Cheap, Thermally Efficient Non-Structural Wall Detail

[Nick]

Morning All,

 

As a few of you may already know, I've just started planning a build which is going to center around a huge double height room (8m x 10m floor plan minimum). Main priority is to get no-frills well-insulated space on a tight meter-rate although good acoustics would be a bonus.

 

It seems so far the only way I'm going to get this amount of clear, open space without huge costs (although I may well be wrong), is a steel portal frame construction which there is amazingly little info about on the net. I'm still reading everything I can find and pricing up different possibilities for the main structure, but it seems the key problem is going to be insulating the steelwork at ground floor level to prevent cold bridging and protecting it from fire. I've taken a lot of the inspiration & methods from the "barnhaus" design from a couple of years ago which was also a steel portal frame, but the walls on that build were 450mm straw bale and built inside the steels from what I can tell. That would lose me 500mm of floor space all around and also I'd rather stick to a design that will be simple for builders to put up as this will help my chances of getting a good labour rate for the bits I can't do.

 

The attached is my first attempt at a wall design. I haven't started looking through the regs in detail yet and although I do work in the construction industry, I'm a total newbie when it comes to anything other than glass. All I really know is what I've read in the last couple of weeks so would appreciate your help in picking this apart and understanding a better way to go about things. Open to all methods/materials/ideas at this stage.

 

Wall Detail A.pdf

 

Construction is (Inside to Outside):-

12.5mm Plasterboard

18mm Ply

300mm x 97mm JJI Engineered Timber I Beam. 2x 150mm Rockwool In Between Studs.

18mm OSB3

25mm x 50mm Treated Battens

20mm Rainscreen Cladding (Rough allowance as I don't know if it will be timber, aluminium e.t.c.)

 

Based on the JJI book, this should achieve around 0.12 W/m2K. Could possibly add some 50mm PIR in there to reduce further and smooth any cold spots from the timber.

 

My very rough costings show the raw material costs at about £70/m2

 

My hope is that by putting the steel in the middle of the wall and allowing enough space to pack around it with 60mm PIR (possible along with intumescent paint), I can fire-protect and insulate it in one go without losing too much internal space.

 

By using Rockwool instead of PIR, the acoustics should be slightly better although there is no cavity or decoupling so I don't know how well it will really work overall. There will need to be vapour layers in there somewhere which I haven't drawn.

 

Thoughts please?

 

 

 

 

[Onoff]

Would you not also want a service cavity detail internally?

[Nick]

I did consider it but the ply is doing 2 jobs for me the way its drawn now. Stiffening the joists and giving me something to fix to from the inside.

 

If i were to put it on battens id lose the Stiffening of the joists and if i were to put the plasterboard on battens it would be harder to fix to. Keen to avoid adding more cost with another sheet of ply or OSB although not the end of the world.

 

You think there would be an issue running services through the main cavity or just inconvenient?

 

Also any suggestions on where to put the VCL? Can i just tape between the joints of the external OSB, and should i be worried about the screws for the battens going into the boards?

 

[SteamyTea]

Have you looked into pumped cellulose insulation?

It has good thermal resistivity, a high SHC and good sound insulation.

 

 

[Nick]

8 minutes ago, SteamyTea said:

Have you looked into pumped cellulose insulation?

It has good thermal resistivity, a high SHC and good sound insulation.

 

 

 

I havent... will do though!

 

Is it something you can install yourself? The reason im quite liking the timber I beams is that although there is a lot of work in fitting them, they're lightweight enough that a few people can do it without too much need for plant or expensive access.

[SteamyTea]

Not sure about self installation in the UK.  You certainly can in the USA.

 

I am a bit of a fan of I Beams and H columns.  Was admiring a stairway that used them just yesterday.

 

A lot of people also use wood fibre board for insulation.  I think it can also be used structurally too.

 

As for having a service void, you could fit conduit inside the wall panels and then just pull the cabling though.

 

Have you thought of your cold bridging and air tightness details yet, especially around windows and doors (which should be easy for you).

[Crofter]

1 hour ago, Nick said:

 

Also any suggestions on where to put the VCL? Can i just tape between the joints of the external OSB, and should i be worried about the screws for the battens going into the boards?

 

 

That would be the wrong side of the wall... did you mean tape the ply?

