Monthly payment on $100,000 30-year mortgage at 4% is about $475 ($5,700/yr). I've seen people say that marginal tax rate after considering ACA subsidies is about 22%, so $1,254. Add in $4,000 in interest and you're over 5%. Seems like early in the mortgage, it would be worthwhile (adjusting returns for risk); but the situation while holding the mortgage would look rosier as time passed.