If you must do this, then don't use the money on moonshots and pick something with low volatility.

Even if you don’t invest it, if you have other credit (car, credit cards) obviously paying those first is always a better choice too.

Many utilities give an 8-9% interest rate on bonds, with very little risk (Comcast isn't going bankrupt soon). That said, there's no such thing as zero-risk arbitrage.

Why bother?

Renting a house is more or less unheard of. If I want a house I need to buy one (or: “rent from the bank” as it were)

And the market price is basically “what two high income earners can afford if they pay interest only”.

So: if I want to live in a house at all, I need to buy it using borrowed money, and in the window I expect to live in a house (say 10-25 years) I couldn’t make a large dent in the principal regardless, because of the extremely low interest rates (around 1% now for mortgages) and the high prices that come from that. I don’t like it, but I don’t have a choice.

Also regulations strongly favor buying with borrowed money over renting. Interest can be deducted while those who rent from a landlord obviously pay interest too - indirectly through their rent - but in their case it isn’t deductible. So anyone who is forced to rent (e.g because they can’t afford the huge 15-20% down payment or aren’t paid enough to get the mortgage) is pretty much subsidizing the tax deductions of those who take mortgages.

I have a few older friends kicking themselves for not buying a property when they could afford it, rents have risen and there is a very real prospect that they will have to move out of the community they spent their lives in.