[Nick]

7 minutes ago, Crofter said:

 

That would be the wrong side of the wall... did you mean tape the ply?

 

I read (think it was the Passivehaus design guide) that because of the number of protrusions through the inner wall on a steel frame, putting the VCL on the outside was acceptable as it's less complicated to design in.

 

I'm thinking now though that it might be best to put another OSB on the inside of the wall, then battens to form a service void, then the Ply (maybe reduce it to 12mm) and the plasterboard.  That way I could move the VCL to the inside and it would be simple to join up with the floor & ceiling if I repeated the detail there.

[ProDave]

My 2 pence worth in no particular order.

 

On the inside you NEED a service void (please tell me you were not planning to run cables in the insulation through the JJI's)

 

So inside out:

Plasterboard

25mm by 50mm battens vertically following each JJI this gives a service void inside the sealed envelope for cables etc. Use thicker battens where you want to run pipes.

Vapour control / air tightness membrane so the whole of the building is sealed on the inside

OSB (cheaper than ply usually) only needs to be say 9mm

JJI joists filled with insulation. Knauf Earthwool Frametherm 35 is a lot nicer to work with.

9mm OSB again it does not need to be thick, it's not for racking strength (your steel frame provided that) just to hold the insulation in.

Breathable membrane

Counter battens You usually need vertical and horizontal battens to give free air flow.

battens

cladding

 

 

Arguably you could forget the inner OSB layer. A house near me that I am wiring shortly has JJI joists filled with Frametherm batts, then just the air tightness membrane (helping to) hold the batts in, battens then plasterboard, but I think personally a solid layer to keep the insulation in place is a better solution.

[TerryE]

+1 on service cavity and cellulosic filler. You can also get a special airtight version of OSB that has a plasticised film on one surface and is designed for use as air tightness barrier. Also the is some advantage in using a breathable outer layer such as Panelvent cover by a tenting fabric.  Consider using Foamglass as a thermal isolation layer.

 

You will also need the whole thing signed off by a decent SE.

[Nick]

19 minutes ago, ProDave said:

My 2 pence worth in no particular order.

 

On the inside you NEED a service void (please tell me you were not planning to run cables in the insulation through the JJI's)

 

So inside out:

Plasterboard

25mm by 50mm battens vertically following each JJI this gives a service void inside the sealed envelope for cables etc. Use thicker battens where you want to run pipes.

Vapour control / air tightness membrane so the whole of the building is sealed on the inside

OSB (cheaper than ply usually) only needs to be say 9mm

JJI joists filled with insulation. Knauf Earthwool Frametherm 35 is a lot nicer to work with.

9mm OSB again it does not need to be thick, it's not for racking strength (your steel frame provided that) just to hold the insulation in.

Breathable membrane

Counter battens You usually need vertical and horizontal battens to give free air flow.

battens

cladding

 

 

Arguably you could forget the inner OSB layer. A house near me that I am wiring shortly has JJI joists filled with Frametherm batts, then just the air tightness membrane (helping to) hold the batts in, battens then plasterboard, but I think personally a solid layer to keep the insulation in place is a better solution.

 

Thanks Dave. All very helpful stuff.

 

With you on the service void 100%

 

Would you be happy switching the Ply that's backing the plasterboard to OSB? I've always used ply to go between joists & battens as it seems a bit stiffer and more hard wearing but I'll bow to your knowledge.

 

Finally (I know this is pedantic) but have you ever bothered using sheets of different thicknesses when building a wall to try and cut down sounds of different wavelengths, or not worth the effort?

[Nick]

6 minutes ago, TerryE said:

+1 on service cavity and cellulosic filler. You can also get a special airtight version of OSB that has a plasticised film on one surface and is designed for use as air tightness barrier. Also the is some advantage in using a breathable outer layer such as Panelvent cover by a tenting fabric.  Consider using Foamglass as a thermal isolation layer.

 

You will also need the whole thing signed off by a decent SE.

 

Cheers. Will look into all of those suggestions tonight!

 

Will definitely get it signed of by an SE don't worry. I don't trust my drawings and I'm the one who has to live there

[Dudda]

I'd go with ProDave as that's the closest to what I'd be suggesting although I'd probably just pump it with cellulose insulation to save time rather than earthwool although earthwool would be a tiny bit cheaper. 

