We are in the process of speccing a timber frame house to build in Scotland. It’s all very exciting, and we are pretty certain of the design after lots of work on that. What we can’t decide on is the spec for the timber frame. We can have u values of 0.25, for another £1800 we can have 0.19, or for 0.12 it’s £7k extra. I know there’s more to it than u-values, but all else being equal which should I go for? I reckon the answer’s 0.19, but gut feeling is probably not the best way of deciding, and we’re not in the position to just say sod it and pay the most expensive. What do you think?

how big are the windows? makes a big difference. Nothing is clear cut, aspect, location, etc all play a part

Lets put this into perspective.

0.06watts per M2/k for £1800.

To save 1 watts would take 16.6m2 of wall area. I think I would spend the money on the windows and look at a triple glazed option. Windows and door are the weak spots in the fabric of building.

What cold bridging? What air-tightness?
What passive gain?

I would get someone who really knows….

aecb.net

The windows are big on the south elevation, pretty small and few elsewhere. The house is tucked into some big trees to the north, but it’s more open on the other sides and its 500′ up a hillside. I can’t work out how much to spend on this against spending the money elsewhere in the spec, on windows, doors etc.

Would SAP survey be any use for this type of info?

Triple glazing is not the way forward: In our climate the payback is a long time, and modern double glazing is perfectly adequate. You are better off going for better insulation and thermally broken window frames. Ensure the design minimises cold bridging, or even removes them altogether (practically speaking), depending on the design of the structure.

I’d go for the lowest u-values you can realistically afford.

500 foot up a hillside!

I would spend it on the windows and doors. Make sure the preformance is suitable for the exposed location with suitable detailing.

Timber frame or SIPS construction?

Ensure the design minimises cold bridging

Timber frame

The windows are big on the south elevation, pretty small and few elsewhere.

How much are you factoring in for an Aircon unit? 😉

Timber is still a cold bridge and in fact a lot of builders put too much unnecessary timber in (types of timber lintel etc) which reduces their effectiveness

Over how many years would it take to save the money on fuel?

B r has the nub of the question; we intend to be there for ever, but if the payback is twenty years I’m still less interested. The aircon might be a point, but about twice a year given the weather there the last few years. It’s a timber frame, built on site but factory cut. The specs are here: http://www.fleminghomes.co.uk/uploads/Specification%20Options%202012.pdf

The best you can.
But for an idea get a free heat loss program heat loss your largest room but alter the u value each time.
Note the difference it makes work out roughly how many hours you use the heating per year, work out the cost of your fuel per kw multiply it out and see how long pay backs are.
It will give you an idea but does rely on a few assumptions.

The aircon might be a point,

I was joking, but in case you’re not – go for the cheapest as reducing energy consumption isn’t an issue for you.
There’s no point fitting triple glazing either if all the money saved over winter is going to be lost on A/c. 🙂
I’ve heard people say that looking at installations in terms of payback is not a good way to approach the solution.

Don’t worry, we won’t be putting in aircon when we can open the windows. The plan is to up the thermal mass where possible too, so that we minimise the greenhouse effect.

To save 1 watts would take 16.6m2 of wall area

With a 16.6 degree temp differential, it would only be 1m2.

Not air on but you should consider heat recovery.

The best way to improve the efficiency of your building is to install an intelligent heating system. This will save you far more money than any eco bling such as triple glazing or solar panels.

😀

Not looked at the pdf yet but i assume your going for mechanical heat recovery ? Aircon would be a complete waste of money if you insulate correctly.

I’m 400ft ASL south of glasgow and wind is the big thing here. Airtightness makes sense though i have seen some on the likes of Grand Designs where the airtightness seems to simply mean no flow out/in from the inner skin of the building. Unless your using kingspan everywhere you really want airtighthness on the outer skin. (allowing of course for condensation control).

Where roughly are you building ?

Surely the best way is to insulate everything to buggery and then have solutions that are dynamic enough to work with the changing conditions.

We’re in Perthshire, so pretty chilly (16.6 degrees difference inside to out on a very regular basis), pretty wet and windy too, though not so much as the west coast perhaps. I’d got as far as ignoring solar etc (for the meantime), insulating well and considering mechanical heat recovery ventilation. We have enough plot to put in ground source, but I’m not sure it stacks up against a good modern gas system for our needs (four of us needing plenty of hot water to scrub off the mud from various outdoor escapades.

And you will of course be insulating the 20×20 bike shed your building ?

Or better still, a workshop under the house.

Something funny there pjm?

And you will of course be insulating the 20×20 bike shed your building ?

My workshop is insulated to a U value of approx 0.2 (150mm Celotex on walls and ceiling, with 100mm on the floor) – tis very toasty inside once it’s heated up a bit.

Something funny there pjm

I was thinking about the 23% return, on outlay, that my Solar PVs have produced this year. So far I’ve generated 3345kwh.

3345kwh = £1502 plus add an additional £55.19

Electricity supplier also owes me £250 as I’m not using as much electricity. So lets call it £1800 and I still have 3 weeks until the first year is up. Maybe the weather will improve?

