27 Apr 2009

On Cavity Wall Insulation

The house I am currently living in was built in 1960. I have long been pondering what exactly to do with it: the choices being the demolish it and rebuild a new, or to renovate it.

It’s got lots of glass. A very sixties house. Even though it’s double glazed to a reasonable standard (mostly aluminium frames), it gets cold and the heating system is barely up to the task of keeping it warm in winter. It will readily burn 20 litres of oil a day in winter on space heating and that translates to something like 150kWh/m2/annum under normal occupancy conditions. Or ten times the PassivHaus standard of 15kWh/m2/annum, now the target of choice for low energy designs.

It has cavity walls — that’s easy to tell because all the bricks are stretchers — but whether the cavity itself has insulation in it I have no idea. So, when researching an article last month about grants for cavity wall insulation, I contacted two agencies who undertook grant work and both said they would send a surveyor out within six weeks. Last night at 8pm, a guy rings and asks if he can come by at 8 am this morning. He actually arrives at 7.22. He’s gone by 7.27, having drilled one hole through the brickwork, found evidence of some polystyrene beading and then announced that “I’d no need to worry because I’d been done already.”

I am left wondering. What was all that about?

I await the visit of surveyor No 2. And more thoughts about the Cavity Wall Insulation Business.

The scheme contact details are as follows. One is run by the Energy Savings Trust, called the Affordable Energy Scheme (Tel: 0800 512012); the other is called the Heat Project (Tel: 0800 093 4050), which is funded by the energy suppliers. They offer pretty much the same thing, which is subsidised cavity wall and/or loft insulation. Anyone can apply for this grant-aided work, which has to be carried out by approved installers, and the rate paid is fixed at £189 which gets you 130m2 of cavity wall insulation or 55m2 of loft insulation – or twice £189 (=£378) if you want both. If your project exceeds these amounts, you are charged pro rata at £4.50/m2. If your project is smaller, then the rate is lower.

8 Apr 2009

PassivHaus ventilation questions

Picked up this interesting paper off a thread on the Green Building Forum. It hails from Canada and dates from 1969, but it has an air of authenticity about it. It answers a question I have long wondered about – just how much fresh air do we need to survive and prosper?

Every time we breathe in, we draw in just under 1 litre of air. If it’s fresh air, we draw it in at atmospheric conditions, which equate to 21% oxygen and 0.03% CO2 (or 300ppm – since 1969 this has increased to 380ppm, but that’s another story and it doesn’t effect the calculations here). Our lungs absorb oxygen and give back CO2, so that what we breathe out consists of 16% oxygen and 4% CO2. In the course of an hour, while at rest, we breathe in and out around 500 litres or 0.5m3.

If you are asleep in an enclosed space of, say, 16m3 (typical size for a small bedroom), then, over 8 hours, you will have added 4m3 of exhaled air so that, when you get up in the morning, around 25% of the air volume in the room will have passed through your lungs. That means that CO2 levels should have reached (4 x 0.25) 0.5%, which equates to 5,000ppm.

My little experiment last week, sleeping in a bedroom of around 30m3 on which I shut the door and closed the windows, got the CO2 detector to register just over 2,000ppm, which falls fairly accurately within these parameters. So far so good. The paper suggests that 4,000ppm CO2 is unpleasant and represents really poor IAQ. I would concur. I thought 2,000ppm was stale and stuffy.

Now what is interesting to me about this paper is that it concludes that a safe level of ventilation is around ten times what we breathe in and out. At rest, we breathe around 8lts per minute: the recommended ventilation rate is 80lts per minute. That’s enough to ensure that indoor CO2 should never get above 500ppm, which is unnoticeable. That’s an incredible safety margin. In fact, you can control indoor CO2 levels with much lower rates of ventilation, as people rarely if ever stay in one room for more than 8 hours at a time. So bear in mind that the minimum accepted ventilation levels have a huge margin of safety built into them.

How do these figures compare with an average leaky house, which experiences around a complete air change every four hours. Say the house is 300m3. That means that 75m3 of air is changing every hour. Which works out at 1.25m3 or 1250lts per minute. Enough to keep 15 people breathing easily, at the recommended ventilation levels. And that’s with the windows closed.

