just a curious question
lovecoldair
Save a Little
I would think so if your ducts are in the attic.
Maybe a reduction of 7%.
Instead of 140'F maybe one would have 110'F at best
So duct cooling loss would be reduced to
110- 60 / 140 - 60 = 50 / 80 = 63%.
37% Savings
Duct cooling loss might be 18% of the total cooling energy.
.37 x .18 = .067
7 % savings of $500 annual cooling cost would be about $35 / year.
I would think so if your ducts are in the attic.
Maybe a reduction of 7%.
Instead of 140'F maybe one would have 110'F at best
So duct cooling loss would be reduced to
110- 60 / 140 - 60 = 50 / 80 = 63%.
37% Savings
Duct cooling loss might be 18% of the total cooling energy.
.37 x .18 = .067
7 % savings of $500 annual cooling cost would be about $35 / year.
Assuming you are talking about a powered attic ventilator, aka, PAV.
If the floor of the attic is tightly sealed from the conditioned space, and there is enough passive ventilation for the fan to pull air into the attic, you may see some slight savings. Studies have shown that, in most cases, the fan costs about as much to run as it saves.
If you have any combustion appliances, like furnace and/or water heater, that are located in the attic, or draw combustion air from the attic, a PAV can be very dangerous, or at least shorten the life of the appliance.
Most homes are not built with the attic floor sealed all that air tight from the conditioned space in the home, so a PAV will draw air out of the house. The quantity of air drawn from the house will vary from house to house.
If your home has several different ceiling heights, a bunch of recessed light fixtures, etc., a PAV is likely a VERY bad idea from an efficiency and IAQ standpoint.
I have seen homes that were being depressurized by PAV's so bad that they did not cool well, gas water heaters in the attic were being backdrafted, and the home had huge dust problems.
Even with a reasonably tight home, a PAV will virtually always increase infiltration in the conditioned space.
Trying to deal with attic heat by ventilating the hot air is backwards anyway. Most of the heat that enters the attic is radiant heat from the roof. The hot air is what you get after the radiant heat energy heats all the objects and surfaces in the house. The objects and surfaces then transfer some of the heat to the air.
If you really want to make a HUGE difference, block most of the heat from entering the attic in the first place.
Have a radiant barrier installed on the bottom side of the roof deck. Foil is the most effective, but a good spray on barrier is usually the most cost effective as a retrofit solution.
http://www.ornl.gov/sci/roofs+walls/radiant/index.html
http://www.eere.energy.gov/consumer/your_home/insulation_airsealing/index.cfm/mytopic=11680
If the floor of the attic is tightly sealed from the conditioned space, and there is enough passive ventilation for the fan to pull air into the attic, you may see some slight savings. Studies have shown that, in most cases, the fan costs about as much to run as it saves.
If you have any combustion appliances, like furnace and/or water heater, that are located in the attic, or draw combustion air from the attic, a PAV can be very dangerous, or at least shorten the life of the appliance.
Most homes are not built with the attic floor sealed all that air tight from the conditioned space in the home, so a PAV will draw air out of the house. The quantity of air drawn from the house will vary from house to house.
If your home has several different ceiling heights, a bunch of recessed light fixtures, etc., a PAV is likely a VERY bad idea from an efficiency and IAQ standpoint.
I have seen homes that were being depressurized by PAV's so bad that they did not cool well, gas water heaters in the attic were being backdrafted, and the home had huge dust problems.
Even with a reasonably tight home, a PAV will virtually always increase infiltration in the conditioned space.
Trying to deal with attic heat by ventilating the hot air is backwards anyway. Most of the heat that enters the attic is radiant heat from the roof. The hot air is what you get after the radiant heat energy heats all the objects and surfaces in the house. The objects and surfaces then transfer some of the heat to the air.
If you really want to make a HUGE difference, block most of the heat from entering the attic in the first place.
Have a radiant barrier installed on the bottom side of the roof deck. Foil is the most effective, but a good spray on barrier is usually the most cost effective as a retrofit solution.
http://www.ornl.gov/sci/roofs+walls/radiant/index.html
http://www.eere.energy.gov/consumer/your_home/insulation_airsealing/index.cfm/mytopic=11680
I have always wondered about that...it makes sense about the PAV pulling conditioned air up thru the attic and ventilating it to the outside...That would definately be a concern if you had too much cfm and not enough vent space to draw the air from, it would suck air from the 1st or 2nd floor up thru the attic...
