Insulation of Timber Framed Wall

[benben5555]

Hi,

 

I've had quotes from several timber frame companies for our new build house. They all (more or less) offer a standard package of 120mm PIR insulation between the studs. From the research I've done this does not seem like it will be sufficient to reach even the minimum requirements for Part L.

 

What is the most common and cheapest arrangement here? Is it to line the inside face of the wall with a subsequent layer of insulation? This will then surely make it difficult to have a service void or to be able to fix things to the wall in the future?

 

We are planning part brick and part timber cladding. I'm designing the building myself so we don't have an architect to ask.

 

Thanks

 

 

[Russell griffiths]

That is probably the budget package to get you interested. 

Pits down to the spec you want. 

You need to do the research on what insulation levels you want and find a frame to suit, if I was going timberframe then it would be minimum 145mm frame, 140mm pir with 50mm pir on inside. 

But that’s what I would use to build a house I was only going to live in for 5-6 years, any more than that and I would up the specto a twin stud wall and 300mm plus of insulation. 

[benben5555]

Would this be the correct detail? Funny thing is according to the Celotex website 120mm between the studs only gives a U-Value of 0.25. But add another 25mm internally and it jumps to 0.16.

 

 

Edited October 24, 2021 by benben5555

[ProDave]

Define your requirements and design to that.

 

Are you intending to buy a timber kit from one of the established suppliers?  If so I hit a brick wall as soon as I mentioned taking one of their standard offerings and adding more insulation. They ended the conversation.  Instead I got it designed by a competent architectural technician who knew his stuff and stick built by local builders.

 

If you end up over sheeting the inside of the frame, you then batten in line with each frame member, to make your service void.

[dpmiller]

If a TF supplier doesn't have the flexibility to give you what you want, you want a different supplier. Whilst everyone has their standard packages, I had no problem getting pricing on options to suit my requirements...

[Thorfun]

in our research we found that the prices the TF manufacturers add on for factory fitted insulation was stupidly expensive and so we decided to go with an open panel timber frame and will fit the insulation ourselves. this way we save a lot of money and also can then choose the insulation we want. 

 

might not work for everyone though.

Edited October 24, 2021 by Thorfun
typo

[Iceverge]

I echo what @Thorfun said. Open panel, self insulate.

 

Then add a vapour membrane + 45mm insulated service cavity and nail the airtightness. It's cheaper and more effective than adding more insulation. 

 

Easy improvements from here would be to increase to Frametherm 035 or 032  and then to increase to a wider main stud. Blown cellulose for the final win. 

 

 

[Iceverge]

What the timber frame manufacturers suggest get through the regs using the absolute minimum amount of timber and in factory labour. 

 

I would like to see any results of anyone getting a satisfactory airtightness result from this construction. I'm happy to be challenged on this if anyone knows better.

 

The timber is exposed to the full variables of relative humidity within the building as there's no vapour barrier. With a rigid insulation like PIR how does the subsequent expansion and contraction of the studs not lead to a leaky structure and moisture laden air coming into contact with the back of the OSB sheathing? How does the structure cope with PIR shrinkage? 

 

It also has poor heat protection for the timber clad areas. (brick exterior takes care of a lot of this otherwise) 

 

[benben5555]

Thanks for the information. It's useful to have knowledgable people to speak to!

 

Firstly, I have no idea what 'heat protection' is!

 

Secondly, what would you suggest is better than PIR? Mineral wool as shown on your detail? Is there not an issue with 'slump' with mineral wool when used vertically as it is not self supporting?

 

Also I'm not questioning what you have shown there in that detail but why does the vapour barrier go between the insulation layers, and not between the plasterboard and the first layer of insulation?

 

And in terms of the service cavity what is electrical cable clipped to? I'm guessing it needs to be run against the plasterboard to avoid overheating?

[Iceverge]

3 hours ago, benben5555 said:

Secondly, what would you suggest is better than PIR? Mineral wool as shown on your detail? Is there not an issue with 'slump' with mineral wool when used vertically as it is not self supporting?

 

 

I've not heard of anyone having trouble with these slumping. I think @ProDave used something similar. Apparently the better insulating ones (Slab 32) are better to work with.  Rockwool would be better acoustically and maybe nicer to work with but it's expensive. Blown in cellulose is my favourite however. 

