Does cavity wall insulation really make that much difference? Two completely different EPC certificates.

[atticusclaw]

Does anyone know how much difference cavity wall insulation makes?

We are looking at installing a biomass heating system which should work well for our property (big, isolated, expensive to heat but with a wood supply which would cover a big chunk of our needs). The system is incredibly expensive though and will cost over £20k.


We had to have an EPC carried out because ours from when we purchased the house five years ago was too old. The only thing we've done by way of improving the house since we've bought it is to have cavity wall insulation installed. However, the rating on the recent certificate has changed our energy usage from 231kwh per square metre to 170 kWh per square metres. Can that really be right, that we now use 60kwh per square metre less just through cavity wall insulation? We have seen very little difference in our energy bills and the house is now bloody freezing in the summer as well as in the winter!

On the basis of the new calculations the new system isn't affordable due to the high initial cost . I think I should query it but know nothing about these things.

[PigletJohn]

Cavity wall insulation does make a tremendous difference. Imagine lying on a concrete floor, in a freezing shed. Now imagine adding a two-inch thich foam mattress and a two-invh thick duvet.

Also, unlesx yoy live in a bungalow, the walls of your home will have the largest surface area of any elekent of your home.

However your figures are not right. I think you mean watts, not kilowatts.

How thick is your loft insulation?

How draughty are your windows?

Do you have uncarpeted floors?

Disregard the £ on your bill. How many cubic metres of gas (or litres of oil) did you use per average year, before and then after the CWI?

[atticusclaw]

Ah PigletJohn, I hoped you'd be about!

I think the kwh figures are right.

The house is large (345 square metres) in the main house. Loft insulation is thick. There is 150mm of old insulation down and then on top of that is 200mm space blanket insulation so about 35cm in total.

Windows are double glazed but probably early 1990s. Not draughty though (just ugly). The windows are enormous. In some rooms at the back of the house the windows cover the whole of the back wall space.

Some uncarpeted floors downstairs carpets upstairs.

The old EPC says that we use 231kWh/m2 per year i.e. 79695 kWh per year
The new EPC says that we use 170 kWh/m2 per year.

(These figures are just for the main house)

Oil usage is difficult to tell because its entirely weather dependent and we have made an effort not to use the oil heating and light the wood burner in one room instead (hence me working all day in my coat and fingerless gloves with an electric blanket on my lap) but if we use the heating then just over two tanks a year (2500 litres in each tank). This covers the office too which is another 50 square metres.

[PigletJohn]

here we are

Use this to calculate heat loss/boiler power requirement. Do the calculation twice; once with CWI and once without (you can tinker with other variables separately). It will give a fair estimate of the reduction in heat loss from CWI. IIRC it assumes that your house is heated to 20C and the outside temp is 0C.

I prefer to have oversized radiators, to cater for heating it quickly from cold, and exceptionally cold weather. Modern boilers modulate their power down from maximum, to match the load, within limits, so they seldom run flat out (except for combis). This means they usually run more quietly and tick along, and do not frequently turn on and off.

[atticusclaw]

The office is rarely used though.

[atticusclaw]

Not sure if I've done that calculation correctly. Comes out as 360.5 kw

I think there's something wrong with the EPC certificate. We had a green deal report to accompany it completed by the same surveyor on the same day

The broken down figures on the new EPC say

space heating 32148 kWh per annum (and also recommends loft insulation!)
water heating 4187 kWh per annum

The broken down figures on the green deal report say

space heating 48714 kWh per annum
water heating 6718 kWh per annum


If the figures on the green deal report are correct that would be much more in line with what I was expecting and would tally up with oil use (just over two 2500 litre tanks).

[PigletJohn]

a typical large house boiler size might come out at 35kW, a typical modern house might be 10kW, so I hope you have put in a wrong figure.

OOI I use about 1100 cu. metres gas a year for space and water heating, which is about 12,270 kWh. Averagely modern, averagely insulated, slightly above average size. Perhaps your house is four times the size, or older or badly insulated. Heat losses through glass are very big, even if double or treble glazed. If you do not heat your house much, actual energy usage should be lower than the calculator estimates.

Summer usage here is about 5kWh/day just for hot water. Modern condensing boiler + well-insulated cylinder. Two adults. Combis are a bit less efficient in summer. Your hot water figure looks very high.

Is it possible the green deal figures have been manipulated to encourage you to buy the biomass system?

[PigletJohn]

p.s.

your house is about three times the size of mine.

[atticusclaw]

Don't think so since I was very clear that we already had the supplier lined up and that we wouldn't want to use the green deal loan anyway. The suppliers estimated that we would be ok with a 40kw boiler but that a 60kw boiler was a possibility, they will confirm after the site visit.

The house is cold. Its big, early 1950s built, isolated and permanently in shade since we are in woodland.

Its all very confusing but I have decided to bite the bullet and query the figures with the surveyor. If nothing else I can't see why the two documents would have such different figures on them.

[atticusclaw]

Thank you for all your help by the way

[specialsubject]

EPCs are valid for 10 years. The green steal insists on a new one as part of the kickback.

Be very careful of that supplier.

[atticusclaw]

The EPC isn't valid for 10 years, the government requirement for the RHI is for an EPC which is no older than 2012. The EPC is nothing to do with the supplier at all though so not sure why I should be wary of the supplier.

The issue (according to the surveyor who carried out the EPC) is that we have two children. Apparently the EPC takes the actual energy requirement to heat the house and then reduces that figure by dividing by the number of people who live here (4) and multiplying by 3 because most homes only have three occupants. Which is clearly a complete nonsense because my house doesn't get cheaper to heat if I only have one child. It costs what it costs.

Anyway, the upshot is that we can't put in the biomass boiler which would have a massive beneficial environmental impact because the artificially reduced figures mean that the payments don't cover the value of the installation. Complete nonsense. The whole thing seems designed to create money for EPC certifiers.

Really cross that I've spent £150 on this stupid worthless certificate.

[MsAdorabelleDearheartVonLipwig]

We had our wall cavities filled. The difference was amazing. It was especially noticeable in our living room. The air went from feeling chilly and fresh to warm and still. It was instant. The day it was done you could feel the difference. I'm so glad we had it done.

[specialsubject]

"seems designed to create money for EPC certifiers"

you got it. There is a reason the green deal has had a very low take up.EPCs ARE valid for 10 years for sale or rental, but part of the scam is to make you pay for a new one.

much of the EPC is nonsense; I've seen recommendations including a wind turbine in a tiny sheltered garden, and solar panels that would take a century to pay off. The 'savings' are now shown over 3 years to make them look bigger. About the only use for the EPC is to show the area of the house and whether it has a condensing boiler.

removing VAT on energy saving improvements would be much simpler and cost a lot less. And therefore has not been done...

[Legwarmersforboys]

My neighbours had cavity insulation fitted & ever since have had problems with mould & damp in their house.

[PigletJohn]

I had cavity wall insulation installed, and ever since I have had lower fuel bills and a warmer house.

[Legwarmersforboys]

We have had it done & no problems, it's just worth knowing that it can cause issues in some houses.