I just hate being put on the spot when I'm still trying to get my head around U Values, value for money and the implications of selecting one U Value over another U value.

So the Timber Frame company called to ask which insulation package I'm interested in. They offer a range of wall and roof U Values staring at 0.24, which I've discounted as being far to poor. So I'm focusing on their better offering of 0.12 or 0.10 for the walls and 0.18 or 0.13 for the roof.

Starting with the walls - If I got for the 0.12 it will cost me £9,900 and 0.10 will cost me £12,000.

The roof - If I go for 0.18 it will cost me £4,800 and 0.13 will cost me £5,550. All based on the use of PIR.

Is it worth the extra for the better u values. Is PIR the way to go, or would the use of other insulation offer better value for money?

The self build has a floor area of just over 300m2 and the insulation package prices above are supply only, which I would then fit, alternatively I can get it fitted by them at an extra cost.

I've started to populate my spreadsheet and will do the 'what if' scenarios once I have all the data/areas/sizes I need from the architect.

I suppose I'm putting the cart before the horse. but I just wanted your thoughts on the use of PIR (as suggested by the TF company) and if anyone had anything could suggest a better option. Also, with one eye on cost, is PIR the best value?

£2850 is the difference: if you have an idea of how long you are going to live there (and it is a long time) and that money isn't already allocated (!!) then I would say spend it - for a start how much will heating cost you in 10 years time - no spreadsheet will tell you that! In short, I would counsel a qualitative decision and save yourself a lot of time number crunching.

I've started to populate my spreadsheet and will do the 'what if' scenarios once I have all the data/areas/sizes I need from the architect.


You don't need a high degree of accuracy at this stage - it's a comparison you are after - so basically all you need is roof area, wall area, % of wall are that's glazed and the GF area - just treat it like a big box as the internal conditions are pretty similar.

As for predicting future fuel costs, then again all you are looking for is a simple payback - fuel costs are not going to drop over ten years that's for sure

As above - PIR is almost certainly the most efficient

Regards

Barney

So to answer the question about wall and roof build up .....

Walls - u value 0.10?

25mm larch cladding
25mm battens
50mm PIR
Breather membrane
140mm frame filled with 120mm PIR (questioning the 20mm gap)
50mm PIR
25mm battens
12mm plaster board

1.5 storey, room in the Roof - u value 0.13?

State
Battens
Breather membrane
Counter battens
9mm darling OSB
140mm trusses filled with 120 mm PIR (again questioning the gap)
50mm PIR
Vapour membrane
25mm batters
Plaster board

So I was thinking I could fill the 20mm gap to improve the u values. I'd also like to make sure the insulation was sealed into the messages of the frames, I assume I'd use a low expansion foam for this?

You comments and suggestions to improve the above build up next welcome.

One final question, is there an online u value calculator to check the finished value of the above build up?

Just a small construction point for the larch. The 25mm baton is this a counter batten on a 50x50 with the PIR between? Or is the batten sitting directly on the PIR with fixings through the whole lot to timber frame/sheeting.

I ask as if you are using 50x50 batten with PIR between just be careful as usually 44x44 or 44x47 and I ended up using 40mm PIR

The thickness of PIR check with suppliers I ended up having to use 2 layers due to delivery times, two layers is handy for the floor, I had 150 PIR 50mm layer cut between pipes with 100mm over the top. With your roof you might end up with a 40mm&100mm boards or pay extra, 2 layers a pain x2 the work!

Between timbers, I agree rigid board insulation is only an option if all edges abutting timber, and all joints between boards, are filled full-depth with squirty foam - laborious, fail-prone and expensive. Otherwise you'll have very numerous straight-through shortcuts for air convection driven by considerable temp difference, and you'll get nowhere near the quoted U value. Even fag-paper thickness open joints are convection channels.

Cut roll insulation is better - but still hard to get to sit perfectly with just slight compression everywhere and no gaps - the temptation to 'stuff' is great.

Between timbers, I'd only use blown-in cellulose - Warmcel - or sprayed-in foam-up Icynene, which has very similar properties (e.g. breatheability) and almost as good eco-credentials as Warmcel. Don't believe the bad press about Warmcel slumping - there's good practice ways to absolutely avoid that, which I'll expound if needed.

