U-values - your opinions please

[acresswell]

Your thoughts, please:

We're doing a new build.  We've now got detailed drawings from Architect, and we're about to send them off to building control.

As specified, the U-values are:
0.15 Walls
1.20 Windows (max... probably between 1.0 and 1.1 in reality)
0.10 Floor
0.13 Roof
(all in W/m²K)

The wall insulation is pretty fixed (not willing to change the width of the cavity at this stage!) but I could still make minor changes to the others if necessary.  Cost is always important... so I told the architect that I'd rather scrimp on the roof insulation (where I can get into the loft to add extra insulation at a later date) than elsewhere.

What do you reckon?  Is there any area that stands out as a weak point?
Have spent a lot of time looking at thermal bridging and reducing that as much as possible, and trying to specifiy decent airtightness details (which I'll follow up on site)

Thanks,

Adrian

[A.L.]

hello adrian,

no obvious weaknesses, your floor U-value lokks rather good, have you got dimemsions/exposed perimeter length/insulation details for this?

with these levels of insulation ordinary infiltration rates could easily be 2/3rds of total heat loss, have you a target for infiltration/do you intend to fit mvhr?

[EccentricAnomaly]

What A.L. says. If you're not project managing yourself have you managed to get anything about airtightness into the contract? How big are the windows and which way are the facing?

[zeus]

Hi

I'd look at the roof in particular .... can't see any reason why 0.09 shouldn't be the target, particularly as it's a very cheap area to address ....

Regards
Z

[acresswell]

Thanks for your comments...

We do intend to fit MVHR, so we're very conscious of airtightness.  Main construction will be done by one contractor(up to weatherproof stage), then we'll take responsibility for the electrics, plumbing and plastering.  Since the wet plaster will be an important part of the airtightness, we can't expect the first contractor to accept an airtightness clause...  but we will of course be able to inspect their work from the inside before the plaster goes on.  We plan to fit things like airtightness tapes ourselves, since I'm sure that we'll do them carefully.

We're on an awkward sloping site with a retaining wall, so will have raft foundations (which have a dual function in that they are structurally linked to the retaining wall and stop it tipping forwards).  The foundations have to be dug a little way down, so we'll actually have 450mm of floor insulation (expanded polystyrene) in order to bring up the floor level to a suitable height before topping with a screed. It's cheaper than raising the floor level some other way and then using celotex, etc.  dimensions are approx 8mx12m. We're using the denbydale passivhaus detail of filling the cavity (200mm) with expanded polystyrene below DPC level to reduce thermal bridging.

Zeus,  we're aware that the roof values aren't particularly good...  but it's an area where we can do things retrospectively.  We'll have 195mm rafters and are filling the gaps between rafters with the thickest celotex possible.  In most bits of the house we can always add more below the rafters at a later stage... and in those bits where we can't add any more retrospectively, we couldn't add more insulation anyway without compromising on ceiling height! We're worried that the budget is a bit tight for everything we want to do, but we do plan to beef up the roof insulation as soon as funds allow.

The biggest windows are south-east or south-west facing. We'll have one monster window (1.8m x 3.9m) but the others are fairly typical (e.g. 1.2m x 1.2m). There will be relatively few windows on the north side of the house (because it's facing the hillside)

Hope this all makes snense!

[wookey]

acresswell - that all looks excellent. I've taken same approach as you (steal a lot of denby dale details, pay for shell, do electrics/internals etc mayself later). The disadvanatge of this is that you can't specify an airtightness level, and I know for a fact it's not as good as if I'd done it. Sounds like you are keeping all the membrane internal which helps (so long as it doesn't fill with holes over time). Mine is between timber and insulation so I can't check or fix it now it's covered (and punctured with helical nails) I suspect this isn't such a clever detail from an airtightness POV, although at least it's guaranteed not to get any worse.

Next time I'd work out a way to make them meet some kind of airtightness guarantee. Just insisting that they coudn't cover anything up with out inspection would help a lot I think.

Why not put some insulation on top of rafters? Warm roof is always best. Very easy to do whilst building, and better design than putting more on the inside later.

