What U value would you like to achieve?

<blockquote><cite>Posted By: tony</cite>What U value would you like to achieve?</blockquote>


In an ideal world, I'd be going to 0.12/0.13, but looking at cost v benefit (and given my failure to think about this early enough in the design process!) I think I'd be happy with around 0.17. I've been told by some that airtightness and cold bridging are just as important and its wrong to look at U values in isolation.

I'd love to say I have an airtightness target that I can quote and know what it means...... but i'd be lying.

For more insulation, it's a mistake to think that means deeper studs - in fact it's a mistake, full stop, to put all the insulation between the studs (or rafters, if it's a sloping-ceiling job). That creates severe straight-through thermal bridging which really degrades the value-for-money/effectiveness you get out of your insulation.

Let the studs be as small as is structurally necessary, fit insulation between. Then the extra insulation thickness you require, in board form, fixed seamless across the outside of the studwork, cladding or render outside of that. That way, the thermal bridge that the studs create, is well killed by substantial insulation across their otherwise cold-exposed outer edge.

If you like that, can suggest a whole-wall build-up to maximise cost-benefit and performance.

Posted By: nplusonePeter, I've been to several home shows etc and the message re build tight, ventilate right has definitely sunk in - its just that I haven't got a figure in my head as to what I should be aiming for, and if I did, I wouldn't understand what it meant. In practical terms, I've been told what I can do to maximise airtightness is don't just trust whoever is fitting the insulation to do it well (either inspect what they are doing, or do it myself, or use spray foam). Also tape joints etc. As to whether or I need MHVR or whether passive stack will do, that's probably an entirely different thread

To give you some idea the easiest way is to think in terms of air-changes-per-hour (or ACH). That is measured using a fan to pressurise the house to a set pressure (usually 50 Pa) so it's just a figure of merit, not a number that means anything directly in the real world. Anyway the passive house figure is < 0.6 ACH and some other systems use < 1 as the limit. The building regs say you must have mechanical ventilation if your airtightness is better than 3, IIRC. Oh and to complicate matters, building regs use a different set of units.

If you aim for passivhaus, then you will need to take account of airtightness specs of doors and windows as well as your build. You need to design for airtightness (red lines on drawings etc) and then make sure it is built to the design. It is possible to get figures better than 0.6 but you have to set rules and enforce them closely (full-time supervision) and probably do a lot of details yourself unless you are lucky with your builders. In short, make it an obsession. It might be easier to aim for 1, but realize that most builders don't have a clue.

Spray foam doesn't have much place in an airtightness plan, except perhaps for special airtightness foams in a few places. Incidentally, don't have your insulation as part of your airtightness plan. If it is airtight (e.g. warmcel), so much the better, but don't rely on it.

I would plan on MVHR. I think passive ventilation is another subject you would have to obsess over and it's probably an obsession too many.

my critical path item right now is stud thickness and insulation material (to progress design of founds for Stage 1 building warrant, and to progress electricity connection), that's the immediate focus.

It sounds like you are rushing too much; if there's any way to do it, I'd say slow down and learn more first, whilst you can still change details or even basics to improve them.

my quandary is thicker studs and thinner boards or vice versa - and what to fill the studs - and roof structure - with. If I can get a decently performing wall and roof structure without having to invest in the thicker frame, I'd be delighted

I would definitely think about putting extra insulation on the outside rather than the inside. I believe it will simplify design and construction. I think the frame depth may come down to the relative cost of different types of insulation. There's also a technique called Larsen trusses that might be worth looking at. Rockwool is probably cheapest and could be used throughout but you would need to design some way to support the outer skin of the building. Warmcel between studs only. EPS can probably be used for the outer insulation without further support. Ditto woodfibre at greater cost.