[kennelm]

My house has dual HVAC zones, and when the weather gets really hot (upper 90s with humidity), the unit in my attic struggles to cool the upper level. The inside temp starts to creep into the upper 70s, and the unit runs non-stop. I had 2 attic fans installed a few years ago, and that helped. I noticed that my attic is insulated to R30, and I am wondering if going to say R38-R40 would make a noticeable difference with my upper unit A/C. I guess at some point there are diminishing returns on insulation.

Would it be worth the cost to get a contractor to blow in more attic insulation? If so, what would be a reasonable cost per square foot to go from R30 to R40?

 

[yuri]

Have you had the unit properly serviced? Freon level needs checking etc etc. More insulation is better but you have to phone some contractors for prices. The ducts in the attic should be checked to make sure they are properly sealed, now is the time to do it and not when it gets HOT.

 

[COLDIRON]

upper 90s

Every unit struggles in 90-100 degree weather and runs most hours of the day.
Depending where you live design conditions in a lot of areas of the USA are 95deg.

In addition I would say tighten up your house, insulate good, and make sure your equipment is clean and running as good as it can.

 

[biggles]

the coil on that air handler can only do 18F difference on the entering and leaving air and you need to verify that the return air is true space air and noting from the attic is being be sucked in to jack that return up.do you just have a single return grill in the 2nd floor hallway,and does it go right into the back of the air handler.then the suply ducts are they insulated flex runs with minimal elbows and turns.the charge has to be up to operating levels if anything less your tonnage is less in doing that 18F split.if your condenser is being banged by the sun that adds to the operating conditions compared to a shaded area...start with the ducts then get the charge checked. 18F air splits on the air handler(drop) as mentioned then 10F on the condenser(rise)

 

[beenthere]

Have the unit sericed.
Also have the duct work checked for leaks.

Then also seal up any air leaks you can find in your house.

 

[kennelm]

As recommended, I am getting the unit serviced. Meanwhile, no one has offered an opinion on the merits of increasing the insulation from R30 to R40. Would this be a worthwhile investment? Or is R30 "good enough"?

 

[kenmac]

just opinion. Every little bit you can do will help. I doubt you'll really notice any difference & pay back will probably take years.. You already have R30 which is far more than most ( atleast in my area) All your duct work ,coil etc, is in a hot attic. If the duct work is metal . Your a/c has to cool all this metal down.. If I were going to insulate anything, it would be an outside wrap of the duct work & coil

 

[hvaclover]

Attic fans?

The air being displaced in the attic has to be replenished from somewhere. I know you have R30 but you are actually pulling air from the upper floor into the attic. You are actually adding more infiltration load.

 

[beenthere]

As recommended, I am getting the unit serviced. Meanwhile, no one has offered an opinion on the merits of increasing the insulation from R30 to R40. Would this be a worthwhile investment? Or is R30 "good enough"?

If your attic reaches a temp of 150°F, and you keep your house temp at 70°F.
Increasing from R30, to R40, lowers your heat gain by 1,000 BTUs total, on a 1500 sq ft ceiling.

Not a cost saving thing for cooling alone.
And only helps the A/C minimally.

Sealing air leaks in your home will do far more to save you money.

 

[kennelm]

Attic fans?

The air being displaced in the attic has to be replenished from somewhere.

Isn't that what the soffit vents and the end gable vents are for?

I know you have R30 but you are actually pulling air from the upper floor into the attic. You are actually adding more infiltration load.

Well, I noticed an improvement after the vents were installed. It definitely pulls that super-heated air from the attic...

 

[beenthere]

Is it making the first floor unit work more though?

Seal your attic access(weather stripping).

I like power vents for attics also. But, they do pull air from the occupied area, if the house isn't sealed right.

The sofit vents, are generally only installed)(sized) for natural convection. Not powered exhaust.

 

[hvaclover]

Isn't that what the soffit vents and the end gable vents are for?



Well, I noticed an improvement after the vents were installed. It definitely pulls that super-heated air from the attic...

Yeah but it's only an incremental improvement and your unit is running more in proportion to the improvement . Put plainly, your paying more for the slight improvement in longer run time.

 

[kenmac]

The sofit vents, are generally only installed)(sized) for natural convection. Not powered exhaust.