Everything else is in the detailing. Foamglass or marmox blocks in footing, window and door bridge minimisation, eaves junction/rafterfoot similarly. It seems relatively cheap to get windows under 1.0 now (but very expensive for less than 0.9) and there are hordes of 3G suppliers so not too hard to find what you want at acceptable price.

You clearly have the right ideas - best of luck :-)

[A.L.]

hello again,

just a note of caution,

twice in your last post you referred to expanded polystyrene (EPS), this is not suitable for below dpc work as it gets wet and loses thermal resistance

check that you have actually specified extruded polystyrene (XPS) this has a closed cell structure and is unaffected by moisture.

indeed the denby dale house refers to 'closed cell polystyrene' - www.building.co.uk/passivhaus-diaries-part-seven-the-walls/3146658.article

Below ground we've used polystyrene solid closed-cell insulation within the walls, which will provide stability if there is any ground movement. Also, because it is made using closed cell materials, it will not absorb any moisture if groundwater gets into the cavity. As soon as insulation gets soaked its effectiveness is reduced, increasing thermal conductivity.

[acresswell]

Archie,

thanks for flagging up  the difference.  I got the two mixed up for a moment, but there are 2 different products on our plans...

The 200mm width of polystyrene filling the cavity below DPC is indeed specified as "Closed cell Polystyrene" on the plans.
The 450mm height of polystyrene below the floor is indeed Expanded polystyrene... but the damp proof membrane runs below it.

Thanks for the help,

Adrian

[Brandon]

I do quite a bit of work with (and know socially) the brothers who run Pen Y Coed, the Warmcell installers (amongst other things) they were working on a development where the timber frame contract had a clause in it where by the frame with doors windows and felt only had to achieve 0.6 on hando ver, the warmcell then dropped this to below 0.4, and in a couple of cases 0.3, so it can be done, and I for one would not consider getting a house built without an airtightness claus being in the contract.

I have seen too many houses that have a lot of insulation drawn in, that on paper achieve great u values and airtightness, but in the real world the detailing is shocking, and as such the buildings under perform drastically.

[acresswell]

I like the idea of an airtightness clause, but I'm afraid it's likely to scare off a number of builders (or, more likely, lead to them upping the price) since they'll only be used to airtightness testing after plastering. That means I don't want to include such a clause unless I can be sure that it'll make a difference.

If I did include a clause, does anyone know what would be a reasonable figure to expect for a house built with masonry cavity walls?  (brickwork outer leaf, dense block inner leaf, before plastering, before laying of the ground floor screed, ceilings would be plasterboarded but not skimmed)

My suspicion is that there will be so much air leakage through the blockwork itself (before plastering) that it'll be difficult to track any other leaks effectively.  I will have the advantage that because the floor screed won't have been laid, it should be relatively easy to inspect where the services come in, to check that they've been sealed carefully.

Keen to hear other thoughts on this...

[brackwell]

I am thinking new build regulations require a air test.
Ken

[acresswell]

I'm quite happy to do an airtightness test but the plaster will form the bulk of the airtightness layer, so what I'm querying is if there's any point doing a test before the walls are plastered... since I'd have to do one at the end, anyway.

[Brandon]

surely if you pass an airtightness test at the shell stage, then that will do, as as you say, the plaster will improve it.

[Baz]

If you do a test before it is necessary or appropriate and fails it just tells you that you need to fnnish the job  first which you knew anyway. Thus you have wasted £200-500 which has an energy / carbon equivalent which has to be recovered by the savings that the extra test actually revealed that wouldn't have been found by the later test. It's no good saying it is saved by the overall savings of the house it has to be the actual value contributed by the test result.
I think you would find the carbon payback period to be centuries. Better to plant an acorn.

[djh]

If I did include a clause, does anyone know what would be a reasonable figure to expect for a house built with masonry cavity walls?  (brickwork outer leaf, dense block inner leaf, before plastering, before laying of the ground floor screed, ceilings would be plasterboarded but not skimmed)

My suspicion is that there will be so much air leakage through the blockwork itself (before plastering) that it'll be difficult to track any other leaks effectively.  I will have the advantage that because the floor screed won't have been laid, it should be relatively easy to inspect where the services come in, to check that they've been sealed carefully.

There are professionals on the AECB forum who will know the answers to these questions. http://www.aecb.net/forum/