Insulation level will affect SEER level.

The higher the R-value of your insulation the lower the required SEER value of your AC units.

R-value is R-value, so providing you install at least the recommended R-38 ceiling insulation for your location, using foam will only add unecessary cost to your project without adding benefit.

So if you can adequately cool the house with a 13 SEER unit at R-38, then you should be considering blown cellulose or fiberglass and not foam if you want to save money all around.

One thing is that, if you're putting your air handler and ducts in the attic, it might make sense to foam the rafters and go with the lower SEER. You'll decrease the load on the HVAC system, especially during the summer (like the wicked hot one that's finishing up).

I disagree manhattan42. It definitely makes sense if you make the attic unvented which makes the entire attic conditioned. This will lower the load of the AC. Insulating has nothing to do with seer rating. Give the same set of conditions, the 15 seer AC will be approximately 15% more efficient than the 13 seer. If the electric bill cost $100 to power the 15 seer AC, it'll only cost $85 with the 15 seer. Tonnage of the AC will probably be lower with an unvented air tight attic, but the tonnage requirement doesn't change between a 13 seer, 15 seer, or 18+ seer.

Athensmom - I know you have a pretty big house but $8k seems like a lot to go to seer 15. We went from builder basic 13 to highest end seer 15 for $7k (2 units for 7 tons). Just a Seer jump would have been less than $3k. The other "stuff" was dual fuel, variable speed, 2 stage, zoned master suite.

When you are making $8k decisions, it really helps to have an energy star consultant. It might be too late. We paid $850 or so for the certification which included an energy audit. The audit gave the savings for different decisions - like going from seer 13 to seer 15 saved $50 a month (I don't remember the actual numbers). It really helped to make decisions and it made a couple of things no brainers. It wasn't perfect but it was helpful. The certification in my area gives me 5% off NG and electric for life which pays for the certification over 8 years.

I'd love to post a link to mine because it would help - a quick check showed that either change saves $100 per year although it doesn't really break down attic foaming only so there is some subtraction involved. It gives foaming the entire house a savings of $250 but that assumed crawl space and sealing that.

There are 2 issues when going to seer 15 that you should consider - Seer 15 equipment is generally better than seer 13 because it uses better parts. That is a manufacturer usually puts the cheapest parts in a seer 13 unit. There are some better seer 13 units but mostly the better parts are in the more efficient units. This should make for more reliable operation.

The other issue is that your a/c units won't last forever but your foamed attic will.

My personal opinion is that foaming an attic is an expensive way to insulate a home. Sure it seals well and puts the ducts in the envelope (usual local practice). But at the expense of the foam and the much larger conditioned space. My rafter surface area is double my ceiling area and assumming both are insulated to the same R value - that means there is twice the heat loss with a foamed attic. You can insulate both but that just drives the cost up even more.

So based on my numbers - you have an 80 year payback on the foamed attic. Hard to believe and maybe it isn't true. There is the argument that the air handler will last longer in a cooler attic. Probably true. Could build an insulated box for air handler for

"It definitely makes sense if you make the attic unvented which makes the entire attic conditioned. This will lower the load of the AC. Insulating has nothing to do with seer rating."

Untrue.

Increasing the size of the area to be conditioned increases the load.

The size of the unit (SEER) is determined by heat loss/heat gain which is directly related to insulation.

Insulating the attic in this case will not only increase the costs to insulate, it will also increase the costs to operate the AC system.

Double whammy.

Thanks David. I don't know whether it is too late or not - we haven't broken ground yet. I think the cost differential as far as the units go is partially that we have 3 units, not two (one upstairs, one downstairs, one master suite) - total 8.5 ton. The price includes equipment, labor, dryer and exhaust fan venting. This is not a vendor I am familiar with - just who my builder used to get his preliminary bid.

I also looked at the estimate again (I was working from memory) and the differential is between a 13 and 16 SEER, costs as follows:

16 Seer Carrier Puron Heat Pump 21250
14 Seer Carrier Puron 18700
13 Seer Carrier Puron 13600

Dual Fuel adds 1025 per system.

Thanks for the thoughts.

Also, the estimate is very vague and doesn't specify which equipment etc. This was just done for putting together a preliminary estimate for cost to build but there seems to be much too high of a jump between the 13 seer and the 16 seer, even for 3 heat pumps! I have asked for clarification of precisely which models were quoted, and it will be interesting to see what is said.

Seer has nothing to do with capacity of of a HVAC system. A 2 ton 13 seer will put out the same BTU's as a 2 ton 18 seer HVAC system. The 18 seer unit just uses less energy to do the same amount of work. Just think about moving a a couple hundred pounds of rock from point a to point b. You can either drag it across the ground, or you can put it on a wheel barrow and roll it there. Same amount of work being done but one does it more efficiently (higher seer).

