Large two story house (5000+ sq ft).  Upstairs is difficult to cool down in this 10 year old house when outside temps are 100-115   Electricity in summer is less than $400. Trying to get the upstairs livable, but getting different opinions from different companies. Any suggestions as to the best bang for the buck?  Adding floor insulation (currently at R26), spray on RB, foil type RB, foam spray insulation, adding vents (apparently I am under-vented for size of attic).  Thanks for opinions.

Posted By bi0hazard on 13 Oct 2011 12:17 AM
Large two story house (5000+ sq ft).  Upstairs is difficult to cool down in this 10 year old house when outside temps are 100-115   Electricity in summer is less than $400. Trying to get the upstairs livable, but getting different opinions from different companies. Any suggestions as to the best bang for the buck?  Adding floor insulation (currently at R26), spray on RB, foil type RB, foam spray insulation, adding vents (apparently I am under-vented for size of attic).  Thanks for opinions.

Venting does almost nothing for cooling the roof deck or attic.  Power venting does, but it does so primarily by pulling conditioned space into the attic- forget venting. (Venting's true function is and always was for purging moisture, but in cooling dominated regions it may be doing the opposite.)

Taking the floor insulation up to R50-ish with a cellulose overblow on the existing insulation is usually cost-effective anywhere, if it will fit all the way out to over the top plate of the exterior studwall, and will provide more year-round savings than radiant barrier.  (R50 takes an initial blown depth of 15-16" from the top of the insulation to the gypsum below, and it'll settle to ~14" over a decade or so if they did it right.  Blowing over batts or blown fiberglass/rock-wool is just fine.)

The cost effectiveness of any of these measures varies with location.  See:  http://www.ornl.gov/sci/ees/etsd/bt...et2010.pdf

Note that at code-min R value the effects of adding radiant barrier are relatively small. But the type of insulation makes a difference- low density fiberglass doesn't cut it, since it's somewhat translucent to infra-red radiation, and 1-2" into the top layer the temperature will be several degrees above that of the attic air, under a 120F roof deck. With cellulose that doesn't happen.

If you have ducts/air handlers in the attic, sealing the duct leakage is key,  followed by insulation.  In "typical" Y2K construction duct leakage is blowing away 15-20% of the total air conditioning output.   In a 115F attic even with sealed ducts you'd be losing low double-digit percentage of the AC output to the attic, but if insulated to at least R8 it begins to come under control.

Spray foam on the roof deck and air-sealing the attic is pretty expensive, but also works well.  If you go that route, don't let the contractors talk you into removing the existing attic floor insulation except where they need to in order to properly seal the soffits it's all good. Also, pressurize the house with a large window fan and verify that you don't have air coming out the attic vents before you let them pack up & leave- it's remarkable how easy it is to miss stuff in an attic-sealing retrofit. In US zone 3 (DFW) or less you can use open cell, but in zone 4 or above you'd be required to use close-cell only to avoid moisture issues on the roof deck.

A couple of other thoughts:

When it's time to re-roof this house, consider using only third party tested CRRC rated cool roof shingles/materials. It's not usually an up-charge, but it limits you to somewhat lighter or less-saturated colors. See http://www.coolroofs.org/

If a lot of the heat gain is via windows & walls, there may be some treatment necessary there too. Exterior window shades can be quite effective, and may be necessary on the E and W sides. On the S side most of the energy summer sun is reflected off the window, but shading them with awnings makes a measurable difference. If the wall siding is a darker color, painting it a lighter color (particularly on the S & W sides) may be worth considering. Don't fall for any nano-sphere ceramic paint or additive scams- evidence of efficacy is lacking. (3rd party testing generally indicates that they aren't effective at all at controlling energy transfer in building assemblies & temperatures.)

Greetings,

I have yet to find anything better than a properly installed RB sys for this type of application.
But I need more info as to the construction.
Is it:
1 1/2 story.
Standard 2 story.
Is it finished off. (drywall installed)
If finished, how much insulation in attic. what kind.


