I just purchased a 2300 sq ft brick home built in the 20's. It 2 walls... or 2 bricks thick with a good 3" gap between the bricks. Of course there is no insulation. I did a major revamp of the interior already, made a mistake and didn't use poly(?) foam board insulation on the exterior walls(under the drywall.) I need new windows and a new roof. I want to know if I can, and if it would be cost effective to have fiberglass blown in between the brick when I do the roof and windows. I have very little construction knowledge, but my thinking is that the framing for the windows could be drilled on the sides and bottom to blow in the insulation. Also when the roof is replaced maybe I could accees a space to blow more in from the top? Anyone have knowledge or an opinion? Thanks!

Blowing insulation into this space is probably a bad idea. Exterior masonry walls should have at least a 1" airspace to facilitate drying. The fiberglass could become saturated, not perform well and cause other serious problems. If you can access from the top, 2" of insulated sheathing against the interior wythe would be ideal but I doubt this will be possible. Focus on air sealing the new windows but more importantly, the basement/crawl and attic ceiling plane. Beef up your attic insulation with blown cellulose to R50 or higher.

I also think that you could boost the walls energy performance considerably by filling the 3" space with closed cell polyurethane spray foam. This would perform much better than the fiberglass over the long term and would provide a barrier to moisture infiltration. However, this could put the long term durability of the brick at risk because of the elimination of the drying space.

Perhaps you could perform a combination of the two by only spray foaming the inaccessible areas. If so, stick with closed cell.