Party garage wall insulation methods - help!

[Gooman]

I'm planning to convert the far end of a double-length garage that's attached to the house on the left-hand side and has a party-wall with next-door's garage on the right-hand side.

 

Most of the far end wall will be gone and replaced with bi-fold doors. At the front end there'll be a blockwork partition wall between the new space and the remaining garage/workshop/storeroom.

 

The left-hand wall will go (in the converted space, not the remaining garage) and it will be open-plan to the kitchen.

 

The party wall is the problem bit.

 

Building Control have told me that as it's a party wall I need to achieve appropriate sound insulation levels, which they recommend doing by erecting a 10mm stud frame. I'll then need 100mm PIR insulation which will be taped to provide a vapour barrier. Then (presumably) I need a 25mm service void, so more battening. Then plasterboard.

 

I hate the idea of battening twice - sounds like a real faff. Any bright ideas?

[Mr Punter]

What is the wall makeup for the party wall? Is it 2 x single 100mm skins with a 50mm cavity, a 215mm solid wall or what?

 

What BC are suggesting does not sound outlandish.

[Gooman]

As we're not in the property yet I'm not 100% certain ... I believe it's just a single 100mm skin as it's just a single storey shared between the two garages.

 

I guess my other alternative is to see if I can avoid having any electricals or rads on that wall and so avoid the need for a service gap.

[scottishjohn]

I would not skimp on the sound proofing  or insulation if just single skin wall  and the 100mm stud frame must not touch or be fixed to party wall  or it will transit sound .

 

Edited August 14, 2019 by scottishjohn

[Gooman]

Ah, I think I misunderstood BC's comments ... it's a 10mm gap from the party wall, not a 10mm frame. The new stud frame needs a 10mm gap and it's attached to the floor, ceiling and walls either side rather than the wall.

[Temp]

+1. Most likely they mean a 100mm stud wall with 100mm PIR between the studs. Whole lot set 10mm away from the existing wall. The BCO may not have thought about a void for services, that's up to you.

 

You could use thicker studs instead of battening. 100mm CLS is actually about 94-95mm wide so 100mm insulation wouldn't sit flush both sides if that sort of thing annoys you. Wickes sell 140mm CLS which would give you a rather deep 40mm service void. Perhaps fit 110mm insulation between 140mm studs leaving a 30mm void? See what you can get locally.

 

One advantage of battening is that it gives you a second opportunity to flatten a wall if the studs are bowed.

 

Best way to build this is probably to fit timber to floor and ceiling first then fit alternate stud - PIR - Stud - PIR. That way the studs can be tight up against the PIR with no need to cut the PIR to fit between studs.

 

You might also consider two layers of plasterboard for extra sound proofing.

 

Have they already converted "their side"?  If not did the BCO say anything about what you need to do to prevent fire spread through shared roof?

Edited August 14, 2019 by Temp

[Temp]

1 hour ago, Gooman said:

The left-hand wall will go (in the converted space, not the remaining garage) and it will be open-plan to the kitchen.

 

To me that sounds like the difficult bit. Presumably this wall is brick/block and supports the roof of the house so a steel beam will be needed. Possibly also a brick pier or steel post with foundation in what was the corner?

[Gooman]

6 minutes ago, Temp said:

Have they already converted "their side"?  If not did the BCO say anything about what you need to do to prevent fire spread through shared roof?

 

Nope, that's still a double length garage. 

 

BCO didn't mention anything about fire spread ... what would be expected there?

[Gooman]

3 minutes ago, Temp said:

 

To me that sounds like the difficult bit. Presumably this wall is brick/block and supports the roof of the house so a steel beam will be needed. Possibly also a brick pier or steel post with foundation in what was the corner?

Yep - I'm expecting both to be required.

[ADLIan]

Your BCO needs to go back to building school. PUR is no good for sound insulation - must be mineral wool between the studs for acoustics. There will also be a thermal requirement for this wall if the other side is an unheated space.

[Gooman]

 

4 minutes ago, ADLIan said:

Your BCO needs to go back to building school. PUR is no good for sound insulation - must be mineral wool between the studs for acoustics. There will also be a thermal requirement for this wall if the other side is an unheated space.

 

I think he was meaning that the 10mm gap with the stud frame not attached to the wall would be sufficient insulation.

 

The PIR insulation (see first post) is the thermal insulation.

[ADLIan]

But that new stud wall will add little in the way of acoustic insulation . I would expect double layer of plasterboard (to add mass) and mineral wool between the studs (as an acoustic absorption material). Perhaps check exactly what standard of thermal and acoustic insulation, with numbers, your BCO is asking for.

[Gooman]

As this is a conversion within a self-contained dwelling, and not a "room for residential purposes" in the meaning of Part E, doesn't that mean that Part E requirements don't come into it and that Building Regs for sound won't apply?

 

If so then obvs I still need adequate measures for my own satisfaction, but I don't need to satisfy Part E and it's not a BC issue.

[ADLIan]

That's why I'd suggest getting chapter and verse from your BCO as he's the guy that enforces the Regs. If he's relaxed about this then you may be disappointed by the acoustic performance of your wall, perhaps have a look at the some of the solutions in Part E and adopt some of these measures.

[Temp]

The room maybe a conversion but in the process you are effectively moving the boundary between the habitable rooms of two dwellings. The regs for such walls are tighter than for internal walls.

 

My guess is the BCO feels his proposed solution (single brick, 10mm gap and 100mm PIR/PUR, plasterboard) is acceptable to him. Whether it it is good enough for you or meets the letter of the Approved Document for sound transmission is another matter. 

Edited August 14, 2019 by Temp

[Gooman]

Thanks all for your good advice.

