Advice on Self Build

[Smirker]

Hi A.L,

175mm of rigid PUR insulation, not fibre wool (140mm in stud + extra layer to the interior) we have been told should achieve 0.15.

Airtightness we are aiming for <1 so three is the worst case scenario. I am accutely aware that we need to focus on "fabric first" as a means to reduce energy consumption and therefore cost.

Perhaps we should try to increase the insulation in the walls further to 200mm?

[A.L.]

hello again,

using lambda 0.025 for PUR (more realistic than sometimes quoted 0.022) gives U=0.171 (105mm brick, 50mm unventilated cavity, 12mm OSB, 140mm TF+PUR, 35mmPUR,12mm plasterboard, 2mm skim coat) assumes 15% timber fraction as BRE recommendation

200mm PUR (extra 25mm) gives U=0.144

250mm PUR (extra 75mm) gives U=0.111

[Smirker]

Thanks for the calcs A.L.

We are not going brick skin but render (might be able to use a self-coloured/ insulated one?) We have a juggling act of trying to maximise insulation but not making the walls too thick. The plot is only 9.45m wide but 45m long. With 1m between boundaries each side the width of the house is only 7.5m. we want to maximise insulation without unduly affecting internal space. Our friends self built with timber frame and their walls are 450mm thick! Not an option for us.

Would upgrading to higher spec insulation (PIR) help in this regard? I think over 200mm of insulation and the walls will start to become too thick.

Thanks for everyone's help thus far, we really want to get this right first time.

[Sean]

have you considered a proper multi-foil ?

there's a somewhat rambling thread on the GBF relating

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=125&page=1&Focus=44665

[Smirker]

Cheers Sean, will take a look. My understanding of multifoils is that they are not all they're cracked up to be but that opinion could be wrong!

[A.L.]

hello again,

Cheers Sean, will take a look. My understanding of multifoils is that they are not all they're cracked up to be but that opinion could be wrong!

- thats my understanding to,

tri-iso super 9, claimed to be equivalent of 200mm fibreglass may only be as effective as 75mm -www.planningportal.gov.uk/uploads/br/multi-foil-insulation_july2005.pdf

given that it requires a 25mm gap on both sides it is really 75mm thick,

at around £11 per m2 it may be a bit dear


using render will reduce the 200mm wall u-value from 0.144 to 0.151, effect of any 'insulating' render is insignificant

substituting PIR at lambda 0.022 gives U=0.136 from 0.151

I understand that 89mm timber frames are structurally adequate, 140mm was largely introduced to accomodate extra insulation!

reducing the frame to 89mm but leaving 200mm of PIR gives U=0.122

my concern with rigid insulation is fitting into the timber frame in an airtight manner, perhaps consider a sprayed polyurethane like Walltite from BASF

[Sean]

tri-iso super 9, claimed to be equivalent of 200mm fibreglass may only be as effective as 75mm -www.planningportal.gov.uk/uploads/br/multi-foil-insulation_july2005.pdf

your understanding is sadly flawed, clearly you are not aware that the report you have used to form your opinion was based on test results taken from sites where the installation methodology was shown to be incorrect

[Richard Owen]

my concern with rigid insulation is fitting into the timber frame in an airtight manner, perhaps consider a sprayed polyurethane like Walltite from BASF

It's not difficult. Either cut the boards tight and friction fit or cut them loose and seal them in with spray foam.

Foil tape all the joins and the job's a good 'un.

Running a layer of insulation over the top of everything (need only be 25mm) helps as well.

[titan]

I don't know where the building will be situated but if noise is a potential issue ( traffic, aircraft, wind, neighbours  etc )   PU has virtually no sound deadening qualities.

[Smirker]

Thanks, good points raised here. Re the frame, it is being built by English Bros who cut the insulation with a laser saw then shoehorn it into the frame. I think 140mm stud is their "standard" but it might be worth asking about 89mm and increasing insulation as suggested. The reduced wood cost compensating for the additional insulation. So render actually worsens the U-value (albeit by an insignificant amount)? would not have thought that. We are planning on filling the frame than adding 50 or 60 mm of extra insulation and having watched the Denby Dale stuff on Youtube will be placing emphasis on junctures to avoid cold bridging etc. I will be happy if we get air tightness to <1.

Sound insulation not too much of an issue as we are not on a main road and back onto fields. we plan on using Fermacell rather than plasterboard in the interior. The Architect (my Father-in-Law) is dubious about the amount of heating we will require and wants us to put some heating in bedrooms which I think will prove unnecessary. Still pondering over Ecocent plus boiler for UFH or thermal store+solar thermal. Decisions, decisions.