If a 2 story, finished, the following is appropriate.
In two stories the percentage of savings goes down because the ratio of wall to ceil'g area changes.

Add'g add'l bulk insulation will probably result in little to no savings and certainly will not improve the comfort level

Vents are for removing moisture and are minimal at best for lowering energy costs. Although, if you are under ventilated it can allow the moisture level in the insulation to be higher which increases the energy flow. (higher bills) Installing a quality ridge and soffit vent would help. cor-a-vent.com

With the temps you are talking about the RB is much more efficient than bulk because as the temp rises the bulk insul becomes LESS efficient.

Following is an excerpt that comes from an article I wrote regarding heat gain and how to determine which is the best course. It is the drywall temp you want know because this determines the amount of energy being radiated into the house. Nothing else matters.

In summer you can determine the temperature of your ceiling drywall by taping a thermometer to the drywall surface.
This chart is based on a 75 deg floor temperature. The chart can be validated by using the emissivity data and formula from Mark’s Mechanical Engineering Handbook. FG values are for insulation between joists and include joist heat transfer. The RB value is for the joists surfaces covered with the RB and a furring strip to separate the RB from the drywall. “A” is the dry wall temperature. “B” represents the Btu’s radiated for the FG installation. “C” represents the Btu’s radiated for the RB installation. “D” the Btu difference between the FG and RB. Cellulose will be slightly better than FG.
Although the mechanics for side walls will be slightly difference this method can be used for approximate comparisons.

Summer Winter
“A” “B” “C” “D” “A” “B” “C” “D”
150 88 5 83 75 0 0 0
140 75 4 71 70 5 .3 5
130 61 3 58 60 14 1 13
120 49 3 48 50 22 1 21
110 37 2 35 40 31 2 29
100 26 1 25 30 38 2 36
90 15 1 14 20 45 3 42
80 5 .3 5 10 52 3 49
75 0 0 0 0 58 3 55

The 110 deg is high lighted to represent a 95 deg day. The 30 line is highlighted to show the similarities of the summer winter conditions. Note the jump when the temperature gets down to zero degs. Because of the rapid drop off in FG efficiency as the material thickness is increased it is difficult to extrapolate the RB and FG data for “R” value comparison. Compared to the advertised “R” value for FG the RB “R” factor could exceed “R”100 value by a considerable amount, and it is impossible to have a “R” value of 100 much less 100 plus.

In your conditions I would not be surprised if the ceil'g temp went to 120 degs.

In your situation the BEST method would be to install 1/4" furring strips to the bottom of the drywall along the joist line. Install a perforated single sheet RB to the strips. Install furring strips across the joist line and then dry wall. This should bring you down to about 5 btu/sq ft/hr. If you install the other RB over the existing insul it will bring it down lower. fifoil.com

Greetings,
The chart compressed during transmission. The series is four so just divide the line sequence in half and move second set to right to line up with header.

Thanks for the responses.

I called 4 companies over to look at the house (2 story, no drywall in attic).

Two were only aluminum foil only companies. Of course, the both said I need aluminum foil RB. One of them wanted to add floor insulation the other didn't. They both wanted to add vents, saying I am undervented (either replace my passive vents with 4 wind turbines or replace my non-functioning electric fan with a solar powered fan.

One spray-on RB company came out. Of course, he said aluminum foil is is no good, old tech... wanted to install spray on RB, insulation, and vents.

The interesting part was a company that installs both foil and spary RB. He actually recommended floor insulation, sealing ducts, and new vents. He said RB may not be needed, but I wanted extra comfort upstairs, he would recommend spray-on RB b/c its cheaper and would also help decrease temps.

So everyone wants to put in vents. I'm undecided about foil versus spray-on RB.

Dana, I think windows are a big problem on the west side of the house. Temps are 105-110 until 8-9 PM in the summer so I think this is letting in a lot of heat. My neighbors have solar screens on their windows. They look horrible (cheap) and they are super expensive (2K installation) . Do you think this is a good way to go instead of RB?