 

You're mentioning 'acoustic' or noise control quiet a bit. Are you building in an area where external noise is an issue? If so consider fixing 'resilent bars' to the battens of the service void before fixing plasterboard. Would add cost but would greatly reduce any acoustic concerns you have.

[ProDave]

25 minutes ago, Nick said:

 

Thanks Dave. All very helpful stuff.

 

With you on the service void 100%

 

Would you be happy switching the Ply that's backing the plasterboard to OSB? I've always used ply to go between joists & battens as it seems a bit stiffer and more hard wearing but I'll bow to your knowledge.

 

Finally (I know this is pedantic) but have you ever bothered using sheets of different thicknesses when building a wall to try and cut down sounds of different wavelengths, or not worth the effort?

Never tried different thicknesses. Personally I think it's best to stick to one so you just buy a big pile of it and don't worry about running out of one size.

 

Are you saying yo want ply directly behind the plasterboard to "fix" things to? Your electrican will hate you for that.  I would not do that personally but I do see the point in putting something behind the PB just where you are expecting to hang something heavy, e.g flat screen tv (but I have never yet failed to fix one just by finding the studs to screw into)

 

P.S re earthwool vs pumped in insulation.  I was originally going to have the pumped in stuff, but that was a job that needed doing by A.N.Other. I then looked a bit deeper. Earthwool was half the price of the pumped in stuff, and was a DIY job, and gives virtually the same U value overall . Having done it now (well 90% of it, still working on it) I love the fact I am able to fill then board each section and it's all done. No need to get someone in to pump in the stuff which due to a few peculiarities in my build would have been a logistical nightmare to get it all done in one go.
 

[jack]

26 minutes ago, Nick said:

Finally (I know this is pedantic) but have you ever bothered using sheets of different thicknesses when building a wall to try and cut down sounds of different wavelengths, or not worth the effort?

 

I've heard of this sort of thing in critical applications like sound rooms.  I believe the main aim is to avoid having multiple sources of resonance at a particular frequency, rather than reducing overall sound transmission levels.  I doubt it would make much difference if you're just trying to reduce external noise.  300mm of a dense insulation like cellulose (which is what we have) or wood fibre will cut noise like you wouldn't believe.  

[Nick]

1 hour ago, ProDave said:

Never tried different thicknesses. Personally I think it's best to stick to one so you just buy a big pile of it and don't worry about running out of one size.

 

Are you saying yo want ply directly behind the plasterboard to "fix" things to? Your electrican will hate you for that.  I would not do that personally but I do see the point in putting something behind the PB just where you are expecting to hang something heavy, e.g flat screen tv (but I have never yet failed to fix one just by finding the studs to screw into)

 

P.S re earthwool vs pumped in insulation.  I was originally going to have the pumped in stuff, but that was a job that needed doing by A.N.Other. I then looked a bit deeper. Earthwool was half the price of the pumped in stuff, and was a DIY job, and gives virtually the same U value overall . Having done it now (well 90% of it, still working on it) I love the fact I am able to fill then board each section and it's all done. No need to get someone in to pump in the stuff which due to a few peculiarities in my build would have been a logistical nightmare to get it all done in one go.
 

 

1 hour ago, jack said:

 

I've heard of this sort of thing in critical applications like sound rooms.  I believe the main aim is to avoid having multiple sources of resonance at a particular frequency, rather than reducing overall sound transmission levels.  I doubt it would make much difference if you're just trying to reduce external noise.  300mm of a dense insulation like cellulose (which is what we have) or wood fibre will cut noise like you wouldn't believe.  

 

Thanks chaps. Just done a bit of reading onto the cellulose now.

 

Really like the idea of it filling the void completely and drying solid which will stiffen the construction. Overall U-value will be a little poorer at 0.13 but not too concerned about that.

 

Less keen on the idea that it could be corrosive to metal and could potentially absorb moisture and degrade if there is ever a leak.

 

Much of a muchness really though - will have to come down to price in the end as once I seal the wall I'll forget all about what's in there. Whatever it is has to be 100x better than the flat we're in now so i'll never notice. Either of you have any idea of a rough m2 or m3 rate?

