Airtightness and Part L

m1b2k3

Hi all, we are in the process of building a new house. I am just wondering about how airtight your house has to be to comply with the Part L regulations? We were told by our engineer it must be airtight to comply? No wall vents or  window vents and using a hrv to ventilate the building? Is this correct?

Metric Tensor

Your engineer is, in my opinion, totally correct to advise you as he has - but he is probably sparing you the nitty gritty of the fine detail. I'll try to summarise below:

a. There is no such thing as totally airtight - even in a submarine. So there is a measurement of "how airtight" the house has to be.

b. The worst airtightness you can get away with according to the building regulations is a very "leaky" value. (Anything below 7 m3/h/m2)

c. BUT if your house is that leaky it will be very difficult to meet the rest of the Part L regulations so in practice in order to meet all the Part L requirements your air-tightness value will need to be much lower (Probably somewhere around 2 m3/h/m2 - many people would even consider this too high!)

d. When you have a house with "good" air-tightness ventilation is VERY VERY VERY important and although it is not a legal requirement it would be foolish (in my opinion) to try to achieve proper ventilation by whacking a few holes in the wall. In my opinion, and that of your engineer, the only way to properly ventilate a modern "air tight" house is with a purpose designed ventilation system delivering proper quantities of fresh air to every room - rather than relying on the direction the wind is blowing outside a hole in your wall.*


* Notice I've said "in my opinion" a lot. This is because some people disagree that mechanical ventilation is necessary and say they can/will ventilate the house by opening windows - which is fine in theory - but most people don't feel like opening the window at 11pm on a windy night in December when it's -3 outside.

sydthebeat

have you had a preliminary DEAP assessment done yet?

if not, you need to get one done ASAP.
the DEAP report can then tell you the different specifications required to comply ie one with MHRV and one without.

its still possible to have wall vents, but by doing so you need to upgrade the other factors to compensate for not having the heat recovery of a MHRV. in my opinion the cost of these upgrades amount to the same as teh MHRV system itself, so you may as well use the MHRV system and get the benefit of it.

m1b2k3

Thank you both so much for your responses. yes we have had the [font=Open Sans, Arial, Helvetica, sans-serif]preliminary DEAP assessment completed. Our engineer has said, as you have Metric Tensor, that it is most cost effective to meet the regulations by putting in an air to water and an MHRV system.[/font]
[font=Open Sans, Arial, Helvetica, sans-serif]Our engineer has been great and its not so much that I doubted what he was telling me was the truth, we are both very warm people and I suppose like the idea of fresh air in the house, not through a machine. But on the flip side we both work during the day so we cant leave the windows open all the time (or in all weathers) so I would prefer to have the MHRV in place to keep air circulating. [/font]

[font=Open Sans, Arial, Helvetica, sans-serif]Our friends are building a house and their engineer has a more lenient take on the building regs. And that is fine, each to their own I just wanted to see what people thought here. I had read the building regs but since I am not an engineer it didnt clear anything up for me :-) [/font]

Calahonda52

m1b2k3 wrote: »

Thank you both so much for your responses. yes we have had the [font=Open Sans, Arial, Helvetica, sans-serif]preliminary DEAP assessment completed. Our engineer has said, as you have Metric Tensor, that it is most cost effective to meet the regulations by putting in an air to water and an MHRV system.[/font]
[font=Open Sans, Arial, Helvetica, sans-serif]Our engineer has been great and its not so much that I doubted what he was telling me was the truth, we are both very warm people and I suppose like the idea of fresh air in the house, not through a machine. But on the flip side we both work during the day so we cant leave the windows open all the time (or in all weathers) so I would prefer to have the MHRV in place to keep air circulating. [/font]

[font=Open Sans, Arial, Helvetica, sans-serif]Our friends are building a house and their engineer has a more lenient take on the building regs. And that is fine, each to their own I just wanted to see what people thought here. I had read the building regs but since I am not an engineer it didnt clear anything up for me :-) [/font]

OP, in the interest of glasnost and perestroika, I don't see where MT says what is bolded above.
Maybe I missed it.

m1b2k3

Apologies i misquoted

Metric Tensor

Thanks CH - I may have inadvertently given that impression.

What I was trying to say OP is that installing HRV is, in my opinion, vital when a house is built to the current building regulations (in the way that most people do wrt air-tightness).

HRV systems are expensive so there are cheaper ways to meet the regulations but in my opinion they will give you a house that's uncomfortable to live in.

Also the heat pump is possibly not the cheapest way of meeting the regulations (either up front or long term!) - it does seem to be the most commonly used heat source these days. Whether it's the best or not is a debate for a different day and depends on your definition of "best".

adrianm1234

Metric Tensor wrote:

Also the heat pump is possibly not the cheapest way of meeting the regulations (either up front or long term!) - it does seem to be the most commonly used heat source these days. Whether it's the best or not is a debate for a different day and depends on your definition of "best".

Metric Tensor I agree with the HRV with the airtightness.Heating is my problem.We are at roof level and we're going with A2W HP but in back of my mind I have a lingering doubt about getting it as not convinced it's the cheapest way of meeting regs either. What's your suggestion though?

Metric Tensor

Who did your DEAP/BER calculations? You need to ask them to run the calculations for different options because if you remove the heat pump from the calculations you need to replace it with some other form of renewable contribution so your house still complies with the regulations.

Natural Gas / Oil boiler accompanied by solar panels is an option that "might" work but you will have no idea how many panels or what efficiency boiler to use without updating that DEAP calculation.

Belkin1975

adrianm1234 wrote: »

Metric Tensor I agree with the HRV with the airtightness.Heating is my problem.We are at roof level and we're going with A2W HP but in back of my mind I have a lingering doubt about getting it as not convinced it's the cheapest way of meeting regs either. What's your suggestion though?

Which system did you go with. Just starting to break ground in the next fortnight and we are at odd re the heating system. Has anyone tried grant combi?