Thanks!

You need to understand the heat transfer. The sun is heating up the roof deck. The underside of the roof deck is transferring its heat mostly via radiant heat. The radiant heat is heating the ducts and much of it is passing through the fiberglass insualtion and heating the ceiling. The hot air in the attic plays a small role in heating the house.

Your comfort is being affected by 2 things. Higher air temps on the second floor and the increase in surface temps of the ceiling. I think surface temps of walls and ceilings are overlooked in the comfort equation. Keeping the surface temps lower will result in less radiant being tranfered directly to the occupants. More insualtion or radiant barriers will lower the ceiling surface temps.

Air sealing should be at the top of your list. In the summer the home is subject to a reverse stack effect. As you cool the air in the home is becomes denser and falls. It creates a negative pressure at the upper level and pulls in hot air from the attic. The cool air is escaping from the leaks at the lower points of the home. The stack effect is greater in taller buildins. Thus a 2 story home has more stack effect than a one story. Plumbing stacks, soffits and other "attic by-passes" are the largest leaks. I put it at the top of the list because it needs to be done before adding insulation and it makes the insulation work better.

Do you have valuted or catheral ceilings on the second floor?

R 26 is not enough insulation. Fiberglass insualtion is the most air porous insulation and allow is the least effective at blocking. Adding cellulose on top will block the radiant heat and does a better job of trapping air so it is a better choice. Shot for an R 40 or better.

Foil barriers stapled to the rafters would be an added layer. The Florida Solar Energy Center studies show declining benefit of radiant barrier as the level of insulation approaches R38. The barrier will save less than 10% and could be less than 5% when the attic is well insulated.

Ducts in an attic are a big problem. All the joints need to be sealed. Keeping the heat off the ducts is a problem. This may make the use of a radiant barrier worth the added expense.

Attic venting should accomplish 2 goals. Remove excess heat and keep the humidity levels down. Passive vents of 1sf of vent per 300 sf of attic floor will accomlish this. Power venting will pull air from the house and will not change the temps very much. Plus as stated above the primary heat flow is radiant which passes through the air and is not affected by venting.

Power venting as mentioned above is referring to fans that blow outward. Use two fans, one blowing in and one out and you aren't pulling anything from the house. Will it reduce temperatures yes - enough to pay for itself - probably not. If you have a fan at all, better to spend the money on a whole house fan that will allow you to go without AC in some weather.

I have cathedral ceilings.

I've been told by the 5th company that my insulation is at R38. Go figure. He wants to put more insulation, to get above R50. I've been told cellulose is a bad product compared to fiberglass, by company that installs both.

Every RB company that came out has told me that my intake/soffits are good but I need more exhaust/vents to cool the attic. Most installers want to put on turbines except one who wants to put a ridge vent along the whole length of the roof.

Any other opinions? I think Foil is better than spray on RB at this time. Not sure about vents. Probably will add insulation.

THANKS!

What is wrong with cellulose in their opinion?

Duct sealing is extremely important to net efficiency (and air quality- especially if it has fiberglass insulation, or is in a fiberglass-insulated attic.) If the ducts & air handler are in the attic, that's job-1, followed in short order by more attic insulation.

Exterior screens on the W windows are DEFINITELY a good idea, probably necessary, but raising the R value of the attic is more important.

Cellulose is superior to low density fiberglass in an open-blow attic application. New-school fiberglass such as Optima or Spider at high densityworks well, but to hit the necessary 1.8.lbs density requires blowing in mesh, raising the installation cost considerably. Low density goods are not sufficiently air-retardent to maintain a stable R value. Cellulose at open blow densities works just fine. Just be sure to specify "borate only" or "sulfate free", to avoid any metal corrosion issues in the event of a roof leak. (In the event of a roof leak you'd have to scoop out & replace some of the cellulose, but it's cheap stuff and easy to install- a DIY project with a rental blower.)

