I just want to note that not everybody is telling you to replace your windows.
Like I said, in the real world, air leakage is more important than R-value (which doesn't take air leakage into account). Old windows, if the weatherstripping is in good shape, no leaks around the frame or the sashes, with storms installed as well... pretty much as good as new windows, IMO.
Most of the heat loss, in either case, is infiltration, not radiation.
I'd have to agree with frenchie.
I'm in British Columbia, where there have been 10's of 1,000's of "leaky condos"--buildings/houses that were super-insulated without enough thought to dealing with water penetrating into the wall cavity. There were so many claims for rotting/moldy houses that the provincial home insurance corporation went bankrupt...
CMHC (the Canadian Mortgage and Housing Corporation) has since launched a whole series of seminars to teach the construction industry here about the lessons learned in this fiasco and how to avoid repeating them.
I took their 65 hour course a few years ago.
Here are the main points that apply to Master Brian's situation:
1) Old houses with large eave overhangs, proper exterior window sills, and hollow (uninsulated) walls seldom rot or go moldy.
2) The biggest mover of water vapour into a wall cavity is via *air* movement vs. vapour diffusion through the surface of the wall itself. i.e. air leakage through a 1" x 1" gap (or, equivalently, a 1/16" x 16" gap say along a baseboard or window jam) will deposit several *pints* of water into a wall cavity during a heating season. A non-vapour barriered (but air-sealed) wall will only deposit a few ounces of water during that same time. Therefore a perfect air barrier (with absolutely no cracks/gaps around penetrations in the wall like windows and doors) will prevent rot/mold much more than putting poly on the inside surface of your walls.
3) A hollow wall will disperse moisture during the dry season. The interior side of the cladding will effectively be at "room temperature" and will therefore give up the moisture quite quickly. The unrestricted convection air currents within the wall will then disperse moisture enough to avoid rot/mold.
4) Convective air currents within a wall "pump" heat through the wall. The air adjacent to the warm surface warms up, rises within the wall, cycles over to the other side of the wall (the cool side), gives up its heat, sinks, and then moves back over to the warm side.
5) Insulation works by stopping these convective air currents within a wall. Loose fill insulation (fibreglas batts, cellulose, open-cell foam) have enough air resistance to stop the weak convective air currents, but offer little resistance to wind- or stack-effect pressure differences. As frenchie said, they act as simply a dust filter (hence the black stuff you see on insulation batts whereever there are air leaks).
6) Old houses rarely have a reliable air barrier. Spaces between clapboards. Spaces around window jams. Stucco butting up against wood which swells and contracts during the seasons.
7) Because we know that *air movement* transports the majority of moisture into old walls, we need to ensure that there is a mechanism to get that moisture out of the wall before rot/mold sets in. In old houses, the hollow walls (with their convective air currents) are the mechanism that protects it.
8) If you install insulation into these old walls without also installing a perfect air barrier that seals all wall penetrations, then you will be allowing moisture to still penetrate the wall (via wind- and stack-effect driven air movement) but will be removing the major mechanism for removing that moisture (convective air currents).
This is a *huge* problem and is blamed for the outbreak of rotten/moldy houses in the pacific northwest.
To summarize, to keep you from turning your 100 year old house into a rotten/moldy mess, you should either:
1) Leave it as-is and swallow the monthly heating costs as an ongoing cost for keeping your house rot- and mold-free.
2) Tear off all the siding. Install loose insulation in the walls by whatever method you prefer (e.g. blown cellulose). Add an air barrier (Typar/housewrap) to the outside of the house with particular attention to sealing *all* seams and *all* the wall penetrations to the air barrier. Reinstall siding. NOTE: Tar paper is *not* considered an effective air barrier by CMHC unless all seams are taped. In that case, it will cause moisture problems if there is poly on the inside surface of any of the walls. Picture a tuna sandwich left in a sealed ziploc bag...
3) Blow *closed-cell* foam into your walls. No need to remove all the siding. No need to install a separate air barrier. Closed-cell foam *is* considered an air barrier if it's over 2" thick, so you can get the same results as #2 by *blowing* closed-cell foam into your walls. Foam board is *not* an equivalent because the gaps around the edges prevent it from acting as an air barrier.
Does this help?
.../j