I've never seen a legit AC contractor oversize to cover himself. I'm not saying they're not out there, but you should not have a problem finding someone to do the calculations properly.

You have to find an HVAC contractor who will do a real Manual J load calculation. In some parts of the country it is very difficult to find one.

How much extra are you paying for foam insulation? Will the roof deck be foamed? Is this open or closed cell foam? You want to make sure the foam is properly installed and you will have a reasonable return on investment.

Make sure you foam the deck. We are doing the same thing, about 3,000 sq ft.

Open cell foam allows water to exit, closed cell foam will trap the water

next to the roof deck.

invest in an unbiased load calc and present the calc to contractors

to bid the job. weed out the contractors who use rule of thumb

sizing, or who expect you to double the costs of the system

by purchasing more expensive unneeded tons.

shop hvac companies wisely. this is an important choice

that will affect your comfort daily.

bigger is not better, nor is more expensive equipment the solution.

correctly sized hvac system with correctly sized and designed

ductwork is the goal.

best of luck

Thanks for the good advice everyone.

Icynene is one of the products we're considering and my understanding is that it will allow moisture from something like a roof leak to pass through it. We will be foaming the roof deck.

The unbiased load calc was something presented to us awhile back that I had forgotten about. Thanks for the reminder - I just ordered it. Apparently the guy has been doing them for as long as I've been alive - and I'm no spring chicken - and all speak very highly of his numbers.

Icynene is the name brand of the foam. types of foam are open cell

and closed. the former allows moisture to pass thru and has the lower

R-value, the latter no moisture passes thru and higher R-value.

the amount of inches = required R-value.

avoid "average fill of xxx inches", specify exact inches.

also be leary of companies who claim 1" acts/performs as R-10

and other such bs statements.

install has to be air tight for full benefit and no problems with

performance.

best of luck

I'm curious about this as well.

I assume the OP was talking about interior spray foam, not exterior sheet foam. Hopefully whomever did it understood that in a hot roof system they need a layer of closed cell against the sheathing and then fill out with open cell to avoid moisture and other problems.

Finding HVAC contractors (and code inspectors) who understand what they're doing is tough, even in Minnesota. They know what they've been told for designing systems but have surprisingly little knowledge of how things actually work and even less knowledge about energy efficiency. Get too close to the edge of the little box they've been operating in for the past few decades and they get nervous.

I'd think that they'd want to understand what they're doing and explore better alternatives but that's not the current HVAC industry. Of all the trades I've worked with they seem the most antiquated and the most stuck in doing things only they way they've always done it. They don't want their world complicated with improvement.

jrbrig, we did end up going with the Icynene contractor and have been pleased with the performance of the house so far. It took some doing, but the HVAC contractor finally agreed to lower tonnage somewhat. I would have preferred to drop about 2.5 tons but he would only drop 1.5 ton from his initial recommendation.

It's difficult to compare directly because we more than doubled square footage from our old house (and run two dehumidifiers in the new basement that I'm told are power hungry). That said, I don't doubt the ROI was worth the price of the foam - it'll just take much longer to break even than the foam guy estimated (which is not surprising).

We moved into the new home in July so our sample size is small, but over the last few months our elec bill has been just under double the old house's bill. I knew it would be higher because the new house is so much bigger, but honestly I had hoped for better than that.

My "what-if" explanation in my own head is that if I had successfully convinced the HVAC guy to drop another ton or 2, my savings would be more. The current units barely broke a sweat when cooling the house in August when it was mid 90s, so I think that extra size is costing me. At least that's what I tell myself.

@dbrad, how old was your previous house? Is your new house primarily LED lighting? If your former house was more than 15 or so years old I would have expected about 30-40% savings per sq foot so your new energy bills for twice the sq ft about the same as your old house.

In your attic did they do closed cell against the sheathing and then fill out with open?

Just ran some numbers. Keep in mind that the sample size is small in the new house because we just moved in, so I just used a July/Aug sample from both houses. I haven't checked the dehumidifiers' power usage with my kill-a-watt meter yet (like everything else, still lost in the move) but I found some info online for similar units and used a conservative estimate to subtract out their estimated usage.

New house @ 4800 sqft = $0.056

Old house @ 2300 sqft = $0.101

Old house was a production tract home built around 2000 with a mix of LED/CFL/Incandescent. New house is all LED but has probably 2-3x as many lights per room as the old house.

They used open cell throughout. The underside of the roof deck was discussed and they preferred to let moisture migrate through if the shingles developed a leak. Otherwise the sheathing would rot before it was discovered. Made sense to me.

I've been tracking temperature/humidity in the attics with SensorPush sensors. The house has 4 separate attic spaces - 3 of the smaller ones average 2-3 deg above room temp & 48% humidity. The 4th attic space is rather large and has a window - during the hottest part of the day it's about 7-8 deg warmer than room temp (cools to room temp at night like the rest of the attics), but humidity will spike to 70-80% from 2-4pm every day in July/Aug (house has tree shade on E & W sides, exposed S side though). Because the humidity spike is brief I haven't been that concerned about it, but the plan next summer is to move one of the dehumidifiers to the attic and set it to run above 50% to catch those spikes. It'll share a drain with the hvac up there.

It'll be interesting to see how it performs over the winter.