Probably the least expensive thing you can do is check the venting of your attic space.

Ahh, and also make sure all the heat and a/c ducts are still sealed! Hadn't thought of that one! Thanks!

He wasn't talking about sealing ducts (which is good too), he was talking about venting your attic :) The temp in your attic should (in a perfect world) be the same as outside. On hot days the hot air in your attic needs to be able to escape vents near the top and be replenished from soffit intakes below. This will lower your cooling requirements in summer.

For sure check your attic insulation thickness is a good place to start. If you have a crawlspace, ensure that is also insulated correctly.

http://www.energystar.gov/index.cfm?c=home_sealing.hm_improvement_insulation_table

If you have a basement where the rim joist cavities are exposed, seal those up well either direct spray foam or rigid foam/spray foam combo. Also seal where the sill plate meets the basement wall (that is a common leaky spot).

While in attic, find all of the ceiling light boxes and spray foam those also, plenty of air leaking out of those. Also spray foam or caulk penetrations and bath fan housings. Sealing leaks gets you the most bang for your buck.

Where do you live - what's the winter temp like?

I agree attic insulation and venting.

Also air infiltration (cracks / gaps).

Checking all sources of air infiltration is a great place to start, and its relatively cheap to make the fixes. For example, if the 2nd floor has ceiling lights you need to go up into the attic and make sure the boxes are sealed to the sheetrock and no air can pass from the attic into the house. Any electrical receptacles in exterior walls should also be sealed around the edges to stop air infiltration from outside.

Bruce

Storm windows (more budget friendly), LED bulbs, programable thermostat

http://www.usatoday.com/story/money/business/2015/07/02/study-home-efficiency-upgrades-pay/29649163/

https://www.energystar.gov/campaign/waysToSave

geoffrey_b- North Florida- summers are hot hot hot but winters tend to be mild. If it dips below freezing it's usually just overnight.

An energy audit of your home can help you identify and prioritize areas for improvement. A blower door and IR camera are great tools. Check with your local utility providers to see if they have will subsidize any of the cost of the audit or improvements.

don't expect a lot from these low cost audits. while they will provide you a lot of numbers & provide a list of contractors, most fail at showing the homeowner

where the leaks are, and how to seal them long term.

but hey, you get some 'free' light bulbs & weatherstripping.

blower door testing is a great thing, I've been doing it for 15+ years.

but the person doing the testing is the key to what you'll know at

the end.

I'd suggest, if you go the utility audit route, to have a note pad &

a roll of painters tape. this way you can write down leakage sites

& stick a piece of tape on the leakage area. takes longer, but

its your house & your audit.

same goes with duct testing. go into the attic while ducts

are depressurized, you'll feel the leakage.

duct takeoffs at supply plenum, plenum (supply & return) connections

to equipment, etc.

then post back your findings & we will walk you through how

to seal each one long term. not much sense in doing things that

don't last, its a pita to say the least to access these areas.

make the first time of sealing be the last time.

air/duct sealing is your fastest payback. but don't use

foam as it doesn't seal long term. and spraying foam around

electrical isn't a good idea. I've spent a lot of time digging foam

out of areas that were supposed to be sealed. then sealing with

products & methods that have stood the test of time.

tell us more about your house.

Sealing a home should be an comprehensive process. If you seal a home and do not allow for fresh air mixing/transfer, you can wind up with a 'sick' house. One that will not exhaust stale air/odors and cannot fully utilize the bath/kitchen vent fans. That means a system for air transfer needs to be developed utilizing air mixing and heat transfer equipment with the heating/cooling systems. That is a job for the pros.

Doing things that maximize natural forces, like a ridge vent with proper soffit vent area will use no energy other than nature. Hot air rises. Having the vent at the ridge and the fresh air intake for an attic at the lowest point means nature causes the air in the attic space to be continually replaced, no matter what the out side temperature.

Insulate the attic(using the sealing ideas already provided above) and you have a 'Forget it' attic. I did add blow-in insulation to our attic---12" worth.

Our house was built in 1965. Subsequent owners have added insulated siding and add on storm windows. Those two things have almost completely the major leak points around windows. The windows are the original 1965 wooden sash double hung styles units. With the added storms, they function almost as well as double pane modern types. When we change them, it will be for the ease of use/cleaning, not for energy savings.

The advice about rim joist insulation is spot on.

Weatherstripping exterior doors offers a lot of options. The following video is pretty good.

Adding a storm door can also help sealing.