timber frame extension

[Hopking]

I have just started construction on my own timber frame extension and need some advice on wall thickness.

 

I know that wall thickness is determined by U-value but what I am unsure about is the minimum wall thickness achievable.

 

The houses existing walls are 350mm thick but as the extension isn't massive I want to have the thinnest walls possible to conserve space  and pass building regs. Also perform well in with heat loss and sound proofing. The exterior will be cladded so no masonry outer skin. Am I able to do a single skin  timber wall? I would prefer not to have to pay for a calculation for the necessary wall thickness so any advice on how to work out the u-value would be great.

 

Any info and recommendations for insulation much appreciated.

 

Happy to answer any info that's needed

Thanks in advance

[ProDave]

Work it backwards.  Assume you will make the wall 350mm thick.

 

Timber cladding, battens and counter battens are likely to take 100mm

 

A 25mm service void, plasterboard and a racking layer of OSB is going to be 50mm

 

So that leaves you able to build with a 200mm frame and get 200mm of insulation in there.  I almost guarantee that will be a lot better than the original masonry wall.

 

Now you need to choose what insulation in that 200mm to meet or exceed building regs.

[Hopking]

16 minutes ago, ProDave said:

Work it backwards.  Assume you will make the wall 350mm thick.

 

Timber cladding, battens and counter battens are likely to take 100mm

 

A 25mm service void, plasterboard and a racking layer of OSB is going to be 50mm

 

So that leaves you able to build with a 200mm frame and get 200mm of insulation in there.  I almost guarantee that will be a lot better than the original masonry wall.

 

Now you need to choose what insulation in that 200mm to meet or exceed building regs.

Hi proDave.

I agree that that method will work for matching to the existing wall thickness but i would like the thinnest walls possible in the extension to get as much space as possible out of it.

 

I've been a chippy for 5 years now but I've only worked in loft conversions and garden rooms. From my experience with this work I have built exterior walls that have just been 4x2 cladded with OSB then felted and battened ready for cladding or tiles.

 

Is a second skin or a service void necessary in ground floor extensions or can I just build a single skin  timber wall that will have a finished thickness of around 150mm?

[PeterW]

1 hour ago, Hopking said:

Is a second skin or a service void necessary in ground floor extensions or can I just build a single skin  timber wall that will have a finished thickness of around 150mm?

No… but it will be a reasonably thick wall anyway unless you go with expensive insulation or compromise the uValue

 

working from outside:

22mm fibre cement cladding

25mm batten

Tyvek or similar

11mm OSB

120mm stud / full full PIR

25mm batten/overlay PIR/VCL

12.5mm plasterboard

 

That is 215mm and about the minimum you can do structurally and gives around 0.17 as a uValue. 
 

You can run the inner battens horizontally between the sockets to give you the service void. Lots of aluminum tape on the PIR joints creates your VCL. 

 

 

[Gus Potter]

5 hours ago, Hopking said:

I have just started construction on my own timber frame extension and need some advice on wall thickness.

 

I know that wall thickness is determined by U-value but what I am unsure about is the minimum wall thickness achievable.

 

The houses existing walls are 350mm thick but as the extension isn't massive I want to have the thinnest walls possible to conserve space  and pass building regs. Also perform well in with heat loss and sound proofing. The exterior will be cladded so no masonry outer skin. Am I able to do a single skin  timber wall? I would prefer not to have to pay for a calculation for the necessary wall thickness so any advice on how to work out the u-value would be great.

 

Any info and recommendations for insulation much appreciated.

 

Happy to answer any info that's needed

Thanks in advance

Spot on.

 

Please be careful here. Like all things TF takes a bit of learning about.

 

Structural stuff:

 

First thing is building stability. With TF you get about a 25% contribution from the external leaf of masonry, no masonry.. debatable contribution from the outer skin. Next is insulated plasterboard on the inside. If normal plasterboard straight onto the studs you get a contribution (last resort when stuck on the sums) from that provided you are sure it is not going to get wet. Insulated plaster board.. no as the fixings tend to bend in the insulation.

 

You have openings.. these can drastically reduce the racking resistance and if large bifold doors ect these can completely change the way the forces act.. often you need to check that the existing building can cope with the extra sideways load and pulling / tension effect it has on the existing house walls.

 

Depending on where you have the openings a tall narrow TF panel can generate significant uplift at the bottom.. so you need to check the tie down strap nails and make sure you have enough ballast in the underbuilding to resist the tension in the straps generated by the over turning effects in the panels.

 

Turning now to the vertical loads the studs need to carry. You can put a surpising amount of load down a TF stud when is is prevented from moving along the plane of the wall, its minor / weak axis as the OSB restrains it. But when you couple this vertical load with a wind load it has a reduced capacity.. quite a lot. You need to check the case of wind pressure bearing down on a pitched roof coupled with the weight of the roof.. often it is okunless you live in a windy spot and the roof is not flat. But if the stud length is over 2.4m in height this can suddenly becomes and issue as the TF is holding the external skin.

 

Last but not least.. you need to look at your windows and doors.. how much defection can they cope with both vertically and horizontally.

 

Some detailing stuff:

 

Make sure you get your cladding details sorted. You may want to put a chamfer on any horizontal battens to channel water to the cavity where it can be properly dealt with... it's these apparent minor details that matter a lot for a durable build.

 

Have you checked to make sure you don't have a fire boundary condition?

 

Make sure you get the cladding ventilation and insect grills nutted out both top and bottom, soffit details and so on.