What about an air tight, PassivHaus style structure? Assume something like 0.6 ach at 50 Pascals – that in fact is the PassivHaus standard for air tightness. It is estimated that at normal atmospheric pressure, the actual air changes are 20 times less than they are at 50 Pascals, so we could assume a real air change of 0.6/20 = 0.03 ach. Again, lets assume a 300m3 house. 300x0.03 equates to 9m3 per hour or 150 lts per minute. That’s the air leakiness that you get with a 300m3 PassivHaus. It is still surprisingly high. In fact, it’s still fine for two people (at the very safe 80 lts per minute each). So rumours that you would suffocate if you lived in a PassivHaus with no ventilation are perhaps just a little exaggerated. In fact, you would survive just fine.

These calcs do call into question the insistence (from PassivHaus practitioners) that you have to have MVHR installed in an airtight house if indoor air quality isn’t to suffer. The problem with air quality is actually more localised. You can induce it over a prolonged period (like 8 hours sleep) in an enclosed space (like a bedroom), but you would struggle to notice it in a whole house. MVHR is very much a whole house solution to a problem that affects individual rooms. Why ventilate the living spaces at night? Why ventilate the bedrooms during the day?

All good questions. The debate will run and run.

2 Apr 2009

Lumens per watt: a useful graph

Was looking into the background of lighting efficiency when I came across this chart on the web. Useful. BTW, Part L defines energy efficient lighting as anything that equals or betters 40 lumens per watt. Click on the chart to make it open up larger in another window.

Thin line between wonder products and cons

I am stirred to write by some correspondence I have been having with Nick Grant, a water maven and a leading light in the AECB. Nick has recently come out against rainwater harvesting (RWH). He now reckons it’s a complete waste of time and energy and lists no less than eight reasons why you should avoid it. These being:

1. Very cost ineffective — about £15/m3 life cost for RWH assuming pump doesn't fail after 2 years. (By way of comparison, mains water costs around £2/m3.)

2. Doesn't save enough water to be meaningful, and can't be installed in enough places.

3. Doesn't work where needed. Thames gateway — no roof area/person, nor rainfall, even less in a dry year when needed to save water.

4. Can fail, leading to mains top-up running to waste and so undoing all the savings that could ever have been made.

5. Problems flushing loos when power off or pump broken

6. Uses 2-4 times the energy of mains, but still only a little compared with water heating. However that debunks the idea that all houses should have RWH to save all the CO2 needed for pumping mains.

7. “Ah but what about all the chemicals and other environmental impacts.” Again, argument needs to be that rain is significantly better than mains to justify the green premium, but is actually worse.

8. “Ah but I want to save water, but don't want a low flow shower” (perhaps meet CSH 3).

He is not alone. There is a considerable weight of opinion gathering to dis rainwater harvesting as a green con. I am not so sure, but then I don’t know that much about it. But it strikes me that there is now a fairly long list of supposedly eco-products out there that have been slated for being worthless. I can think of:

• multifoil insulation
• polyurethane foam insulation sprayed under roofs
• micro wind turbines
• electric heat pumps
• green electricity tariffs
• magic energy savings devices (as made by EPS who have a very strange website indeed
• green roofs

And then there are products which some people advocate, whilst others think are a complete waste of time. Think maybe limecrete. Or hemp. Or biomass boilers. Or micro-CHP. Or perhaps triple glazing. Or even mechanical ventilation with heat recovery.

I have no doubt that people making and selling these products don’t start out to con anyone but that, quite naturally, they sell their good points and ignore the bad ones. Any visitor to Ecobuild can’t have failed to notice that every single exhibitor trumpeted their green credentials — and why not? But logically, some products must be greener than others. Thus far any attempts at producing an authoritative overview — such as the BRE’s Green Guide — just produce howls of protest from people who don’t like what they see there.

It makes it all very hard for the consumer to make sensible choices. And I can’t see it getting any better, as more and more green products arrive in the market place and there are no agreed systems for testing their claims. Seeing as the government has been offering grants for some of these dubious products, and including others in its Code for Sustainable Homes, what hope is there for us mere mortals to make sense of any of it?