Neophytes - do you remember what brand/style you used in your garage rafters? My Pop put in a A-frame wooden shed with a large porch/veranda in his backyard. He put in 3 all-weather ceiling fans on the vranda and has several rocking chairs out there. I told him he should have had a ridge vent put in at least over the porch area, but he hasn't yet. I could probably put the RB in myself and I bet it would cool things down some. I know when you first turn the ceiling fans on, theyu just bring down HOT air from the rafters and with the wood so hot up there, I bet rediant heat keeps things warm on porch sitters for a while.
OOC, I think any brand would do for what you want. Feel free to spend a few dollars on things for install convenience, such as foam or bubble core. The core makes it look different, is really a placebo in terms of effectiveness, but might make the stuff easier to handle or install.
Sometimes when I sit underneath a black restaurant umbrella and feel the heat radiating downward onto me, I wish they had RB there too. But that would be the type of thing an engineer would do not a decorator<g>.
Best of luck -- Pstu
Sometimes when I sit underneath a black restaurant umbrella and feel the heat radiating downward onto me, I wish they had RB there too. But that would be the type of thing an engineer would do not a decorator<g>.
Best of luck -- Pstu
PSTU - thanks for the reply. As an engieer, I would certainly agree that an RB on the underside of the umbrella (or shiny silver umbrellas) would be a wise choice. PLus, you can look up and see down the shirt of the blind date you're with to see how expensive the bottle of wine needs to be). 
If the attic vented already and you put fan on the roof. I would say yes.
If you NO vents ...........soffit vents, gable vents I would say no.... you will pull condition air up and out of the house. The exhaust fan is only half of what you need.
To complete the install you need a place for the air to enter the attic (in the attic) then your good to go.
If you NO vents ...........soffit vents, gable vents I would say no.... you will pull condition air up and out of the house. The exhaust fan is only half of what you need.
To complete the install you need a place for the air to enter the attic (in the attic) then your good to go.
Sorry, I missed these examples;
""Examples of Use of Present Value Tables
Example 1
I live in Orlando, Florida in an 1800 square foot ranch style house. I have R-11 insulation in my attic, and the air-conditioning ducts are in the attic. A contractor has quoted a price for a radiant barrier installed on the bottoms of my rafters and on the gable ends for $400. Would this be a good investment?
For this type of radiant barrier, the appropriate table is Table 4. For Orlando with R-11 insulation, the present value savings is listed as $0.32 when the air-conditioning ducts are in the attic. Multiplying this value by 1800 square feet gives a total of $576. This value exceeds the quoted cost of the radiant barrier of $400, and thus this would be a good investment.
Example 2
I live in Minneapolis, Minnesota in a 2400 square foot two-story house. I have R-19 insulation in my attic, and have no air-conditioning ducts in the attic. A contractor has quoted a price for a radiant barrier installed on the bottoms of my rafters and on the gable ends for $250. Would this be a good investment? Would investment in another layer of R-19 insulation be a better investment? A contractor has quoted a price of $564 for adding this insulation.
For this type of radiant barrier, the appropriate table is Table 4. For Minneapolis with R-19 insulation, the present value savings is listed as $0.08 when there are no air-conditioning ducts in the attic. Since the house is two-story, the ceiling area is 1200 square feet. Multiplying $0.08 by 1200 gives a total of $96. This value is less than the quoted cost of the radiant barrier of $250 and thus this would not be a good investment.
For adding another layer of insulation, the appropriate table is Table 6. For Minneapolis, this table gives a present value savings of $0.57 for adding a layer of R-19 insulation to an existing layer of R-19 insulation. Multiplying this value by 1200 square feet gives a total of $684. This value exceeds the quoted cost of the insulation, and thus this would be a good investment.
""
""Examples of Use of Present Value Tables
Example 1
I live in Orlando, Florida in an 1800 square foot ranch style house. I have R-11 insulation in my attic, and the air-conditioning ducts are in the attic. A contractor has quoted a price for a radiant barrier installed on the bottoms of my rafters and on the gable ends for $400. Would this be a good investment?
For this type of radiant barrier, the appropriate table is Table 4. For Orlando with R-11 insulation, the present value savings is listed as $0.32 when the air-conditioning ducts are in the attic. Multiplying this value by 1800 square feet gives a total of $576. This value exceeds the quoted cost of the radiant barrier of $400, and thus this would be a good investment.
