet is about 5' off the ground and has access doors to our utility room (conditioned space), and inside the closet is fully open to the attic. I've sealed the doors to the utility room to help with air leakage." I'm not sure I understand this completely, but it sounds like a nightmare. Be sure there are no leaks between your unconditioned space and your conditioned space.
"My home has soffits in the kitchen and built-in closets which have lower ceilings than the main home. I can see all these locations are not air sealed and not insulated." Heavens to Betsy, Todd, you're facing a lot of work. And you're asking about a radiant barrier in your attic?
"Thanks for all the great advice, I really appreciate it." Glad to help!