My husband was in jail for 1.5 years and wondering if he had to pay child support at that time. In Washington state.

The child support order would still have been in effect unless modifed by the court which ordered that child support be paid. Of course normally imprisoned CS obligors are not in a position to pay.

Unless a modification was awarded by a court the arrears would accrue.

If he had money in the bank, he could have continued to pay, or a garnishment could have been done, unless he got the order modified, the amount due keep getting larger each month.

It's already been paid but was wondering if that's usual. It seems people don't tend to go to court to get child support modified- is that because it's expensive to get it modified or the court doesn't usually grant modificaitons? Just wondering...

Many just don't think about it or some don't know they can. And for others, the trouble or cost

Since most child support is awarded by formulas based on income, courts will grant a modification for a change in circumstances. So if people don't go for a modification it's there missing a bet.

... is that because it's expensive to get it modified or the court doesn't usually grant modificaitons? Just wondering...

Both. Obviously hiring an attorney to seek modification is costly. And in many places one has to prove a significant change in circumstances, which can be difficult to do.

In your case, his being in jail, it would have been difficult to do. Inmates don't usually have the money to hire an attorney to get child support changed.