Foil RB on the rafters is definitely more effective than spray on RB. Most spray on goods aren't sufficiently low-E to meet the definition of RB, although some at least somewhat effective, and may be better value per dollar than foil since it's so cheap. But in terms of bang per buck it isn't as good taking the R value up to R50+ using moderate-density IR-opaque goods such as cellulose. Going to IR-opaque goods the hottest layer in the insulation is now at the top of the insulation, where it can convection-cool to the attic air, whereas with low density fiberglass the hottest layer is a couple inches in, and approaches the temp of the roof deck. If you go with an all fiberglass solution, radiant barrier has a larger effect, but at R50+ that effect isn't huge.

Until there's a dual-fan approach that is fully adjustable for pressure balance, a powered ventilation scheme will increase air infiltration drives on the building. Turbine vents will also depressurize the attic drawing conditioned space air in, unless you make truly gia-normous soffit vent area. Ridge venting will depressurize the attic also, but not as much. As previously stated, venting will not dramatically affect the issue- the attic air temps may drop several degrees, but the roof deck temp won't drop much, and both need to drop to affect the temp at the top of the insulation layer.

Dana

I dont see how a double fan will really improve things much. The fan blowing in will create a positive pressure nearest the fan that will gradually decrease. On the other end you have a fan blowign out creating a neative pressure. If attic floor is not air tight you will have the high pressure pushing air into the house and in other part with negative pressure you will be pulling air out. I also see degrading the R value by the air currents that will washo voer the porous insulation. Then there is the cost to install and run the fans.

I think it is more prudent to spend that money on other energy efficienct projects. I hear people talk of getting the air temps in the attic as close to the exterior air temps as possible. I dont feel that is neccessary. Since we know that the primary heat transfer in the summer is via radiant heat from the roof deck to the attic floor. Lowering the air temps is not going to change the radiant heat flow. Passive attic venting will remove excess heat and humidity. If the attic floor and duct work has been sealed and attic very well insualted then powered fans will have a very small effect to the point of not being economically feasible. The farther south a home is the more one should at least weigh the option of a radiant barrier.

At this point if a person were still considering the dual fan setup they should ask themselves what else could I do with that money and would it save more. Would planting shade trees, installing awnings, or window films provicde more comfort and energy savings.

An additional consideration in venting an attic in Texas and other areas in the south is extending the lifetime of the roof. A roof in south Texas (maybe not DFW) might last 15-18 years, while a roof in Connecticut might last 30 years or so. I think the difference is usually attributed to the much higher roof material temperatures in the south that cook some of the volatiles out of the asphalt shingles. If a new roof for a modest sized house costs $9,000, and is replaced every 15 years, that is $600 per year in roofing costs. If the roof lifetime could be extended to 20 years by using $50 per year worth of electicity, then that would be a net saving of $100 per year in roofing expenses, before considering any savings in cooling costs.

I have not looked for any documented proof of extending roof lifetimes with attic venting, but there is certainly anedotal evidence that supports that premise. Just another factor in the discussion.

Also don't overlook the fact that air conditioning duct work is running around those very hot attics, and it is hard to get enough insulation around those big ducts to keep from heating up the cool air before it reaches the room vents.

Greetings,

The following advice is based on 30 yrs experience with RB and correcting the problems caused by other insulations. Who ever is urging you to go to R 50 is blowing smoke in your ears. The following chart will tell you just how little you will save going to R 50. It's a money racket and they prey on your fears of spending too much for energy. If the contractor who told you that you have R 38 is correct then adding anything to it will be a waste of money. The best thing is to add the perforated RB sheet over the existing. It can reduce your AC run time up to 50% or more. Nothing else can do that.

