Insulation, Radiant and Vapor Barriers - Rigid foam board insulation

Help, I need more insulation because of high heating costs!
My house is 2x4 framing with steel braces at corners--no OSB, with 1" foam board between framing and siding. When I install new siding, I want to add as much additional foam board as possible before putting new siding. Has this been done? Can I add 2 inch foam boards over the existing foam before siding? Can I go even thicker with foam insulation? (I know I would have to account for eave and rake overhang )

Although it seems unlikely that your hose has no sheathing, removing the old siding would present an opportunity to blow insulation into the wall cavities. The blown insulation would be more effective than added rigid foam.

Hi Chris, thanks for the quick reply. But the wall cavity is already full of fiberglass batts. I dont wanna change that. Starting from inside the house, it is drywall, fiberglass between 16 oc 2X4's, then 1" foam outside, then alum siding.
There's no vacant wall cavities. I want to add more, say 2" more of foam, and then install new siding, probably vinyl.
Can I put OSB over the existing 1" foam, then 2" more foam, and then vinyl siding?
Yes, there is no wood sheathing--rather common and even though it is about 25 years old, the idea was sorta ahead of its time. Structural integrity comes from let-in steel bracing at all corners. Yes you can come in with a can opener.

Foam needs to be against each other or it will act like another VB.

I'd go with the blown insulation in walls if void and then do the radiant barrier foil over insulation in the attic. Look at link on my post above yours.

Sorry RB will not do any thing for you in a year. Don't waste your money.

I recommend checking out Fine Homebuilding, Issue 194, May 2008.

There is a great article "Remodelling for Energy Efficiency". It describe three increasingly energy efficient renovations conducted by a professional architect.

In the 3rd renovation, the builder filled the 3.5" cavity with cellulose, then added two layers of rigid 2" polyiso foam creating a R-40 exterior walls. (So, drywall, cellulose, sheathing, housewrap, 2 layers of 2" foam, furring strips, wood siding)

To me, the trick, and shown in the diagrams, is extending the water plane of the windows and doors. They used exterior sill extensions. I'd like to learn more about how do this detail work, as I am also considering adding additional rigid foam to the exterior under the siding. I haven't found any commercial products for extending the sills, perhaps they milled it custom.

Good luck!