Change the roof or change the A/C system?
June 19, 2012 2:46 PM   Subscribe

Cooling the second story: Replace the roof or dual-zone the A/C?

We live in a two story house. It was in bad cosmetic shape, but we are handy and the price was great. So, since we are as yet a young family, we just fixed up the first floor and moved in with the second floor as yet in need of work. This is in Houston (super hot and super humid) Texas.

Now I am looking towards fixing up the second floor so the kids can have their own rooms in a couple years. The house has a single zone 4-ton A/C unit. The air handler/furnace in the attic is old (a decade or more). The outside part (compressor?) is new, we had it put in because the old one was dead. As it is single zone, the first floor gets to a nice temp and the second floor stays too hot for comfort until a couple hours after the sun goes down. The metal ducts in the attic have some of their insulation falling off, and what remains is as old as the air handler, at least. We did blow in attic floor insulation, so that part is about as good as it will get.

The roof is 20-30 year old dark asphalt shingles, so it is nearing its end of life as well. But it survived Hurricane Ike, so it is still truckin'.

So: Knowing that I'll need to do both eventually, which would you do first towards making the upstairs cool and livable: Replace the roof with a metal roof with those foam insulation/radiant barrier panels underneath, or replace the air handler, ducts, and compressor (even though it is newish it can't support dual zones) with a new A/C system?

I think the roof would be cheaper, but I'd sure feel dumb if the A/C crapped out. But then I'd feel dumb if the roof blew off in a storm and the new A/C stuff got soaked.

Which would you do, and why?
posted by BeeDo to Home & Garden (7 answers total) 3 users marked this as a favorite
I think I would replace the roof and look at not only better insulation but other passive solar things such as roofing color. I have read that a white or light-colored roof makes a difference. If the difference is large enough, it might extend the life of the AC and might also mean you can get away with a smaller/cheaper AC when the time comes.

I was an environmental studies major. I am a big fan of passive solar design, for a long list of reasons.
posted by Michele in California at 2:55 PM on June 19, 2012

If the roof is in good shape, you can paint it. I've never done it, but allegedly it will dramatically improve the temps in the attic and probably the upper floor as well. It's cheaper than either of your other options so there's no reason not to give it a whirl, as long as you're comfortable doing it yourself.

It'll require cleaning the roof, probably with a power washer, and then priming and painting with the elastomeric coating according to the instructions.
posted by Kadin2048 at 3:20 PM on June 19, 2012

Response by poster: Sadly, most of my roof is a complete death trap. Waaaay too steep for me to walk on. So I'd have to leave all the labor to the professionals.

That paint link is fascinating. I had no idea that existed. Have you used it or seen it used?
posted by BeeDo at 3:27 PM on June 19, 2012

Speaking of coatings, I habitually rave about the products from Ames Research. I have about a 500 square foot section of flat roof on my house I rehabilitated with products from Ames, the Blue Max and Maximum Stretch products. I spent maybe 1000 bucks total on product and, if I had to do it over again, I would have spent more to go all-out and embed their roofing fabric to build up my base layer more. The only reason I didn't is because of poor experience with other products. (Like those found at the local big box hardware store.) I wasn't sure it would work and, at the time, I had to special order from 1000 miles away. But work it did! And even though I already had "cool" gray roll shingles on the roof when I bought it (which were leaking because of ponding areas), the UV-reflective white coat made a significant difference in the indoor temperature on my top floor despite the fact that the rest of the roof is dark asphalt.

All that to say that I am sorely tempted to strip the rest of my roof to the decking and put down an entire coating-based roof system. It remains cool to the touch in the southern sun (not quite as far south as you but almost) and wind can't drive rain up and behind the shingles or blow shingles clean off. I haven't experienced hail with it yet but my sense is it can't suffer any worse damage than asphalt shingles. The only downsides I foresee:

1) If you have overhanging trees that deposit gunk like I do (hackberries... nasty things but lots of shade...), cleaning or top-coating every few years is necessary for optimum UV-rejection.

2) Your house will definitely stand out with a gleaming white roof.

It's this last item that is the only thing holding me back at this point. (I'll have to get my arguments together to convince my wife.) As this is definitely a non-standard approach and you won't be doing the work yourself, you'd need to find a contractor a bit more creative and brighter than the average to get this done. As far as I'm concerned, the matters of energy efficiency, imperviousness, and cost-effective installation are all settled. All that remains in question is the longevity and cost of maintenance over time. But if your curiosity is piqued, please do look more into it. (And maybe post about the experience somewhere if you go through with it!)
posted by ferdinand.bardamu at 6:00 PM on June 19, 2012

And as far as your AC goes, I'd consider revamping the current system to service only the 1st floor and adding a split system for the top floors. This way your main unit will only be servicing the 1st floor and thus should operate more efficiently. Meanwhile, the 2nd floor will be completely separate so you can only run it when and as you need it. Split systems are more efficient in general and you can have each bedroom on its own zone.
posted by ferdinand.bardamu at 6:10 PM on June 19, 2012

Ducts (and the air handler) should never be in non-conditioned spaces. Never, ever. That's your big problem. A new roof and a multizone unit will not fix that fundamental issue. As is, you're pouring your money into your attic. Two solutions, neither of which will be cheap:

1) move all ducts and internal HVAC equipment into conditioned spaces. This is likely a major undertaking.

2) insulate your roof and seal the attic space, converting it from a non-conditioned space to a conditioned space. This is maybe a slightly less major undertaking.

If neither of those are palatable, at the very least strip the insulation from the ducts, seal all seams and joints, and re-insulate ducts with new insulation that isn't falling off.
posted by 6550 at 1:38 AM on June 20, 2012

Seems to me that you NEED a new roof, but you don't (yet) NEED a new AC. Upgrading the AC doesn't do anything for the roof. But if you replace the roof and manage to make it more energy efficient, you may not need to do the AC at all.

Ducts shouldn't be in uninsulated space, but if the attic is well insulated and the ducts are well insulated, then you shouldn't have too much of a problem. Ducts in unconditioned space lose their cold air in three ways:

1- They leak. If they leak into conditioned space, no big loss. If the leak into unconditioned space, yes, it's wasteful. But ducts should be sealed. Not with "duct" (or duck) tape, but with either the metal foil tape or duct mastic.

2- Conduction. The hot air warms the walls of the ducts, which "sucks the cold out" of the air. The bigger the difference in temperature, the faster it happens. So if you can insulate the underside of your roof and drop the attic's temperature from 120 F to 90 F, you cut the losses by half or a third. If you also fix the insulation on the ducts, you can drop it to almost nothing, because you slow down conduction. Then you insulate the floor of the attic to cut the flow of heat (and air, if it's leaky) down into your second story.

3- Radiation. The hot sun warms the roof, which radiates into the attic, which warms your ducts. If you have a radiant barrier on the outside of your roof, then on the underside of your roof, then on your ducts, and on your attic floor, you practically eliminate radiant heat transfer.
posted by gjc at 6:36 AM on June 20, 2012

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