 

Make sure you get your fire stops right and that there are corresponding studs behind. Have you decided on how you are going to do the corners of the kit (California.. don't like them structurally) how you are going to insulate and the sequence of work to be able to do it.

 

5 hours ago, Hopking said:

I would prefer not to have to pay for a calculation for the necessary wall thickness

Yes but this may be a good opportunity and avoid a howler.. why do all this work and risk that when you come to sell someone asks.. is that ok and can you prove it?

 

5 hours ago, Hopking said:

I've been a chippy for 5 years now but I've only worked in loft conversions and garden rooms.

The above sounds like a bit of a rant from me but you have a great opportunity here to learn loads and practice on your own house. I started out a while ago.. bought my first flat and learnt how to get started on the practical side of the building trade and earn a living.

 

In summary if you want post some plans and elevations (rough dimesioned sketches will do as you are not tested on your drawing skills on BH) of what you propose and plenty folk on BH will chip in with advice and tips. It may be that you will conclude that it is your best interests to pay for some SE type advice.. but .. do a good job.. let the SE see you can and they may put you on their list of "good guys" and work will flow your way to boot!

 

 

 

 

 

 

 

 

 

 

 

 

 

[Iceverge]

 

 

 

 

This is about the cheapest proper job but a bit thicker. 

 

All nail gun compatible. No cutting PIR boards and foaming etc. 

 

 

 

 

265mm_TF_wall.pdf

[Gus Potter]

8 minutes ago, Iceverge said:

This is about the cheapest proper job but a bit thicker. 

Yes but... setting aside the structural design side... but that calculation is for an individual element of the structure.

 

You need to look at the area of glazing, the floor and the roof to make sure it all complies.. you may need to do what is called a compensatory calculation which trades off the parts that may fall below the standard and beef up other bits.

 

Have a think about this race to the bottom folks.

 

You may be trying to do this on a building notice / Scotland is a bit different but the jist of the rules apply.

 

The thing I say to my Clients is this.. you may have some smart arse telling you the minimum you can get away with but if you want to stetch PD or the building regs.. fine but think about when you want to sell the house.. you are inviting grief.  How are you later going to prove that what you have done is ok.. you could save money now but later you will now realise the true value of all the hard work you have put in.. and that is why it is good to get BC approval rather than doing it on a notice?

 

To insulate properly on a raw kit costs buttons in comparison to your overall asset value. You need longer screws and you often bring the walls in a bit.. but?

 

 

 

 

 

 

 

 

 

 

 

 

[Iceverge]

@Gus Potter I agree. 

 

I hope my use of cheapest didn't imply poor quality. Maybe I should have emphasised the "proper" bit more.  

 

The above wall is very buildable while being economical and fast to build. 

It's possible to ensure a full fill of all the insulation visually and lightly to be much quicker to get a snug fit of mineral wool batts rather than PIR.

The continuous vapour control layer and insulated service cavity is a very robust airtightness method.

A well detailed external breather membrane will be an excellent wind tightness layer. 

Running the service cavity battens horizontally will cut repeating thermal bridges through the studs. 

15mm sound bloc plasterboard will be ok at 600cc and provide better tear out capacity for picture screws as well as sound protection. 

Using 3x2s rather than 2x2s for the external ventilated cavity will prevent splitting. 

 

Better still use blown cellulose and widen the studs to 170mm or 220mm. Bellissimo! 

 

 

 

For a thin wall that would lightly not perform anywhere as well in reality and just scrape past the regs some thing like this from the LABC might be a suggestion. 

 

 

 

[Gus Potter]

1 hour ago, Iceverge said:

I hope my use of cheapest didn't imply poor quality.

Not at all.. I did not read it this way. I can see where you are coming from.. good economic and practical design insulation wise. Please don't take this as being critical.. we are all here to help each other on BH.

 

I laid it on thick in my earlier post to try and highlight the SE side of things.

 

To touch on the technical. The regs in Scotland are different from those in other parts of the UK, tend to be more onerous.

 

You mention various detailing of the construction.. I read this as standard stuff until I came to the bit where you were running the battens horizontally.. now that is clever.

 

Normally as a starting point I would take  a TF wall as having 15% repeating bridges, in practice a heavily load bearing kit will have a bit more than that but BC don't often complain.

 

That said from a buildability point of view you still need the dwangs / noggings so they would need to run vertically and your average builder would probaby stuggle to fix these without splitting the timber. For me it all sound great until you look at buildability, Yes it you were doing a 7 storey student accomodation block you could educate the Contractor but on a one off small build... it can limit your options.

 

For all on BH.. if you get too "fixated" on some detail that will save you a few mm of insulation  it will work against you.. better to make it easy for the builder.. get more quotes and say.. I'll spend the saving on extra insulation in the walls ratherthan asking for complex details around windows  / doors or tight level tolerances.

 

Also for folk building you own house. This is about people management.. yes you don't want to get ripped off but if you do get on ok with your builder you want to keep them to the end.. then they too can see the end of the job and will make an effort to finish it off...their pride will often kick in if you don't get too tight with the money. I used to be a building contractor before I became an SE so speak from experience.

 

Like your ideas about sound performance and just practical stuff like fitting pictures.. that holds weight.. cost the same to screw the plaster board.

 

 

 

 

 

 

 

 

 

 

 

 

 

Edited December 19, 2022 by Gus Potter

[Iceverge]

18 hours ago, Gus Potter said:

Like your ideas about sound performance and just practical stuff like fitting pictures.. that holds weight.. cost the same to screw the plaster board.

 

 

 

This was @Nickfromwalessuggestion some way back. I haven't had an original though in years!!