I've written a little calculator in excel to see what effect changing thermal elements will have on energy usage. The calcs follow the normal accepted practice similar to that used in SAP calculations and It's going OK but I don't believe the results any more than I believe the SAP calculation I have.

For a completely uninsulated old solid brick house with 100mm insulation between rafters in the loft it comes up with:
Walls: 30%, roof: 12%, windows: 34%, floor: 23%

I have upgraded most of the loft to 300mm insulation. The heating upstairs (zoned and reasonably well controlled) no longer comes on at all. Heating bills appear to be less than half what they were immediately before I insulated (though it is very warm this winter). I am convinced the insulation has reduced heating by more than 8% - with that tiny reduction the heating upstairs would still be on but it isn't.

I think the issue is that heat rises. The SAP calculations (and my own) don't appear to take any account of this. How would you go about plugging the effect into your own calculation?

Or does anyone have anecdotal evidence that could prove my calcs that suggest insulating the ground floor but leaving the roof poorly insulated would have a far greater effect than insulating the roof and leaving the ground floor un-insulated?

Posted By: vordWalls: 30%, roof: 12%, windows: 34%, floor: 23%

What sort of windows/glass do these figures use Vord? You've pretty much described my house there
However my roof is actually ERI (ie not EWI!) of 100mm EPS and I have replaced almost all the glass in the top floor for Ug 1.1 DG (from 1 x 3mm pane) and right now we are getting so much solar gain that with the heating on downstairs (needed) but off via TRVs upstairs (pretty much off anyway) it is too hot in the bedrooms - so must be rising heat (ie via hot air, conduction (I have heavy solid ceilings/floors) and radiation).

A couple of months of winter (so far) might not be enough to get a good handle on what's happening.

Still, looking at http://www.degreedays.net/ might help. For the weeks from the beginning of September to the end of the year for RAF Benson in Oxfordshire heating to 15.5 °C required 733 degree days last year and 515 this year. Add a bit more solar gain this year, perhaps, and it could go most of the way to accounting for your heating difference even if you hadn't insulated the loft.

What airtightness did you assume for the house? Have you done anything (other than adding the loft insulation) to improve airtightness over the year?

It would but for the fact that usually sound deadening quilt will be required between upstairs and downstairs


You think SAP is bad try using EPC software... Worse

Very surprised simply adding loft insulation has made such a difference, but well done anyway!

Still off-topic: Can you make purpose-made timber frames and use a 2G unit (low E glass, warm-edge spacers and argon) in the 2ndary glazing? We have done that a couple of times.

Posted By: vordCompletely off-topic, but are there any practical issues in having an E coating on the secondary glazing? How resilient would the e-coating be to a wipe with some window cleaning stuff once a year?

There are two types of low-E coating: hard and soft. As far as I understand it, soft coat is more efficient and is what is used in modern double and triple glazing but it does need to be protected. Hard coat (e.g. Pilkington K glass) is a bit more durable I think and may cope as the inside face of secondary glazing as long as you take care of it if you remove and store it in the summer. Check with the manufacturers, I'm sure there's more than one.

I have been agonising over secondary glazing. I have a couple of beautiful arts and crafts leaded windows on the ground floor over 100 years old. The windows are fixed but I feel some sort of lift out secondary glazing would be useful for maintenance and for clearing condensation or replacing bags of silica.

My father fitted secondary glazing in our house when I was a kid. It used a plastic channel around the glass and clips screwed to the frame that held it in place. It worked very well. In particular, if fitted on a dry day in late autumn there was no condensation until it was removed in spring.

I had thought about a custom frame with fixed glass. My current compromise thinking is to fit aluminium lift out and sliding secondary glazing with bits of custom frame and try to hide it all with a timber moulding nailed to the front.

I'd expect that you need to open those and wipe them every day to remove condensation. At least that's what we had to do in our rental place.

I'd look for some solution that used timber and/or some plastic to frame the secondary glass. Try to get a reasonably airtight seal to the original frame if you want to avoid condensation. Magnetic seals can work quite well - either the long magnetic strips sold for the purpose or the small button 'super' magnets. It depends what you can accept in the way of alterations to the existing window.

Nick's suggestion sounds sensible.

I stick p strip draught excluder between the plastic secondary frame and the timber windows. The screws then provide a compression seal. Works well, but remember there's also moisture ingress from outside you have to consider.

Probably g/f window is mega draughty

I have noticed during the winds how draughty our windows are between glazing and framing. Clean up and seal with glazing silicone?

Posted By: gravelldClean up and seal with glazing silicone?

Yes and no. My panes were just slid into slots in my frames from above - free to rattle around in any wind!!! (as was the way 40 years ago here in Italy). I cleaned up and sealed with silicone - quickly got the knack and did an excellent job BUT such work is strictly temporary. The silicone gets dirty, peels back, goes milky: it is simply inevitable when silicone is used in this fashion. There some newer crystal rather than transparent/clear 'silicones' and these cost more and last commensurately longer. BUT still temporary and how long is the piece of string, depends on the location, cleaning regime, skill of putting it on, prep, and of course your tolerance for deteriorating silicone around windows. I am not saying don't do it - esp as the only better alternative is to refit all the glass, just be aware.

I think I've figured out where I was going wrong with my calcs. I simplified the house a bit while I was figuring out how to get the calcs to work, then didn't add in enough detail (like the attic). That was quite important as the attic walls were lath and plaster on studs without insulation. One of the bedroom ceilings had no insulation.

Assuming constant temperature inside it looks like I would have reduced the heating bills by 38% by roof insulation. (In real life it will be less due to closed doors and heating zoning and the fact I've not quite finished an awkward crawl space, but I'm more motivated to do it now).

It's useful to have a spreadsheet to fiddle around - it won't be a perfect model but should help indicate where I can grab a bit more improvement for little effort.

Posted By: gravelldWhat do you mean by crystal?

Crystal is the colour. ie normally they say transparent or clear but crystal is even more transparent and IME stays that way longer but let's not hijack the thread.

Ok, I suppose that explains it a bit more.