Compressing fiberglass insulation

[phantom]

I have a 2 X 4 framed exterior wall that I want to insulate. I took down 1" thick plaster which I plan to replace with 1/2" sheetrock and fur out the existing studs 1/2" - leaving a 4" cavity to be insulated. What type of batt insulation should I use?

R-13 (3.5" thick), R-15 (3.5" thick), compress R-19 (6.25" thick) or compress R-21 (5.5" thick). The R-13 and R-19 are readily available at the big box retailers.

It's a north facing wall with a New York winter.

[LazyPup]

Compressing fiberglass would defeat its purpose. Contrary to popular belief it is not the fiberglass fiber that creates the insulating effect but rather it is the air trapped in the fibers that retards the movement of heat energy. Compressing the fiberglass matting would reduce the ratio of air to total volume thus reducing the insulating effect

[Drywall DIY Guy]

Insulation

Don't compress the insulation - it will lose it's insulating effect.

To help the insulating effect, apply an airtight moisture barrier over the insulation just below your drywall. Tape at the edges and especially tape around outlets. This will create the dead air space that will allow your insulation to perform best. Also, it will keep moisture from forming within your walls. Damp insulation does not perform well.