I am needing some insight on which would be the most cost effective measure -

Foam board on main duct line
Foam board on walls of crawlspace

Recently bought a 1975 two story house with a 3ft crawlspace which houses the heat pump handler and ductwork as well as water pipes.

I have been working on sealing and improving the thermal barrier of the house since I figure it will be the most likely thing to pay off in the long term. I would like to do something in the crawlspace and was thinking about putting foam board on the walls OR lining the duct trunk line with the foam board. I plan on doing both but for this winter likely just one so I would like to do the most useful first.

I like the duct route just because it seems like it would be a good way to further seal the main line but if the walls would be better served...

Around here, 1" foam is $15 per 4x8 and 2" is $40. Would 1" be enough or should I put two layers on? I figure 1" would be good for the ductwork but maybe 2" for the walls?

Also, should I be concerned about missing any potential future foundation issues (none that I currently know of) by covering the walls?

In SW Missouri.

Thanks

 

If the crawlspace is open to the house air, insulate the walls.

If the crawlspace is open to outside air, insulate the ceiling and the ductwork.

Hopefully it's the former.

 

Well, it would fall closer to the latter during the winter and definitely the latter during the summer.
I recently made foam/plywood-backed plugs to go in all of the vents for the winter and a 4inch foam door for the access hole. I haven't foamed the rim yet but it's not obviously apparent to be open to the outdoor air. During the summer I plan to remove the vent plugs although it seems like there are heavy arguments for or against that. Until I get a vapor barrier down it's my understanding that I should open them in the summer though. It's not open to the interior air at all.
I would like to do the floors as well but that's waiting on the vapor barrier also.

So with it being closer to the latter of your options, would the immediate benefit regarding the foam board be the duct trunk?

Always up for any additional insulation input as well.

 

First, get a vapor barrier down on the dirt.

Second, if you intend for this to stay open to the outside, then you need to insulate your ductwork and the ceiling of the crawlspace.

 

I know, I have no excuse for not getting that done
Thanks