Hi Declan !
As “O”and “Ajapale” said already the cheapest way, meaning the method with the fastest payback time, is to insulate and draft-proof the building. If the window frames are in good technical condition i.e not rotten than replace the broken panes with the best double glazing that you can find. Usually it is the metallcoated glass trading under the brand name VISTATHERME but there might be other manufacturers as well. When you have found a manufacturer ask him to fill these double panes with noble gas, usually ARGON. It costs a little extra but it is worth it (pay back might take an extra 3-5 years). Once you are at the windows check with your supplier if it is worth exchanging the draft proofing strips. The hollow ones are certainly better than the folded-strip ones, the same as on the door of your fridge. I assume you have timber windows since you spoke about damaged ones. Exchanging the strips on them is really a DIY job, pull out the old ones and push back the new ones.
The trouble with timber windows is that they naturally warp a bit with changing moisture content, so don't set the new panes flush with the frames,give them a little leeway that you can fill with clear silicone (the stuff for outside - weather resistant) before you put in the pane whilst the the silicone is still soft .
Check with your supplier if it is worth doing the other windows as well , at least the ones in the rooms that are usually heated and/or are at the coldest part of the house , usually the north side.
Now to the cavity walls : It's worth filling them , but usually that is already done. Now there are houses with cavity walls that are not filled, in this case the blow-in method is advisable. For material choice go for
expanded foam since there are some troublesome materials like cellulose or glass fibre that could suck up the moisture like candlewicks, become heavy and sack. Wet or sacked-they won't insulate any more. So which foam material ? The choice is between expanded polystyrene (loose beads) or, as you mentioned, polyurethane foam. Beads have the advantage that they are cheaper and recyclable. In case you want to break out parts of the wall - for the extension or whatsoever - you drill in a hole and vacuum the wall empty, the material can be reused .
The polyurethane foam is more expensive and less environmentally friendly due to its content of ozone damaging gas. The manufacturers have worked on the problem, nevertheless it still needs the expanding gas which is a climate killer, less than its predecessors but still. But it has also advantages. The insulation is a bit better ( U-value or k-value ). For exact numbers check the manufacturers homepage ( AEROBOARD versus KINGSPAN ) . In practical terms the difference between them two is to little to bother about , unless you have to follow the building regulations and the space for the insulation material is at the limit .
However, Mr. Byrne , our competition commissioner of the EU , was a couple of month ago investigating price fixing accusations against British and Irish manufacturers of expanded polystyrene. I lost track of what is going on and the outcome of it thanks to the quality of our mass media. The accusations were that the industry is overcharging the consumers ( compared to continental Europe ) by 30% .
To insulate the house from the inside is the last and worst method. As paddyirishman said you loose all the benefit of the heat storing capacity of a solid wall incl. the benefit to regulate the moisture content of the air , I call it a dead insulation. In a funny way : You could either eat the jumper or wrap it around you. However , in some cases (no access to the outside, no building permission , listed building) it might be the only way to save some energy . But be carefull with that, it is difficult - expensive - to get a proper job done this way , thermal bridges at corners,floors,ceilings are difficult to avoid. Thermal bridges will cause condensation and mould - sick building syndrom it is called , check the GOOGLE for it .
And , as ajapale said “hienbloed” is stressing it again and again: close the holes in the walls . No open fires !
Where there is a draft - upwards the chimney - the air that goes out will be replaced by fresh , cold air that has to penetrate the building. At the point where it comes in (the gaps) it will cool down the surface . A cold surface causes moisture to condense - mould.
Besides it's unsightly look it can damage your health and also the house structure itself. And you heat a lot of air that goes up the chimney. If you put in a new boiler anyhow than go for the so called “Condensing Boiler”!
These are enclosed boilers that take the air for combusting from the outside and release the fumes to outside .That means that they dont warm up air just to blow it out the chimney . Manufacturers home pages are to be found at
sedbuk.com/ .I searched a while for mine but it was worth it, a Vokera HYDRA that gives instantenius hot water and heats the house as well . There are others , but it was the cheapest , €1400 plus VAT from Cork HEATMERCHANTS. A condensing boiler does away with the chimney, the hot press , the draft and the boiler room . It pays for itself before you even started it . And they have an efficiency of 90% or even more.
Closing the holes is THE most important thing in insulating. Imagine that you rip out all the sealant strips of your doors and windows. The total gap would be smaller than the opening of your fireplace. So not to close the fireplace is the most stupid thing (energy saving wise). And when you are short of money or you haven't got the time: put a piece of corrugated card board against it , seal it with a bit of masking tape and you feel immediately the difference . Try it . You still can go back. The draft is usually so strong that the card board will stand on its own, sucked to the opening.
Check also
w.irish-energy.ie
Now, Declan , if you find any suplier of triple glazing let us know , there is a market here .