Recently had a TV report of a cel addon attic that caused the ceiling to fall in. Why? Because the cel is too heavy and when it condensates moisture is even heavier. This increase in moisture can/will increase the moisture in the drywall and cause it to fail. The nails cannot support the weight. Two other problems with cel, if boron is the fire retardant then it can eat out the nails and truss connectors, also the moisture causes the boron to seep down and the upper surfaces are no longer fire retardant. I've seen that personally. Cel is definitely not a viable product and the people who sell/promote it will not tell you these problems and even deny it.

The purpose of attic vent'g is to remove MOISTURE. Venting may or may not improve shingle life or reduce costs. Whirley fans are NOT going to remove moisture in winter because you have to cover them, another scam. Ridge vents are almost 100% effective. No other sys comes even remotely close to their efficiency. GO to cor-a-vent.com and learn something, quit listening to these dummies.

I hope the following chart does not get mutilated in transfer.

THIS CHART WAS DISTRIBUTED BY FOILPLEAT WHICH MERGED WITH ANOTTHER COMPANY


The chart below gives the rate of depreciating returns for each added R-l through a given area beginning with averages for uninsulated areas. This same information is available through architects, engineers, power companies, etc.

To find the approximate amount that you would save on your bill, use the following example. Please keep in mind these figures are best results as they do not figure in reduced performance due to moisture from condensation and other factors.

1. Present R factor (2.3 blown or 3.3 batts per inch)
2. Desired R factor (R 30 is recommended; most builders install R 22)
3. Add up numbers in "B Improvement" column from present R to desired R rating
4. Figure average a/c costs. (Figure base 0perating cost for average of 4 people 120O sq.ft. all electric house at about *$8O/mo*. Balance Is a/c costs.)
5. Since heat gain is greatest through ceiling and due to heat retention, figure
60% of a/c cost (60% x Fig. 4 result).
6. Line 5 result X line 3 result gives the approx. cost saving with rock or glass wools. **Cel approx 12% better than FG and foam about 12% better than cel.**
COMPARE THIS TO A SAVINGS OF UP TO 30% FOR THE SUPER RB SYS' SAVINGS.

THE FOLLOWIG A thru C HEADINGS ARE FOR THE 3 DIGIT LIST BELOW

(A)R-Opaque Area (B)U Opaque Area (C) %Improvement over previous R based on original’s area

3 .3333 - EXAMPLE A=3 B=.3333 C= -
4 .25 25
5 .20 l5
6 .1667 10
7 .1429 7+
8 .125 5+
9 . 1111 4+
10 .10 3.3
11 .0909 2.7
12 .0833 2.3
13 .0769 1.9
14 .0714 1.6
15 .0667 1.4
16 .0625 1.3
17 .0588 1.1
18 .0556 1.0
19 .0526 .9
20 .05 .8
21 .0476 .7
22 .0455 .6
23 .043s .6
24 .0417 .5
25 .04 .5
26 .0385 .5
27 .0370 .4
28 .0357 .4
29 .034s .4
30 .0333 .4
31 .0323 .3
32 .0313 .3
37 .027 .2
38 .0263 .2
39 .0256 .2
40 .025 .2
41 .0244 .2
42 .0238 .2
43 .0233 .2
44 .0227 .2
45 .0222 .2
46 .0217 .2
47 .0213 .1
48 .0208 .1
49 .0204 .1
50 .02 .1

* Used original costs reference for dating of document (probably mid ‘80’s).

** NOTE added

The article mentions 30% savings. I do not claim that as the savings can be affected by conditions not assc with the RB. I has seen less and I have seen more. How ever a 50% reduction in run time is quite common PLUS the cooling off of the ceiling means less energy radiated down to the occupants which means more comfort. So called experts are so infatuated with R factors they forget to look at ceiling temps and the effects on the occupants.

Greetings,

Just to make the chart more clear, you are adding up the % of efficiencies from the existing R factre to the target R factor.

Greetings,.

I just noticed the comment about blocking RE as if the energy just "disappears". It does not. The energy is absorbed and radiates to the "coldest" surface, your interior space. It also conducts and the increase, according to A Nat'l Br of Stds report # 77-1274 of a retrofit house using FG and cel, the cel, and using a common standard of 5% increase for every 1% of moisture by weight, had an increase heat flow of 72%.
You go cellulose. By the way the FG had an increase of 48%.