Example 2
I live in Minneapolis, Minnesota in a 2400 square foot two-story house. I have R-19 insulation in my attic, and have no air-conditioning ducts in the attic. A contractor has quoted a price for a radiant barrier installed on the bottoms of my rafters and on the gable ends for $250. Would this be a good investment? Would investment in another layer of R-19 insulation be a better investment? A contractor has quoted a price of $564 for adding this insulation.
For this type of radiant barrier, the appropriate table is Table 4. For Minneapolis with R-19 insulation, the present value savings is listed as $0.08 when there are no air-conditioning ducts in the attic. Since the house is two-story, the ceiling area is 1200 square feet. Multiplying $0.08 by 1200 gives a total of $96. This value is less than the quoted cost of the radiant barrier of $250 and thus this would not be a good investment.
For adding another layer of insulation, the appropriate table is Table 6. For Minneapolis, this table gives a present value savings of $0.57 for adding a layer of R-19 insulation to an existing layer of R-19 insulation. Multiplying this value by 1200 square feet gives a total of $684. This value exceeds the quoted cost of the insulation, and thus this would be a good investment.
""
I had 2 calls this afternoon where I fixed the "not cooling well" problem by turning off the PAV's installed when the houses were reroofed in the last couple of weeks.
It seems most of the roofing companies are trying to sell them with a reroof.
Massive hail storm in April = tens of thousands of houses needing new roofs.
This could get interesting.
One of the houses had 3 solar powered attic ventilators, and a continuous soffit vent.
The ventilators had the house at a 5 pascal negative pressure, despite the continuous soffit vent.
The house also has 2 water heaters and 2 furnaces in the attic, the water heaters were being backdrafted whenever they came on.
Nice and cool in the attics though...
Neither house had any recessed light fixtures, and both are single story homes with the attic access the garage, so most of the "usual suspects" for an easy to fix major source of leakage between the conditioned space and attic are not a factor. That leaves the hundreds, even thousands, of holes in the top plates of walls for wiring and plumbing penetrations, and boxes for light fixtures, etc.
Good luck getting all that sealed in the typical existing home.
It seems most of the roofing companies are trying to sell them with a reroof.
Massive hail storm in April = tens of thousands of houses needing new roofs.
This could get interesting.
One of the houses had 3 solar powered attic ventilators, and a continuous soffit vent.
The ventilators had the house at a 5 pascal negative pressure, despite the continuous soffit vent.
The house also has 2 water heaters and 2 furnaces in the attic, the water heaters were being backdrafted whenever they came on.
Nice and cool in the attics though...
Neither house had any recessed light fixtures, and both are single story homes with the attic access the garage, so most of the "usual suspects" for an easy to fix major source of leakage between the conditioned space and attic are not a factor. That leaves the hundreds, even thousands, of holes in the top plates of walls for wiring and plumbing penetrations, and boxes for light fixtures, etc.
Good luck getting all that sealed in the typical existing home.
For a home with an even # of same sized PAVs, couldn't you also solve that problem by making sure an equal number blew into the attic as blow out?
This assumes that each fan is blowing the same amount of air (with solar, you'd have to make sure the panels were equally clean, pointed at the same place, and received identically times amounts of sun I guess).
This assumes that each fan is blowing the same amount of air (with solar, you'd have to make sure the panels were equally clean, pointed at the same place, and received identically times amounts of sun I guess).
Had some modular classrooms here, had power vented attics. was a complete disaster. I cut open the vapour barriers under the floors in the crawl space and gallons of water drained out. Collected condensate.
Seen some used ones built to US gov spec. Cooled by Bard units with a fresh air intake. Pressurizes the hell out of it, transfer grille in ceiling lets pressure relieve to attic space then out through an attic roof vent.
Seen some used ones built to US gov spec. Cooled by Bard units with a fresh air intake. Pressurizes the hell out of it, transfer grille in ceiling lets pressure relieve to attic space then out through an attic roof vent.
All I know is this..I shut my 2 PAV's off a few days ago, today is 99 and my house is more comfortable, and a little less humid...My upstairs is maintaining 76-77 and it could not do that last week with the PAV's running and the outside temp at 99...I have not ventured into the attic to see how hot it is, but if the ridgevents are doing the job it cannot be that bad...
There were times went I went into the attic after the sun set and noticed the attic was more confortable than my 2nd floor (With the PAV's running)
There were times went I went into the attic after the sun set and noticed the attic was more confortable than my 2nd floor (With the PAV's running)
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