If you can make an air tight seal at the rafter than the load maybe pretty close to equal. Problem is it's usually easier to make an air tight envelope by sealing at the roof. Yes, you have more cubic footage to heat and cool, except it's much easier to heat and cool air that isn't constantly entering and leaving the house via stack effect. Also, much of that increased cubic footage is negated by much cooler attic temps in the summer time. Unvented air tight attics usually run 5 to 10 degrees warmer than the conditioned airspace vs 50 degrees warmer for a typical unvented attic on a hot summer day. This is especially true if your vents and air handler are all in the attic.

manhattan42 does seem to have a mistaken idea about SEER - which has nothing to do with size. Seasonal efficiency rating or something to that effect.

Thomas - "50 degrees warmer for a typical unvented attic". I don't know who has put an uninsulated unvented attic in the south. That has not been done in the last decade (if at all). 20 degrees is an average differential for a typical vented attic on a hot summer day vs ambient. If the comparison is conditioned than I guess it depends how hot but 50 is still too much unless you are conditioning to 70 on a 100 degree day - which would take a really oversized system.

Athens - it certainly isn't too late - I thought maybe you were further along. 8.5 tons is likely too much. I have 7 for 5200 sq ft which includes walkout basement - and this is a tad oversized because Infinity systems are whole ton only. I really should have 6 tons total but we erred on the higher size because of liking our bedroom really cold at night and because they totally overestimate what a basement requires.

If you foam the rafters, that should allow an even smaller upstairs unit.

Your Seer upcharges are still too much even with it being seer 16 and 3 units. Not nearly as bad though. I think I paid $1000 for dual fuel on downstairs system only.

If your third unit is for a walkout basement - consider upgrading to Infinity and zoning the basement. Basements require next to nothing in the winter which drives sizing.

My baseline was Seer 13 for about $13k. Final was $20k for Infinity seer 15, dual fuel main floor and other options listed above (all Infinity standards).

For your comparison- I am installing 3 Trane coastal 10 yr warranty units, total 9 tons, SEER 16, with 8 CFM exhaust fans, 1 dryer exhaust and 3 digital programable thermostats for $19,000.

Here's the big diff with your quote- we are using heaters, not heat pumps, because it is a rare time that we would use heat, so the cost of the heat pumps would not be justified. The main unit is 5 tons, and our AC contractor said the heat pump for that unit alone would be another $3,000 because of its big size (something bordering on commercial size as I recall). It wasn't a big diff on the smaller units.

I think the diff between a 14 SEER and 16 for all 3 units was $2,000 or $3,000.

I have had Trane and Carrier. My Carrier units were expensive, and rusted out twice as fast.

Thanks for all the feedback. Unfortunately Georgia Power only offers a discount to metro Atlanta folks and I don't think we qualify but I will check what they consider "metro". Jackson EMC may as well - I didn't understand their website but will call to check.

I am not sure this is the configuration we need at all but it is a starting point. We have a separate unit 2 ton for the master bedroom, and one is for upstairs and one is for downstairs (one is 3 and 1 is 3 1/2)). This is for 5600 square feet on the upper two floors and does not include the basement which we are not finishing right away.

I have asked for clarification on what specifically was priced. There are several levels of Carrier Puron and it may be that he had to "jump" a level to get a 16 SEER.

We will likely bid it out when we figure out what we want to do and my builder is happy to do so but that this a/c guy is super competitive and he said he would be surprised if they could beat his price but we will see.

He is also asking for clarification on why such a big jump for each SEER level.

It is an additional 1025 per unit for dual fuel.

And you bring up a good point as to whether we should do a natural gas heater and electric air which is what we have now. We almost never get below 32 degrees here (you may get colder where you are David).

Building science apparently endorese the idea that ventilating an attic is one of the "Top Ten Dumb Things To Do In the South" - this is if your air handlers and ducting are in the attic as opposed to under the house.

http://www.buildingscience.com/documents/insights/bsi-015-top-ten-dumb-things-to-do-in-the-south

For info on proper attic ventilation, if you choose to go that way:

http://www.buildingscience.com/documents/digests/bsd-107-understanding-ventilation-in-hot-humid-climates

Athens - 2 things.

If you rarely get below freezing then you don't really need NG at all. My NG is locked out to come on below 35 degrees. So if you rarely get there, you probably don't need it. What I find is that if you are heat pump only, then you want a good system with at least a variable speed fan. You also want a good install so that you never know the system is on. I'd recommend returns in all bedrooms for instance (I think that is good either way) so that you never have issues with air flow under doors. This usually isn't an issue downstairs since there are much less doors in a typical open plan.

2nd - ok I have 2000 sq per floor (roughly). Foamed windows, doors; R-40 in attic; Energy star (which has a couple extra small requirements); and my total was $4k - I only put this because I saw your number in another thread. That included basement walls, interior sound for powder and master bath. I highly recommend foaming the plumbing runs for sound proofing. We have cast iron downpipes (the quieter option) but with modern good toilets (low water use but enough force to not get plugged), the water rushes really fast. You will want to quiet that rush. It is one of those regrets that my builder never brought up. I may even retrofit it and Ill do it in the basement.