Thanks for all responses.

I have noticed a few ridge vents around here. If anyone has new vents (other than the standard), it is usually the turbine. 4/5 companies recommended turbines, and only one wanted to the the ridge vent. The ridge vent's design is seems counterintuitive as far as exhaust is concerned, but I am not a professional. It looks better than a turbine, but it is more expensive (quote $900 for my roof, instead of~ $600 for 4 new turbines). Also, it is very windy in DFW, so I think turbines may actually work It makes sense with foil type radiant barrier, that the heat collected between the roof deck and foil must get out, so I see why I need more ventilation, since I only have about 300 sq in of exhaust for ~3000 sq ft roof area.

My AC and ducts are in the attic, standard here. Only 1/5 companies wanted to mastic seal the ducts ( expensive = $300 for 3 units). Others said the ducts appear OK. I don't know how to know for sure who's right. One company actually took temp readings from my ducts and the guy said it leaks just a tiny amount more than it should but overall it looked good.

As far as insulation goes, nobody does cellulose from the companies I called. They use fiberglass. On person said cellulose is garbage that gets degraded to dust over time and they don't want to install it. What really gets me is that companies are split on whether I need insulation or not. Some say I am at R26 the others R38. How can I figure this out? People charge $600 to add insulation, which is more than I want to pay if I don't need it.

Dana, the solar windows you recommend are popular. My neighbors have them but they are super overpriced and they look awful.

Thanks again for everyone's input.

Please show me research studies that support your claim that all we need is a radiant barrier. Please dont give me the conspiracy mantra. If radiant barriers were a cureall then someone would have found a way to make money off a such a system and it would be in widespread use.

I will go with studies I have reviwed. THE FSEC study reports a nominal reduction in peak and total loads when using radiant barriers. The benefit of the barriers diminish as the level of insualtion increases. The is data from many outher credible sources.

I have not said not to use radiant barriers but rather to know the benefits and dont look at rb's as a cureall. I see the benefit of rb's in southern states. A radiant barrier laid on top of insualtion in a northern climate particualry on a leaky under insulated attic is a waste and disctracts people from doing the right thing. I see that type of install sold in my area along using a utility bill from a sample house as proof that this is the best way to go. Air sealing and beefing up the insualtion would be a better solution.

You also go on about humidity reducing R value. I would say the heat in an attic reduces the humidity level below that of the air outside. In northern states winter can bring very low humidty levels as well.

Air sealing is so important as it accounts for a high percentage of heat loss/gain. It also can be the major source of moisture in wall and cielings. Air leaks from duct to the attic is a no brainer , it needs to be fixed. Most HAVC systems use floor and wall cavities for return air. These can be connected to the attic too even on home with no ducts in the attic.

As you attack one method of heat flow the others can exert more influence. As you reduce convnection then radiant and conduction become the main trasfer methods. If you then reduce conduction radiant becomes bigger. You can inter change any of the methods of heat flow.

Whatever method of insualtion is used it only slows the flow of heat. If we were able to blow heat flow then we would only have to deal with removing heat generated inside and would only need ac and no heat. As it is we can greatly reduce heat loss/gain but not eliminate it. Homes are being built with cellulose and fiberglass that have very low heat loss/gain. In these homes it can be a challenge to get HVAC systems small enough.

I will now go back to ignoring your one sided mislading posts.

Greetings,

The reason that RBs are not more popular is because the gov and utility companies don't want you to use them. The utility companies don't make money on RB homes. The gov loses unearned taxes. I have had several FG contractors admit the superior aspects of RB. Their reason for not installing them is: I can't make enough MONEY.

The FSEC study does not reflect NEW construction RB installation. I have put RB over 24" of FG that was no better than 6" . The reduction in ac run time was 50%+.