 

I think the idea of the different thicknesses is that a noise only gets cancelled out when the material it's travelling through corresponds with its wavelength. Having one 6mm , one 9mm and one 12mm OSB should therefore in theory give better acoustics than three 9mm's. Like you say though, easier and cheaper just to order one size by the palette and not worry.

 

The electrician will be my brother-in-law (though he doesn't know it yet  ) so he'll have to hold his tongue. The room is going to be used as an actual studio on occasion and the ability to have something heavy on a wall bracket rather than having to set up a huge stand makes it well worth sticking the Ply/OSB behind the plasterboard at this stage for the little extra it will cost. Will also add a bit more mass to the wall which doesn't hurt for acoustics.

[jack]

2 hours ago, Nick said:

Really like the idea of it filling the void completely and drying solid which will stiffen the construction. Overall U-value will be a little poorer at 0.13 but not too concerned about that.

 

Less keen on the idea that it could be corrosive to metal and could potentially absorb moisture and degrade if there is ever a leak.

 

I could be wrong, but I don't think the wet-spray approach is used in the UK.  Dry cellulose is generally pumped into a hole at the top of each cavity under pressure.  No issue with corrosion that way I'd have thought.

 

It does hold water, no doubt (we had a roof leak into ours, so I'm more than familiar with its characteristics when sodden!)  I don't know that it's any worse than other porous products in this regard though.   

 

2 hours ago, Nick said:

The room is going to be used as an actual studio on occasion and the ability to have something heavy on a wall bracket rather than having to set up a huge stand makes it well worth sticking the Ply/OSB behind the plasterboard at this stage for the little extra it will cost. Will also add a bit more mass to the wall which doesn't hurt for acoustics.

 

Have a look at Fermacell.  More expensive than OSB + plasterboard, and harder to install due to its weight, but its very dense and strong.  Excellent for weight bearing (from memory, over 30kg point load per screw, and that's not into a stud) and due to its density has better acoustic properties than plasterboard.  We have a mix of that and double-layer plasterboard.

[TerryE]

@SteamyTea mentioned the SHC of Cellulosic filler, what he's talking about is the specific heat coefficient, also loosely referred to as the thermal mass.  The ratio of the SHC to U-value has the dimension of time and drives what is called the decrement delay factor which is the lag it takes for external temperature variation to transmit through the wall fabric.  This profile gives a DDF of days, which means that you can ignore diurnal variations in your heat calculations.  IMO this is more useful that the nominal extra 0.1kWh that you lose from the drop in U-value.  Especially as the main issue that you will face in a room of this size is not so much keeping it warm, but controlling excess heat and how to divert or dump it. 

 

Big rooms usually have big windows and you can easily see solar gains of ½kW / m² on windows which can play havoc with your temperature control if you haven't designed to accommodate this.

[PeterW]

2 minutes ago, jack said:

Have a look at Fermacell.  More expensive than OSB + plasterboard, and harder to install due to it's weight, but its very dense and strong.  Excellent for weight bearing (from memory, over 30kg point load per screw, and that's not into a stud) and due to its density has better acoustic properties than plasterboard.  We have a mix of that and double-layer plasterboard.

 

Was just typing the same..!!

 

go for 15mm Fermacell as it comes in large format boards too so less joins. 

 

Downside is it is damned heavy..!! 

[Nick]

2 hours ago, jack said:

 

I could be wrong, but I don't think the wet-spray approach is used in the UK.  Dry cellulose is generally pumped into a hole at the top of each cavity under pressure.  No issue with corrosion that way I'd have thought.

 

It does hold water, no doubt (we had a roof leak into ours, so I'm more than familiar with its characteristics when sodden!)  I don't know that it's any worse than other porous products in this regard though.   

 

Have a look at Fermacell.  More expensive than OSB + plasterboard, and harder to install due to its weight, but its very dense and strong.  Excellent for weight bearing (from memory, over 30kg point load per screw, and that's not into a stud) and due to its density has better acoustic properties than plasterboard.  We have a mix of that and double-layer plasterboard.