 

 

3 hours ago, benben5555 said:

Firstly, I have no idea what 'heat protection' is!

 

It is how a wall or roof delays the suns heat passing through it in the summer. It's also called decrement delay or phase shift.  Don't confuse it with overheating caused by poor glazing/shading choices.  It's well explained here. 

https://www.greenspec.co.uk/building-design/decrement-delay/

 

4 hours ago, benben5555 said:

Also I'm not questioning what you have shown there in that detail but why does the vapour barrier go between the insulation layers, and not between the plasterboard and the first layer of insulation?

 

You're right to ask. I make tons of mistakes and often think I know more than I do!. 

 

It's here to create a service cavity which allows you to run pipes and wires through the walls around the house without puncturing the vapour membrane (  in most cases also the airtight layer).https://foursevenfive.com/blog/the-service-cavity-making-airtight-construction-easy-2/

 

This membrane is important, it stops air moving through the walls taking moisture into the structure where it will get cold enough to condensate rot the timber. It prevents cold drafts blowing into the house. It must be carefully installed, taped with airtight tape to all windows and doors, floors and ceiling membrane. All penetrations the same. You should aim to have no holes at all anywhere. Effectively hermetically sealing your house. Contrary to cranky old chippies opinions, combined with proper ventilation, you will have a much more durable, healthier building.

 

4 hours ago, benben5555 said:

And in terms of the service cavity what is electrical cable clipped to?

Normally clipped to the service cavity batten AFAIK.

 

4 hours ago, benben5555 said:

I'm guessing it needs to be run against the plasterboard to avoid overheating?

I won't pretend to know this. Maybe an electrician will be along to help. 

 

 

[ProDave]

I used Knauf Frametherm 35, the version that came on a roll.  It is stiff enough that cut to the right size so it is a snug fit it does not slump.  Even like this in a 45 degree sloping ceiling.

 

Right at the start I put one test piece in, and left it with no cover and no support for 6 months and it did not move.

 

[BenP]

Hi. 

 

I can only echo the above - it's the similar route we have taken. Rockwool Flexi between studs and 60mm Wood Fibre board over. Ready for rainscreen cladding / weatherboard.

 

Flexi infill is very simple. Push fit, no gaps and has reasonable heat storage capability. It was under half the price of a sheep's wool or wood fibre flexi batt. A bit itchy to work with but I was actually watching two others do it whilst I started the Wood Fibre. 

 

One thing many forget with infill insulation only is that you end up with so few gaps to actually fill up. See the pic attached and note the amount of triple studs and solid lintels....

 

BP

 

 

[Thorfun]

I'm shopping for insulation at the moment and I have found the Knauff stuff hard to come by so I've gone for Isover 32 batts. it's all the same thing, glass mineral wool so one manufacturer should be the same as another. I had quotes for Isover and Superglass and the Isover was just a little bit cheaper.

 

Rockwool don't seem to do a 0.32W/mK insulation so we didn't look at them for the external walls but will check them out for the internal walls for sound attenuation.

[benben5555]

Funnily enough one of the timber frame companies I got a quote from before phoned me today and when I dug a bit deeper into the quote they actually supply the panels open and supply superglass or frametherm loose for fitting after erection along with 50mm pir for the cavity. Which seems a sensible approach.

[saveasteading]

8 hours ago, BenP said:

See the pic attached and note the amount of triple studs and solid lintels.

Brilliant timing for this discussion. I am this very afternoon designing a section of our project in timber (partial demo and replacement).

 

In ploughing thro the convoluted design guidelines I find the need to apply osb to the internal walls to use as buttresses, and there they are on the photo.

 

Also the need for very heavy timber as lintels. C24 and min 80 thick,  and that would increase for more snow or wind,

and quadruple studs at the door openings (which I now know are called "cripples".)

 

Can you tell me please, if not too  much trouble

The depth of the studs (and hence the insulation quilt)

I am guessing that your osb being on the inside of the external walls is because the outer skin is the Gutex board as shown, which doesn't have the strength. Does that Gutex board get fixed as positioned, and then what outside it?