QUOTE>I have not said not to use radiant barriers but rather to know the benefits and dont look at rb's as a cureall. I see the benefit of rb's in southern states. A radiant barrier laid on top of insualtion in a northern climate particualry on a leaky under insulated attic is a waste and disctracts people from doing the right thing. I see that type of install sold in my area along using a utility bill from a sample house as proof that this is the best way to go. Air sealing and beefing up the insualtion would be a better solution

Spoken like a true novice.

QUOTE>Air sealing is so important as it accounts for a high percentage of heat loss/gain. It also can be the major source of moisture in wall and ceilings.

Moisture in walls and ceilings are a function of DEW POINT And water vapor available. Leaks have nothing to do with it.

QUOTE> You also go on about humidity reducing R value. I would say the heat in an attic reduces the humidity level below that of the air outside. In northern states winter can bring very low humidty levels as well.

The heat can actually increase the condensation. Go find out why. You can have a higher humidity level due to higher heat.

QUOTE>As you attack one method of heat flow the others can exert more influence. As you reduce convection then radiant and conduction become the main transfer methods. If you then reduce conduction radiant becomes bigger. You can inter change any of the methods of heat flow.

Convection only becomes important IF there is a large enough space for circulation. Since multi layer foil breaks up the spaces and because of the nature of temperature gradient thru the sys there is no convection in walls. Small amount in up heat and none on down heat.
RADIATION IS ALWAYS THE DOMINATE MODE OF ENERGY TRANSFER UNLESS SOME OTHER FACTOR AFFECTS IT. Even in conduction, energy transfer from atom to atom is RADIANT. And there is an argument that convection is ultimately tied to radiant transfer.

QUOTE> Whatever method of insualtion is used it only slows the flow of heat.

TRUE and foil stops 95%. You can't show anything that meets or beats that. Brian, an engineer, on another post says that 4 " of U foam, two inches on either side of 2x4 will reduce heat flow 50%.
That's pathetic, when you consider the cost and space used to get such low results.

QUOTE>I will now go back to ignoring your one sided misleading posts.

And I guess I will have to keep teaching you. OH yes, and isn't your comments one sided and misleading? What are you , the kettle or the pot?

Here's an article that will explain it better. I've had my copy for about 37 years
http://www.low-e-ne.com/media/physics_of_foil.pdf

Posted By Roberth on 17 Oct 2011 03:27 PM
Dana

I dont see how a double fan will really improve things much. The fan blowing in will create a positive pressure nearest the fan that will gradually decrease. On the other end you have a fan blowign out creating a neative pressure. If attic floor is not air tight you will have the high pressure pushing air into the house and in other part with negative pressure you will be pulling air out. I also see degrading the R value by the air currents that will washo voer the porous insulation. Then there is the cost to install and run the fans.

I think it is more prudent to spend that money on other energy efficienct projects. I hear people talk of getting the air temps in the attic as close to the exterior air temps as possible. I dont feel that is neccessary. Since we know that the primary heat transfer in the summer is via radiant heat from the roof deck to the attic floor. Lowering the air temps is not going to change the radiant heat flow. Passive attic venting will remove excess heat and humidity. If the attic floor and duct work has been sealed and attic very well insualted then powered fans will have a very small effect to the point of not being economically feasible. The farther south a home is the more one should at least weigh the option of a radiant barrier.

At this point if a person were still considering the dual fan setup they should ask themselves what else could I do with that money and would it save more. Would planting shade trees, installing awnings, or window films provicde more comfort and energy savings.

I agree completely on the fan issue- I was responding to jonr's recomendation for a dual fan, which I believe to be a net-negative unless fully balanced.

Passive venting doesn't remove much heat, but in some climates in can remove some humidity.  But in much of the southeastern US it RAISES the average annual humidity in the attic, and can create condensation issue on ducts or even the ceiling gypsum of an air conditioned house. In cooling dominated climates a sealed-attic approach is usually a better way to go.