I'm restoring a small barn (10' wide x 16' deep, with a 4' x 8' side
lean-to). It's built over a hillside, and supported by steel columns
placed in concrete footings, which replaced the old wood pilings. The
entire frame is 2" x 4" construction, including the rafters.

Here's the problem: I live in NY State, and need to insulate the barn
well in order to use it in Winter, as a home office. The building code
calls for R38 in the ceiling, and a minimum of R11 for the exterior
walls.

The walls are no problem, since I can install an R15 batt insulation,
and although I'd like a higher R value, I can't get it unless I build
out the studs to 2' x 6".

The problem is that the 2' x 4' rafters aren't deep enough for adequate
insulation. So here are my choices, and I need some advice from someone
knowedgable.

Forgot to mention that I'm replacing the roof.

1. Replace the 2" x 4" rafters with at least 2' x 8", or even 2" x 12"
rafters, and put the highest R value batt insulation available,

or,

2. Leave the existing rafters, and use 2 layers of rigid foam
insulation over the new plywood roof sheating. I saw some R12 rigid
foam insulation at Home Depot, so 2 layers of that (R24) and batt
insulation (R 15, gives me R39.

What's the best (and least expensive) solution? Thanks!

wrote:

The problem is that the 2' x 4' rafters aren't deep enough for adequate
insulation. So here are my choices, and I need some advice from someone
knowedgable.

Do you have a flat or cathedral style ceiling?

If you have a flat ceiling, what about just putting R38 worth of
insulation in the ceiling and having it stick up into the attic area?

With a cathedral ceiling you have a bit more hassle, since you don't
want to put insulation right up against the bottom of the roof--you
generally want airflow there. You can add furring strips to the rafters
(to allow for thicker batts), add rigid foam, or use a combination of
the two techniques.

In any case you'll want to vapour-barrier the warm side of the
insulation, and ensure that the attic has sufficient venting/airflow so
that the roof itself stays cold in winter. This will minimize the
chances of ice dams.

Chris

The problem is that the 2' x 4' rafters aren't deep enough for adequate
insulation. So here are my choices, and I need some advice from someone
knowedgable.

assuming you wouldn't be asking this question if you had a flat attic you could insulate, if you want R38 in the roof, you will need 12" of fiberglass PLUS a ventilation space. I would guress the cheapest way to attain that would be to build down the existing rafters with another 2x4 which could be hung with plywood gussets. this system would reduce thermal bridging which is a good thing. then install r 38 batts. foam would be a nice way to go but it is expensive in comparison to fiberglass. if you lay foam on top of the roof, you will need to add another layer of plywood on top of that (though if it's steel, i suppose you could just add some firring.).

BTW, i once owned a house that had 2x4 walls. I firred them out with
2x2's and insulated with r 21 batts. was quite easy and worth it i
thought.