U values External Wall Insulation

ggpowell2

I am putting external wall insulation on my 1920s house to achieve the required U values before rendering or brick slips (not decided yet), but I am conflicted, the minimum I need is 20mm to achieve required refurbishment u values but if I use 110mm I would achieve the required u values of a new build, or of course I could go somewhere in the middle, I'm just weighing up if the difference is worth it?   The difference in terms of money is only a couple of hundred really so is not the main concern, my main concern is the aesthetics, especially as a terraced house, and with things like window cills etc. - please can people share their experiences or advice on what they would do?  Does anyone know what the real terms difference is between 0.3 or 0.16 w/m^2K?

Jeepers_Creepers

Is internal insulation not an option? 50mm on the insides of the external walls would have you as warm as toast. As little as 25mm would be transformative. I mean that - you will KNOW the difference. Your house will be transformed (as long as you also sort out these draughty floors...)

Or, even a combo - 50mm (or even less) on the inside, and no more than 100mm on the out?

Inside is far better than out, as its insulating effects are guaranteed and immediate - it'll just stop the heat leaving your room. Does your house have a wall cavity? If so, the difference betwixt in and out will almost certainly be even greater. (If your walls are solid, then I guess 'outside' will be very effective too).

I can't give you actual figures, but what I do know from experience and logic is that doubling the insulation thickness does not double the insulation 'effect', or anywhere close. 200mm on a wall sounds completely insane to me. Absolutely do it as long as there is no good reason not to; eg it's not a cosmetic issue, or the cost is not much greater. I mean, if there's no actual 'harm' in doing it other than a £ew hundred squid, then do it! But if it means a compromise on things like aesthetics, then stop... You will not - and I really mean, not - detect, sense, see, feel or measure any improvement with that extra 100mm. Your house will be warmer with that 100mm and your energy bills less, no question. But your house will not be noticeably more warm with an extra 100mm, and your energy bills will not be noticeably even less.

Think it through; 100mm of insulation will make a massive difference - that blocks just about anything! You could sit yourself, stark naked, in a 100mm-walled box in the Arctic and be as cosy as anything (although ultimately asphyxiated). Barely any heat escapes - only the teeniest, tiniest, twiddliest, tinkliest small amount comes out through to the outer surface. Of the heat your body exudes, 95% will be trapped inside your wee Arctic box; 5% at most will escape through the insulation (figures plucked scientifically from the air). So tell me what difference will an extra 100mm of insulation will do? You are doubling the insulation thickness in order to tackle the 5% that the first 100mm has failed to contain. Cost effective? Almost certainly no.

There endeth my Points of View for today :-)

shinytop

If you go to stelrad.com there is a detailed calculator for heat loss.  You can model your whole house room by room or just plug in values for one room to get an idea.  Halving the U values on external walls will make a reasonable difference.

FreeBear

Jeepers_Creepers said: Inside is far better than out, as its insulating effects are guaranteed and immediate

I would also agree that internal is "better". Unlike external, there is no messing around moving downpipes or fudging detailing around windows/doors, nor do you have to worry about the roof overhang. If you don't get the detailing right when insulating externally, there is always a risk that water will get down behind the insulation and cause problems.

Insulating internally allows you to "do" round the window reveals (and the doors too). Yes, there are issues with things like radiators, plumbing, and anything hung on an external wall, but these can be overcome fairly easily. In a well insulated home, radiators don't have to be sited under a window, so that gives you opportunity to simplify the CH plumbing. Kitchen walls, put up sheets of 19mm exterior grade ply in place of plasterboard. With plenty of fixings going in to solid brick, hanging wall units should never be a problem. A final bonus - You get nice flat, smooth walls.

Having insulated one room here internally, it has made a huge difference. Losing ~75mm from the room hasn't been noticeable and it gave me the opportunity to do a bunch of other work in there (extra sockets, change radiator for a smaller one, repaint, etc).

OilyByker

100mm on EWI is rarely possible due to pipes, window frames, soffits, all needing extending. 
The latest internal system is only 5mm thick and if combined with OVER FLOOR insulation (under carpet or laminate) then there is a possible 48% decrease in heating required for the averge room. As with external insulation you cant suce
sfully add more that 20mm on polystyrene insulation to the inside either. Electrics, switches, plumbing, radiators, door casings skirting boards all effected. Great for a complete refurb from scratch. Nanotech fabric insulation is the answer, I had it fitted under my carpet on a floating floor. 100% better.