Are these lintels a single piece of timber and what depth (Mine have to be increased to 220)

 

I have another question arising but will open another topic rather than muscle in on this one.

 

 

[Hobbiniho]

if i was a betting man i would say that frame is (6x2) 138x38mm CLS with a 140mm insulation, which is the most used size up here usually with 50mm PIR on the inside and timber cladding on the outside

[Thorfun]

1 hour ago, benben5555 said:

Funnily enough one of the timber frame companies I got a quote from before phoned me today and when I dug a bit deeper into the quote they actually supply the panels open and supply superglass or frametherm loose for fitting after erection along with 50mm pir for the cavity. Which seems a sensible approach.

Might be cheaper to source the insulation yourself so the TF company aren’t adding a profit margin. 

[BenP]

@saveasteading - 140x38 stud. Lintels are 225x50 x3. OSB on the inside to give me a good working surface to VCL, service void/batten out. 

 

Gutex board is currently fixed in place with 2 or 3 thermally broken screws Ejotherm STRH, these aren't structural. On top of this will go the breather membrane tapped for windtightness. This is all held back to the timber frame with 38x50 treated battens using Heco screws (Heco provide a fixing design but they were ££££). 

 

Cladding on top. ?

[saveasteading]

3 hours ago, BenP said:

140x38 stud.

Thanks. That is what I would expect and want, but it didn't look that much.; The design guide accepts down to 38 x 89 @ 600cc which seems tiny. I haven't looked at how small the building and benign the conditions for that.

[BenP]

11 hours ago, saveasteading said:

38 x 89

 

Our internal walls are that! 

 

[saveasteading]

Have done further consideration and re-analysed the U value.

we are now

600 granite (in 3 skins)

25 air gap

VB

100 stud with PIR

VCL

25 air gap and horiz studs

pbd.

 

U value is about 0.15. a lot of this is because the 25mm air gaps give better values than if filled with rockwool

unlikely I know, but narrow gaps do work quite well.

Also Granite is highly rated by heritage building authorities, as providing 'much better insulation than the regulations and standards assume'.

I think this is because of the random 3 layer construction, and that there is relatively little mortar in the faces.

As we don't have to match any standards in a conversion,  we are happy to believe what we have come up with.

Also, the reality of the stone walls is that there is hardly any dampness on the surface, for the wind to evaporate and suck out heat. granite 2.6tonnes /m3  and stones averaging 300 x 300

 

There certainly wont be summer overheating through the walls.

 

 

2 hours ago, BenP said:

Our internal walls are that! 

The Standard states Wall studs to internal racking walls should be at least 38 x 89 size, grade C16 at 600 mm centres. and there does not appear to be any situation that reruires more.

[A_L]

5 hours ago, saveasteading said:

a lot of this is because the 25mm air gaps give better values than if filled with rockwool

 

An ordinary 25mm cavity has a thermal resistance of 0.18m².K/W, a cavity with a low emissivity surface 0.44m².K/W. (possibly the foil layer of the PIR)

a cavity with 25mm of rockwool, thermal conductivity 0.035W/m.K will have a thermal resistance of 25/35 m².K/W or 0.71m².K/W

 

5 hours ago, saveasteading said:

Also Granite is highly rated by heritage building authorities, as providing 'much better insulation than the regulations and standards assume'.

 

Solid walls with a core of rubble fill do have a better than expected thermal resistance, typically 1.0m².K/W  When finished with plaster, pageV

 

5 hours ago, saveasteading said:

and that there is relatively little mortar in the faces.

 

Mortar has a thermal conductivity of about 0.9W/m.K whilde granite is about 2.8W/m.K so I do not think so.

hstp102011-u-values-and-traditional-buildings.pdf

[saveasteading]

25 minutes ago, A_L said:

Mortar has a thermal conductivity of about 0.9W/m.K whilde granite is about 2.8W/m.K so I do not think so.

 

All very good info and a brilliant document I had not seen,  thanks. 

From a very quick look.....and some guessing.    Impervious granite with small percentage of mortar on the faces,  (therefore dry ), huge percentage of mortar in the middle which, as you say, has a decent thermal resistance.

 

I will have a detailed read.