Window U Value and Part L compliance

V0X

We are in the process of designing a house and wanted to use lots of glazing. It seems we may now be struggling achieving compliance with Part L due to demanding u values.

I thought we could increase insulation elsewhere to reduce demands on the window spec however I now believe the windows are treated independently.

Is anyone familiar with the basics of how the minimum u values are estimated to comply with Part L and is there any way to minimise the demands of the window energy ratings other than reduce window sizes.

Thanks.

Metric Tensor

The worst u-values allowed for windows by the building regulations are very poor (a lot of people refer to them as backstop values for this reason). Unless you have a time machine and are buying your windows in 2005 your new windows will be much better than the worst allowable ....

HOWEVER

In order for your entire house to meet the required EPC and CPC values of the building regulations* (in addition to a few other things this is colloquially known as Part L compliance) a DEAP (similar to BER) calculation needs to be carried out which assesses the house as a whole. I suspect this is where you are running into trouble? It is possible, to a certain extent, to trade one thing against another in the DEAP calculation provided all the items stay within the backstop values.

I have done a few houses with a lot of glazing and I haven't had a problem like what you are describing. How much glazing are we talking about? Is it curtain walling? Are you telling your DEAP assessor the exact window type you are proposing? What is the u-value of your proposed windows?

V0X

I don't know off hand but based on our current drawings the assessor is proposing an average u value of 0.8 would be required. While we can achieve this with the windows we have 2 large 5m wide double sliding doors which just can not get down to that level. I really like the large glazing doors so was curious if we were missing any tricks.

V0X

I don't know off hand but based on our current drawings the assessor is proposing an average u value of 0.8 would be required for the glazing. While we can achieve this with the windows we have 2 large 5m wide double sliding doors which we just can not get down to that level. I really like the large glazing doors so was curious if we were missing any tricks.

Calahonda52

The experts with deal with the Part L stuff,

just some observations based on 40 years of this stuff.

Whats the solar gain calculation for the big doors and other glazing?
What aspect are they?
What weight are they?
How often will they actually be opened?
Have you landed out the furniture in the room to see will you actually have full access to them or will they be cluttered?
How easy will it be to move them once you/OH are unable to bench 350 in the gym?

I suspect they are a fairly sizeable part of the budget and therefore warrant very close economic and functional evaluation which goes way beyond :I really like

Proper modern house design is gone way beyond shoehorning to meet our measly [Part L] regs

I don't want the answers, just think about them.

Metric Tensor

OK - to be fair - not just one but two 5m sliding doors is substantial. A substantial amount of trade offs / extra cost will be required on the rest of your build in order to accommodate these.

I guess it's a case of how important they are to you and how much you are willing to spend or alter elsewhere in the house to make them possible.

Sit down with your assessor and ask him what changes would be required to make it pass with these doors at the lowest u-value you can source them.


I echo what CH says above - I know a house with two large sliding doors (although not 5m!) and one of them hasn't been opened in years. In Ireland there will probably be max 10 days in a year where you can leave the sliding door open. For the other 355 days of the year doors will be for going in and out and closing after you to stop the heat getting out, the kids getting out, the dog getting in/out, etc. etc. Maybe a large floor to ceiling window might be sufficient in one of the places? It is your house though and if you really want them it probably can be done - possibly at the expense of other things!

V0X

There is also a wide double height glazing entrance which further creates a big demand... Using large fixed glazing with a side door is certainly a consideration for one section but I guess I was wondering where else we could invest in order to reduce the glazing demands. My understanding is you can not simply increase insulation elsewhere as the glazing is treated somewhat independently. Not sure if I picked that up correctly though.

Point taken about the use of the sliding doors however even when closed they still look fantastic and to have the option of opening them is a nice bonus.

sydthebeat

whats your overall percentage of open areas that designed in the build?
whats current average u value of openings (includes doors)

dont throw the baby out with the bathwater here.

are the opening areas less than 58.9% as prescribed in table 2 of TGD L 2011?

There are methods to ensure compliance, even if the DEAP report shows non compliance.
This could be a letter from the local BCO accepting the non complaince.
this could be certifier accepting the non compliance as being de minimus in the overall context of the build.
you could also have the build assessed under PHPP and provide those results to the BCO as an alternative compliane method.

Metric Tensor

sydthebeat wrote: »

This could be a letter from the local BCO accepting the non complaince.

Have you ever managed to get this syd? An impressive feat if you did!

sydthebeat

Metric Tensor wrote: »

Have you ever managed to get this syd? An impressive feat if you did!

nope ive only ever read about such mythical documents

Gumbo

Metric Tensor wrote: »

Have you ever managed to get this syd? An impressive feat if you did!

sydthebeat wrote: »

nope ive only ever read about such mythical documents

I got asked could I consider such an approach this week on a site in north Dublin. Part L relaxation was been asked for?

Metric Tensor

Is it something that has merit kceire? Are they likely to succeed?

Gumbo

Metric Tensor wrote: »

Is it something that has merit kceire? Are they likely to succeed?

I don't think so.
Their argument is that they can achieve an A1 rating without the aid of renewables and they would like to omit them.

I kinda see heir point but it's beyond my pay grade and I don't want to make the decision, my boss can do that.

I think it could set an unwanted precedent.

fatty pang

kceire wrote: »

Their argument is that they can achieve an A1 rating without the aid of renewables and they would like to omit them.

I can recall a presentation at See the Light a few years ago where a Galway builder noted one of his builds achieved PH/near PH level of energy demand without using renewables. Comment from the floor (a vested interest) noting non-compliance and the room went very quiet before moving swiftly on.

kceire wrote: »

I think it could set an unwanted precedent.

'An appalling vista' comes to mind