Ed is right - mineral wool (incl fibreglass) gives most insulation for the money, albeit at greater thickness. To that, I'd add that similarly, white EPS board or even grey/silver graphited EPS board gives more insulation for the money than any of the foamed insulation boards such as PIR, again albeit at greater thickness. All of these have much better eco-credentials than the foamed board insulations, as well as being very to adequately breatheable, which most of the foamed boards aren't.

Consider buying a bare timber frame of 6" or 7" or 8" deep members, and cross-battening the studs and rafters externally on site with 3x2 on edge, then blowing the whole 9" 10" or 11" cavity full with Warmcel. Guaranteed to fill every crevice, under permanent slight compression, and the cross battening gives plenty of ledges for the Warmcel to lock around and not slump. Pic attached.

Cross battening studs and rafters has implications on how you fix wall cladding and/or roof tiling, depending whether you'd be providing an additional stand-off batten anyway; your roof frame could consist of close-spaced lengthwise purlins rather than down-slope rafters, so the cross battening would end up running down-slope if necessary. Note how purlin construction gives great opportunity for overhanging verge.

Polypearl Plus Tom ?

I've been thinking about this based on something you said a while back actually

using 4" x 2" I was planning a 3 layer frame (so timber vertical on inner and outer faces and horizontal to seperate them on the middle frame (sort of modified larsen truss idea). That gives a 300mm "void" and the cold points are limited to 2" x 2" squares.

Then 50mm grey EPS on inner and outer faces all sandwiched in 18mm OSB 3 boards

Constructed on site rather than as factory "cassette"

inner skin can the be battened for service voids and PB with a vapour membrane on the OSB

Outer skin again battened in 3" x 2" and clad - again a membrane on the outer OSB skin

Fill the lot with blown in EPS beads

Sensible ?

Regards


Barney

.

OK - thanks Tom - to expand a bit, the end walls of the building are planned as dense concrete block inner, 300mm cavity and stone/brick external skin

There is a further dense concrete block on flat sleeper wall at the mid length of the property

Dims circa 10m x 8m 2 storey (with skeilings first floor) and a cut roof - again with 300mm of EPS beads and a further 100mm EPS inside and 100mm eps outside before battening and slates (or perhaps a combination of slates and integrated roof PV and/or solar thermal - but that's low on my list)

The build up above is the front and back of the building

Basically it looks like a barn - it's being built under 106 notice in a garden

I've access to a numbe of Schuco AWS 70 units (mis measured for a commercial job) and I like the barn style with the commercial window details - but they are big, including a section that's 4m high and 3.6m wide - so a lot of glass (but DG) - so the idea of the inner and outer 50mm EPS in adition to the 300mm lattice with EPS Bead was to maintain the overall facade U value - ie better walls to compensate for windows - and the 3 layer 4" x 2" was just to maintin a 300mm void as the end wall cavities

Due to time frames, I'd like to have the fabricated wall and glazing in place and airtight before worrying about the services inside and the cladding outside - was thinking 18mm OSB - I'll take a look at thinner OSB and certainly the strapping

Not sure what the cladding might be at this point - thinking maybe scaffolding boards run through to create laps and V joints onto 3" a 2" battening and then dyed smokey grey/black

The masonry end walls are wider than the building width so "wrap around" the timber facade - a sort of buttress style

So - nice to know that even in GBF land, there is a point where insulation can be overdone

Floor joists run parallel to the facade so don't connect to the timber frame - but it does carry the roof load

Regards

Barney

Warmcell is what I was planning to use! When I can get the planners to see things my way.

Posted By: TriassicInstallers are getting the raw materials from elsewhere

That's interesting - it had seemed to be an unhealthy monopoly, 2 or 3 European-name competitors having disappeared - so how could they go bust?

Their website seems to make no mention of it, nor does companies house. And the article says that they are moving to a smaller unit.
So looks like it is still available.
Maybe some local Welsh business grants have vanished, or there are better ones elsewhere.

Posted By: Chris P Baconremember speaking to a guy in Ireland who was using a German father and son who were based there and they were very particular about which cellulose they would install citing a huge difference in settlement between brands.

Now that's interesting, is there any evidence, research, etc?

Triassic, the 50mm PIR on the o/s is relatively vapour impermeable, so the frame will not breathe to the outside.
With the 50mm PIR on the inside, it will not breathe to the inside either!
Does it need to breathe?