 

Thanks for the suggestion. Looks like solid stuff but TBH, as I'm trying to save cost and do as much as possible solo, it makes more sense to stick with two lighter, cheaper boards I'll actually be able to lift

[Nick]

5 minutes ago, TerryE said:

@SteamyTea mentioned the SHC of Cellulosic filler, what he's talking about is the specific heat coefficient, also loosely referred to as the thermal mass.  The ratio of the SHC to U-value has the dimension of time and drives what is called the decrement delay factor which is the lag it takes for external temperature variation to transmit through the wall fabric.  This profile gives a DDF of days, which means that you can ignore diurnal variations in your heat calculations.  IMO this is more useful that the nominal extra 0.1kWh that you lose from the drop in U-value.  Especially as the main issue that you will face in a room of this size is not so much keeping it warm, but controlling excess heat and how to divert or dump it. 

 

Big rooms usually have big windows and you can easily see solar gains of ½kW / m² on windows which can play havoc with your temperature control if you haven't designed to accommodate this.

 

Agree - definitely a plus. The slightly lower U-value would be fine with me as the mass/airtightness benefits will I'm sure outweigh it. 

 

I'm putting the studio room north facing with as little glazing as possible (all openable though for ventilation) and low G-Value units to keep solar gain down to a minimum. Hoping the reversed heat pump and HVAC will take care of the cooling on the occasions it does heat up. There is a clear path from one end of the building to the other so can always open all the windows doors to vent through the whole house.

[jack]

48 minutes ago, Nick said:

Thanks for the suggestion. Looks like solid stuff but TBH, as I'm trying to save cost and do as much as possible solo, it makes more sense to stick with two lighter, cheaper boards I'll actually be able to lift

 

If you're okay with sexism, they do "one-man" boards: see page 4

[IanR]

Just taking a step back... The title of this thread is "Cheap, Thermally Efficient Non-Structural Wall Detail", however what you are developing here is a structural wall.

 

Have you talked yourself out of the steel portal frame à la "BarnHaus"? or is this still your current direction. If you are still considering the steel portal frame then you could consider a less structural options than the I-Joists to make of the external walls. At the moment there is a double-count in your strategy ie. both the steel columns (suitably braced) of your portal frame and your I-Joists of your wall detail are independently capable of holding up the roof.

 

My current build is a conversion of a steel portal framed cow-shed and in the early days I enlisted some help from Dr. Ed Green to develop early concepts on integrating the portal frame, due to his involvement with Barnhaus. I wasn't interested in the straw bale insulation, so we looked at several other options. and always the plan was to keep the steel outside the thermal envelope and ensure no warm moisture could get to it.

 

Integrating the steel structure into the timber frame structure adds an extra layer of difficulty. The portals of the the steel frame will require racking bracing at wall level (as well as the roof) and as you have drawn it the racking braces will run through the I-Joists. It would be much easier if the steel columns were external to the wall, but this looses you some floor space you are trying to retain. Mine are half in, half out - a compromise that adds complexity.

 

Your choice of steel portal frame is due to room size. What's the roof, ie. pitched of flat, and what way does it run. It's quite possible you could do this with Glulam and I-Joist to avoid the Steel Portal frame, and simplify your structure.

 

With regards to your I-Joist wall detail, and to play devil's advocate, if cost and wall thickness are critical then there's no need for a service void if you are only running suitably sized cables in the external walls. But you shouldn't run plumbing. Also, your VCL could be achieved with a foil-backed plasterboard if your wall build up is so designed. Also, your outer sheathing could be both racking support and the airtight layer with something like Egger DHF board, avoiding the need for any airtight or VCL membrane. Designed right, its a cost effective solution.

Edited January 18, 2017 by IanR

[Ferdinand]

Suspect that you are overthinking the sound insulation if you have 300mm of cellulose.

 

Perhaps go and visit someone's house with that insulation, taking your better half, a trombone and something to make low pitched thuds.

 

Ferdinand

 

Edited January 18, 2017 by Ferdinand

[Nick]

35 minutes ago, Ferdinand said:

Suspect that you are overthinking the sound insulation if you have 300mm of cellulose.

 

Perhaps go and visit someone's house with that insulation, taking your better half, a trombone and something to make low pitched thuds.

 

Ferdinand

 

 

 Good point.

 

Doubt I'll actually go with the cellulose though. Would like to if it was similar cost to rockwool/knauf or slightly more but looking around I can;t seem to find many active suppliers so far unless anyone has any they can recommend?