Even under the best circumstances, attic bonus rooms are hard to cool, especially in older homes that cannot easily be air sealed.

Bonus rooms typically have sloped ceilings and and attic-facing knee-walls. A critical detail is to build the knee-walls so they can be air sealed. This requires a top plate or blocking as well as a back on the wall so each stud cavity is enclosed on all six sides.

The other critical detail is getting adequate insulation in the slopes, while allowing space for ventilation between insulation and roof. This is almost impossible to achieve without furring down the ceiling in order to increase the rafter bay depth.

These challenges go away if the entire roof is insulated, typically by applying spray foam insulation to the underside of the roof deck. This brings the entire attic into conditioned space, and thus the knee-walls do not have to be air sealed or insulated. Furring down the slopes can often be avoided by using high density closed cell foam.

As for conditioning the space… it may be possible to use your existing central AC, but only if your system has excess capacity and there’s a way to route adequately sized supply ducts to the rooms. $3k for a separate system sounds very low when you consider what’s involved in installing a new duct system.

A couple of ductless mini-split heat pumps would be the least expensive option if you don’t want window units and don’t mind if the stair landing and bathroom get hot. But I don’t know any competent contractor who would install two mini’s for $3k.

@Steve, your problem appears
@Steve, your problem appears to be with the design of your HVAC system. Some states now prohibit a single system for two floors unless there’s an automatic zone control system. And even that’s a challenge to get right.

Without knowing a lot more about the design of your house, and where the weak points are, it’s hard to say how best to improve your situation. Your electric bills actually sound pretty high to me for a house that size in central TX, but I don’t know how much you pay for electricity. The usual suspect is leakage at the ceiling plane and any knee-walls. Reverse stack effect (in hot weather) will create a highway of hot air from your attic into the house, especially the upstairs. The opposite happens in cold weather, thus your improved temperature balance in winter.

David and Rhonda — You’re
David and Rhonda — You’re talking about what I’ve been considering.

Some background:
1. 2-story house, built 2008, about 2900 sq ft, San Antonio (hot, humid summers, mild winters), with good insulation as indicated by low electric bills (no higher than $175) in summer or winter months.
2. Carrier heat pump, 14 SEER, works just fine.
3. In late winter/early spring, we can go weeks and weeks without using either the A/C or heater by opening windows at night to cool the house, then closing them in the morning to keep the house cool. But while downstairs is just right, upstairs is, I’m guessing, 5 to 10 degrees warmer.

I figure if I can fix that, I can also reduce the load on the A/C in the summer, when temps can be above 95 from July through September.

So … radiant barriers aren’t worth it, huh? Didn’t seem to me either, and your post convinces me now (without even adding the cost of fixing ceilings workers fell through).

Replacing an 8-year-old roof surely isn’t worth it.

Until now we’ve closed, a little, the vents downstairs while leaving the upstairs wide open. Helps some, but not really enough. Ends up making downstairs, where we mostly live, uncomfortable.

It’s just occurred to me: temps upstairs are fine in winter. That is, if it’s 70 downstairs, it’s about the same upstairs. It’s only when the sun is bright that the temperature difference is felt. That’s telling me the reason the upstairs gets hot is because of heat transfer from the roof/attic. (I need to put my hand on the upstairs ceiling and feel if the ceiling is warm or not.)

Any input would be welcomed.

Thanks.

@Rhonda, retrofitting a
@Rhonda, retrofitting a radiant barrier on an existing home is expensive, especially the labor. With prices exceeding 50 cents per ft2, there’s no way a RB will save enough to justify the cost. Even in new construction where RB-faced sheathing only adds about 10 cents per ft2 to the roof, the payback is marginal except in the hottest locations in the US.

You’re much better off having your ceilings air sealed, insulation upgraded, and ducts sealed than spending thousands to have someone staple RB to the underside of your roof deck.