Install cost £7800

That’s all very well (you must be proud, by the way) but you misunderstand my point: I am not saying don’t do those things, I am saying they should be quite low down on the hierarchy of home design/improvments. First, look at making the house run as efficiently as possible, through both insulation and efficient heating systems. Then, look at adding stuff like photovoltaics, solar panels etc.

I was thinking about the 23% return, on outlay, that my Solar PVs have produced this year.

Only because the government chose to give a ridiculously high rate for your kWhs, nothing to do with how well insulated / efficient your home is. You’re just benefiting from a political (tax) system which gives a benefit to those who can afford PV systems, paid for by everyone who doesn’t.

The elemental u-values of walls/roofs etc aren’t really very useful on their own. The best way to approach a low energy, high comfort building is through a whole building approach. This requires some kind of modelling although it doesn’t have to be very complex – a SAP calculation isn’t very accurate but will give you a guide. Much better is something like the Passivhaus Planning Package (PHPP) which is actually just a spreadsheet, but is pretty accurate and detailed.

I would suggest that high levels of insulation (0.15 W/m2.K or better), high performance triple glazed windows, effective air-tightness and a clear ventilation strategy (MVHR is a great option once airtightness is at a good level) are not green bling. The fact that winter solar gain through triple glazed windows can be more than winter heat loss through them and the fact that they’re passive elements not requiring much or any maintenance or energy to run is a great example.

However, don’t you have an Architect to advise you on this rather than a forum? If you’re building a new home for the future I would be spending now so that you don’t have to later. Get good professional advice, pay for the very best performing building envelope you possibly can afford. This is giving you and your family future comfort and future energy security without having to worry about expensive and maintenance heavy green bling like solar panels and heat pumps.

..and yes, timber does produce a cold bridge. The better the insulation performance elsewhere the more of an issue this becomes. This is where you’ll get condensation and mould so the thermal fabric needs to be detailed very carefully.

Listen to matt_outandabout, he has some expertise! Contact me on paulATpaultestaarchitectureDotcoDotuk and if I can’t help you, I’m sure I can point you in the direction of someone who can.

I would second what Paul has suggested. Don’t know much about Scottish Building Regulations but England and Wales still have to prove building performances through the use of SAPs. Big windows of poor u-value would mean the walls need to be beefed up to compensate. If you’re going for Passivhaus then this is a necessity which any architect should be fully aware of but even with a standard approach it should be second nature.

Okay simply calculations for the wall fabric saving (may be a load of bo%%ocks) using HDD data for Perth and 15.5 basepoint value = 2463 HDD per annum.

U value 0.25 to 0.19 = 0.06

Say 200m2 of external wall area = 12 W/K

0.024 x 12 = 0.288 kwh per degree day
0.288 x 2463 = 709kwh / year reduction in heat loss

U value 0.25 to 0.13 = 0.12

0.024 x 24 = 0.576kwh per degree day
0.576 x 2463 = 1419kwh / year reduction in heat loss

Add boiler / heating arrangement / efficiency / tariff rate to find theoretical saving.

Out of interest 140mm thick timber achieves a U value of 0.74. Also data for the standard wall is working out at 0.234 (offically 0.23) value using the latest BRE software.

Head hurts – Going to ride my bike

monkeyharris, you’ve got a few recommendations already, but here’s another:

http://www.alpineeco.com/Alpine_Eco___Services.html

They know their onions about building in cold and windy places, particularly when it comes to minimising energy demands and maximising benefit from natural resources, light planning etc. Founder is down to earth brummie, very nice chap. Not in your area (quite!), but if you want someone to bounce some plans off…

And another.
http://www.timby.co.uk/, about to build a nice big house/guest house project in Morzine too. I was looking at their showhome today, very cozy. 🙂

baldy is in the right direction, but the triple glazing is a step too far IMHO. The improvement in performance over good double glazing is not worth the extra cost, unless soundproofing is included in the requirement.

TooTall – in pure energy terms maybe, but in terms of comfort and in terms of condensation – if the rest of the envelope is as good as it can be, double glazing will be an obvious weakness.

If there is condensation, then the air flow isn’t being dealt with properly. Triple glazing won’t compensate for that. A decent pair of curtains would probably offset the difference!

Hi Monkey, I run a company that does exactly what you’re looking for (www.alpineeco.com)- building physics calculations and studies. As nedrapier kindly pointed out we’ve been doing this for over ten years and have plenty of experience in cold environments. We use the latest software that can even take into account how much shading is provided by any mountains etc in the area so you’ll get accurate predictions of the cost savings for all options. There is lots of very good advice given here but unfortunately generic answers don’t really work with building design due to the many variables of each project, the bets bet is to calculate things properly.

I can phone or email you to talk in more detail if you’d like?

Cheers,

Rob

PS – you can email me on office@alpineeco.com

@ Footflaps.

I looked at PVs in 2006 but it didn’t pay and to be honest I forgot all about them until this cock up with the FITs tarrif.

The best energy saving I’ve done so far was changing the 6x GU10s for 6xLED within kitchen. £73 for the bulbs and a 264 watts saving. The lights are on say 3hrs a day / 330days a year (north facing kitchen) which equals 261 kwh / per year.

£73 for 261 kwh / year = bargain. 2 year payback / no constantly blowing GU10 bulbs. No brainer.