I have read that new aerogel-type insulation offers potentially better efficiency than ordinary fiberglass, albeit with greater cost. Is this true at a practical level? Should I look at replacing some of my old fiberglass insulation with aerogel, or is it just hype?
It's not hype, but it's not a commercial viability for home insulation at this time.
I was just looking at the prices for aerogel and it's really expensive. It seems that the current use in house insulation is as a wrap. And from the prices I saw, it was $26 per square foot for 2mm thick material. Premium fiberglass is about $0.50 per square foot (2"x6" walls) and premium house wrap is about $0.14 per square foot.
Most (any?) foam insulation will be better than fiberglass. But also more expensive. Regardless, there's no need to go with aerogel unless you are building something that needs to be ultra-lightweight, which houses don't usually need to be. Replacing fiberglass with standard spray foam insulation will be a giant improvement. Not sure you'd be able to justify the cost of the aerogel, though that would depend entirely on other factors (climate, your budget, design of the house, etc.)
All that said, retrofitting fiberglass is a huge project as you need to replace the fiberglass, so you're talking about removing sheetrock and the like to accomplish it.