Yep, he needs air exchange depending on how much air his power vents move or remove...

 

[hvac122]

Isn't that what the soffit vents and the end gable vents are for? As said they were designed for passive ventilation only. They are not big enough for power ventilaters,



Well, I noticed an improvement after the vents were installed. It definitely pulls that super-heated air from the attic...

Yes the attic is cooler but studies have found that your pulling around 20% of that air from the house through the attic floor. This is air that you have already paid to condition. This also could put your house in a negitive pressure which could cause health problems and unsafe conditions.

You would be much better off sealing the attic floor (remove insulation, seal and then put insulation back. Also installing a radiant barrier at the roof line will stop the infrared infiltration through the roof.

 

[Yoyizit]

There is a Law of Diminishing Returns for insulation and a lot of other things; the first dollar you spend on insulation gives you a lot more benefit than the last dollar.
http://www.energystar.gov/index.cfm?c=home_sealing.hm_improvement_insulation_table

 

[kennelm]

Yes the attic is cooler but studies have found that your pulling around 20% of that air from the house through the attic floor. This is air that you have already paid to condition. This also could put your house in a negitive pressure which could cause health problems and unsafe conditions.

You would be much better off sealing the attic floor (remove insulation, seal and then put insulation back. Also installing a radiant barrier at the roof line will stop the infrared infiltration through the roof.

I wonder if we are talking about the same kind of attic fans? The two I have are mounted in the roof and appear to be like 10-12 inches in diameter. How much air can these things move? It never occurred to me that these fans could suck THAT much air -- enough to pull 20% through the attic floor. At best, I figured they might gently exhaust the heated air, which would be replaced with air entering through the soffit vents.

Please tell me more about these health problems and unsafe conditions you alluded?

Also, since my attic has blown-in insulation, I would assume that moving that back to seal the attic floor would be a hard task? What kind of money would I have to pay (say, per sq ft) to do that? And what benefit would I realize in terms of energy savings?

Thanks!

 

[hvaclover]

I wonder if we are talking about the same kind of attic fans? The two I have are mounted in the roof and appear to be like 10-12 inches in diameter. How much air can these things move? It never occurred to me that these fans could suck THAT much air -- enough to pull 20% through the attic floor. At best, I figured they might gently exhaust the heated air, which would be replaced with air entering through the soffit vents.

Please tell me more about these health problems and unsafe conditions you alluded?

Also, since my attic has blown-in insulation, I would assume that moving that back to seal the attic floor would be a hard task? What kind of money would I have to pay (say, per sq ft) to do that? And what benefit would I realize in terms of energy savings?

Thanks!

depressurizing the attic by fan large enough to cause a temp diff is going to create a chimney effect with in the stud space and pull air from the up stairs are, sometimes even the lower floor. Just means you have to cool the new air thats being sucked up ward.

 

[gma2rjc]

I wonder if we are talking about the same kind of attic fans? The two I have are mounted in the roof and appear to be like 10-12 inches in diameter.

Are you talking about a vent that lets the air out of the attic? Or are you talking about a motorized fan?

Also, since my attic has blown-in insulation, I would assume that moving that back to seal the attic floor would be a hard task? What kind of money would I have to pay (say, per sq ft) to do that? And what benefit would I realize in terms of energy savings?

I sealed my attic myself this winter. If you're physically able to climb up there and move around, you can do this yourself.

Just sealing my attic and doing some other minor energy-saving things helped a lot. I compared my gas bills from March 2008 and March 2009. Because of the things I did, my usage went down 19%. That's with just R-19 insulation in the attic. Sealing yours will help a lot.

 

[kennelm]

Are you talking about a vent that lets the air out of the attic? Or are you talking about a motorized fan?

These are motorized fans, thermostatically controlled to kick on at like 100 degrees or thereabouts.

I sealed my attic myself this winter. If you're physically able to climb up there and move around, you can do this yourself.

Just sealing my attic and doing some other minor energy-saving things helped a lot. I compared my gas bills from March 2008 and March 2009. Because of the things I did, my usage went down 19%. That's with just R-19 insulation in the attic. Sealing yours will help a lot.

 

[hvaclover]

You can get 100*f in your attic on a mild sunny day. Those fans are running more than they are off.

Your infiltration, because of them, is a major contributor to the excess cooling load.