I have an old (1950's), victorian esque, 2-story, uninsulated, lath and plaster house that I would like to add some insulation in order to at least curb the hot/cold of the Inland Empire here in California.

All the inside walls and ceilings are lath and plaster while the outside sheathing is wood slats. We do have crawl space into the area between the first floor and the second floor behind the roof (B & C in my diagram). The portion above the second floor rooms (A in my diagram) have an open access to the outside.

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My plan is to put batt insulation on the roof portion of B & C and batt insulation on the ceiling portion of A. The roof beams look to be standard 2x4 with MDF (looks to be replaced less than 10 years ago). I can see some nails poking through the MDF where the shingles are attached.

The problem comes with insulating the portion between B-A and C-A. How can I get a decent R value on that portion at a decent DIY price? I was thinking rigid-foam and placing a 2" first then sliding a 1.5" under it (which would press it into the nails rather than sliding a 3.5" up and scraping the nails). That would only give me about R-15.

Is there a better way to insulate such a small place with a higher R value? Also, what should I do for the vertical walls? The outside walls are just 3"-wide wood runners nailed to the frame. If I remove one of them, I am looking into the wall cavity. No house wrap or water barriers.

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