How to know if render covers 50%

housebuyer7

Hello,

I need to re-render the rear of my property. I am not sure whether I will be required to insulate according to building control. Does anyone know how I will find out, do councils let you request a visit from building control?

FreeBear

It is 25% of the whole exterior. So if you have a terrace house, the whole back wall would be about 50%. A semi, roughly a third (assuming a square footprint).

That said, if you are just removing the top layer and putting another coat on top, it would be a repair. So you'd avoid needing to contact Building Control - Only if you are going back to bare brick would you need to involve BC.

Rerended my property last year (3 bed semi). Chipped off the top layer of render & pebbles, and then applied a smooth coat on top. As only the upper half is rendered, it accounts for ~50% of the total wall area. And as a repair, BC involvement wasn't required. Still... Insulating all the external walls as & when I get time/money.

housebuyer7

I see, I think what I have is two layers (maybe even three) of blown cement render on a Victorian terrace with lime mortar. My understanding was that I should probably go back to brick and re-render in lime. But the cost is a lot and double that of re-rendering in cement. I was wondering if I could save costs with not insulating as the house is already very warm.

housebuyer7

I have internally got stud walls on the rear wall with polystyrene between the stud and brick wall, will this count for my insulation?

FreeBear

housebuyer7 said: I see, I think what I have is two layers (maybe even three) of blown cement render on a Victorian terrace with lime mortar. My understanding was that I should probably go back to brick and re-render in lime. But the cost is a lot and double that of re-rendering in cement. I was wondering if I could save costs with not insulating as the house is already very warm.

In an ideal world, a pure lime render would be best. If you already have some wall insulation on the inside, you can probably get away without adding more.

There is a little cop-out clause in the Building Regulations that says if the cost of insulating exceeds a simple payback time of 15 years, you can get away without putting a layer in. By the sounds of it, you are unlikely to achieve any significant savings on heating. Then again, you don't have to notify Building Control of the rendering - Unlikely they would find out....

housebuyer7

I would like to re-render in lime. I have some other works that I need to notify building control about so I think it may be best if I notify them about this too? If I can use a clause not to insulate that would be good. I am not sure how I can demonstrate the clause you mention?

FreeBear

How much do you currently spend on heating per year ?

Punch in a few numbers into this calculator -> https://www.changeplan.co.uk/u_value_calculator.php to get the current u-value of the wall. Then run the numbers again and aim for a u-value of 0.30 W/m²K or better. That will tell you how much more insulation you need to add.

Now for the fun bit... The mean average temperature in the UK during winter is 4.2°C (1). If you heat the house to 20°C, this gives a temperature differential (∆T) of 15.8°C.

Calculate the total wall area to be rendered (say 10m²).

Your heat loss is a simple calculation of Area x u-value x ∆T - Using the numbers from above - 10m² x 0.30 W/m²K x 15.8°C = 47.4W

If you were to increase the insulation to the point where you hit a u-value of 0.18 W/m² (2), this would reduce your heat loss to 28.44W, a saving of 18.96W or 40% through the walls. A house will typically lose 25-35% of its heat through the walls (we will use the lower number), so your 40% improvement is only 10% of the total heat loss.

Take 10% of your annual heating costs, multiply by 15 years - Is this less than the cost of insulating the walls ?

If so, do the best you can within your budget. Building Control may well accept other thermal upgrades in lieu of wall insulation (more loft insulation perhaps). BC may want a proper SAP heat loss calculation - That, I'm afraid, is going to cost a bit as you'd need to find and pay for a professional. But they should look at all elements of the building (roof, walls, floor, windows, etc) and come up with reasonably accurate figures. Just ask for a before & after calculation to see just how much energy you would save.

1)http://www.sheffieldweather.co.uk/Averages/MONTHLYAIRAVERAGE.htm

2) Target u-value for refurbishment is 0.30 W/m²K, and new builds 0.18 W/m²K. An uninsulated solid brick wall is going to be around 1.90 W/m²K.