SEER is the measure of cooling perfomance of AC measured in BTUs.

The more space one needs to cool the more BTUs will be required.

While ac units are rated by efficiency of the ability to convert electric energy into BTU cooling energy, the fact is, one still needs to spend more electric energy to cool a conditioned space to begin with even though one can do it more efficiently using a higher SEER rated unit.

This amounts to energy dollars wasted, not conserved.

So despite the fact you may be 'saving' energy with a higher SEER rated unit, you can still end up losing money because of the increased volume of unused attic space that needs to be conditioned.

Couple that with the added insulation used to insulate the rafters and gable ends instead of the attic floors, and the owner ends up spending far more than is necessary to keep the building cool...when mere increasing the duct insulation or locating the ducts and AC units in existing conditioned spaces could do better.

Insulating the attic and increasing the SEER of the units is the therefore likely the worst energy 'saving' design option one could take...and will only add to the cost to build and operate the house returning nothing for the investment.

My understanding is that any additional cost of conditioning the attic volume is more than offset by the additional sealing effect of the foam preventing air infiltration, the reduced heat transfer, reduced vacuum effect of pulling cool air out of the house compared to a ventilated attic, and the benefit of having the weakest link of the ac, the ducts and air handlers, in conditioned space (assuming the mechanicals are in the attic). This seems to hold true in the more hot than cold south.
Other potential benefits of foaming include increased structural rigidity, fewer bugs and critters, reduced water penetration, reduced pollen infiltration.

To me the biggest downside is cost, and it is considerable.

Thanks again for all the input.

David - I'll have to check my bid invoice again to get the exact amount. Our downstairs is around 2900 square feet I think, plus this included blown in cellulose in the attic vs batts (which was a considerable amount - we have LOTS of attic as our house is over 100 feet long but not very deep). It also included insulating the basement, or maybe just the basement floor (I remember reading something about r-19 where floors are over unheated space which will be the entire downstairs, although we are doing R-30 in the garage ceiling).

We are also using r-21 batts in the walls which may up the price a bit. That being said, I have no idea whether this is competitive or not as my builder has only gotten one bid from the guy he generally uses.

I highly recommend a trip over to the hvac forum. The first thing the pros over there will ask is if your hvac guy did a manual-j calculation. Then they'll probably tell you to get a couple more estimates.

I'm no pro, but your system does sound a bit oversized to me (this coming from a guy with 13 tons worth of a/c). Bear in mind that oversized is worse than correctly sized. The units will run for shorter periods of time, and the increased cycling is less efficient in terms of energy consumption, comfort, and lowering humidity.

Athens - I forgot you were 2x6 walls. I've extrapolated some numbers and I think you will save $10 a year going from r-19 to R-21 walls. Shocking? Well in your climate, you have little difference in temp between inside and out for most of the year. Summertime highs - the difference is only 20 degrees. Having R-21 with a 20 degree differential is the definition of overkill. Now in the winter, the differential is higher but only at night.

I think we share the same R-19 over basement and R-30 over garage.

Anyway - I'm all for doing some overkill and doing things with long paybacks. But R-21 is probably a 50+ year payback. If you are doing that - then you certainly should do Solar hot water (7-10 years), solar panels (varies but 30 years is a good number), Geothermal is all over the ballpark but usually less than 20 years and could be as short as 5. At $8k - I do think foaming the rafters could be a >50yr payback. If you don't foam the rafters, a radiant barrier is probably a 5 year payback (really cheap to add).

Greenbuildingtalk.com is a good forum is you are interested in killing yourself on the research. A little more environmentally focused than dollars and cents. I personally think you have to chase dollars because I have an economics backgroung. Spending $10k for something that saves $100 a year and reduces energy use is laudable but giving that money to a charity that helps insulate lower income homes that really need it is far more helpful...

Too many decisions with lots of salesmen involved....

I wonder what the cost difference is between the r-19 and r-21 and whether it makes that much of a difference? Need to check!

So David do you have a vented attic then? I keep hearing/reading no one in the south should have a vented attic! But 8k is 8k and I don't want to spend it if it is unnecessary.

Not sure I could get the solar past the Architectural Review Board and don't know how much sunlight I could capture (very heavily wooded lot).

I think there is a federal law preventing ARBs from stopping solar. It might just be NC.

Typical vented attic here. The vast majority of attics in the South are vented. It only recently got allowed by code officials to close up the attic. Not last year recently but in the last 10 years.

I wonder why sealed attics aren't more common in Phoenix if they're that efficient? My attic gets insanely hot in summer.

david_cary ... if you see this, would you please email me from My Page. I am in a nearby area and am looking for subs who do geothermal and other things you have on your house. Thanks !! Angela