 

The BCO has confirmed that he doesn't need any specific acoustic insulation. 

 

I'll definitely plan to add double-thickness plasterboard. If I can avoid putting electrics or plumbing in that wall I'll also include 35mm of mineral wool between the PIR and PB.

 

Does anyone know of a u-value calculator that would help me work out the minimum PIR thickness I need, given that the mineral wool will have some thermal properties, and given the double PB? Seems to be beyond the capabilities of the Celotex and Kingspan ones.

 

For clarity, proposed cross section of wall is now:

 

Existing brick party wall (presumed single-skin, unheated garage on other side)

10mm gap

Timber frame fixed to ceiling, floor and end walls with PIR board between studs

35mm timber frame with mineral wool between studs

2 x 12.5mm plasterboard

Plaster skim

 

 

[andyscotland]

This free online calculator allows for fairly comprehensive calcs http://www.changeplan.co.uk/u_value_calculator.php - haven't thoroughly double-checked its accuracy but used it early in my project and results appeared to be about right.

 

If you want to be able to print off something a bit more professional-looking / save things and come back to them later the BRE tool at https://www.brebookshop.com/details.jsp?id=139470 is £60. Only works on Windows PCs though.

 

If there's an enclosed unheated space on the other side of the wall you can include an allowance to reflect that won't lose as much heat as if it was open to the outside air. There's a section on how you can work that out in BR443 https://www.bre.co.uk/filelibrary/pdf/rpts/BR_443_(2006_Edition).pdf which I believe is still valid for use.

[Gooman]

The Changeplan calculator is exactly what I was after, thanks! That suggests that 50mm PIR (as part of the cross section I proposed above) would give a u-value of 0.286.

 

The enclosed unheated space on the other side is a non-integral garage (as is the space I'm converting) which seems to be specifically excluded by BR443.

 

So now the tricky part is specifying the timber frame. Ideally I want PIR between the studs and a separate layer over to avoid thermal bridging. But with only 50mm PIR that seems tricky, as the timber frame will need to be rigid enough.

Any thoughts?

[PeterW]

1 hour ago, Gooman said:

The Changeplan calculator is exactly what I was after, thanks! That suggests that 50mm PIR (as part of the cross section I proposed above) would give a u-value of 0.286.

 

Not sure that is correct as 50mm doesn't give me 0.286 (using a uValue of 0.022) at 50mm so not quite sure where that is from..? I would go with a 10mm gap with a 90mm stud spaced off the wall using steel studs and nuts, and then 70mm between the studs with 25mm over. A single skin of fireline 15mm plasterboard with skim will be adequate and heavy enough to stop any sound.

 

Minimizing the cold junction at ceiling level will be your issue - what is the design detail for this..? 

[Gooman]

The floor level needs to be raised and that will then require that the roof height is increased. My builder will be doing that work. I'll then be insulating it with PIR between and over the roof joists, extending over where the stud wall will be fixed through to the ceiling joists.

 

My current thinking on the order of work is:

• Builder raises roof and levels floor (but doesn't raise it)
• Builder builds pillars and installs steels (leaving house side wall for the moment)
• Builder removes far, garden wall and builds footing for bi-fold door track (with temporary shuttering)
• I insulate ceiling (but not yet plasterboarded)
• I construct partition stud wall between front end of garage and new room area and insulate
• I construct frame for party wall, insulate and dry line
• I install floor insulation (but not yet chipboard)
• 1st fix - pipework for radiators cut into top of floor insulation, socket wiring within party wall rockwool gap (cable suitably derated), lighting wiring on ceiling 
• I install chipboard floor
• I install battens on ceiling (for cable void) and plasterboard over
• Builder installs bi-fold doors
• Builder removes house side wall
• Sundry insulation and PB  (window reveals, pillars, steels) and plaster skim
• 2nd fix

Rough plan attached ...

[Gooman]

Just occurred to me. If my stud wall is independent from the garage party wall (offset by 10mm) do I need to worry about thermal bridging at all? The footer will be fixed into the concrete floor, but I doubt that's a big deal.

 

If bridging isn't an issue I can have a party wall build-up of:

• 10mm gap
• 75mm studwork filled with 50mm PIR and 25mm rockwool
• 2 x 12.5mm PB
• Plaster skim

According to http://www.changeplan.co.uk/u_value_calculator.php that gives me a u-value of 0.297 and a depth of only 115mm - see image

[andyscotland]

The studs will still bridge from the plasterboard through to the cavity, bypassing the PIR/wool so you will need to include both those insulation layers as bridged by an appropriate fraction of timber.

[Gooman]

55 minutes ago, andyscotland said:

The studs will still bridge from the plasterboard through to the cavity, bypassing the PIR/wool so you will need to include both those insulation layers as bridged by an appropriate fraction of timber.

 

Good point well made.

 

I've skimmed BR443 and unfortunately I'm none the wiser. I assume I need to account for a timber bridging factor of 15%, but it doesn't seem to explain what that means. Is the R value for the elements that are bridged reduced by 15%, or to 15%?

 

OK, read other stuff on this. Man, this gets complex. My head hurts. A different approach is needed I think.

 

So now I'm thinking:

• 10mm gap
• 50mm studwork filled with 50mm PIR
• 12mm PIR surface mounted, joints taped
• 25mm batten filled with rockwool
• 12.5mm PB
• Plaster skim

 

That seems to give a u-value of .261 on a combined thickness of  around 115mm.

 

Edited August 15, 2019 by Gooman

[andyscotland]

1 hour ago, Gooman said:

That seems to give a u-value of .261 on a combined thickness of  around 115mm.

 

I've had a quick go on changeplan (not at my Windows machine so can't double check in the BRE tool). I get 0.296 for that construction:

 

 

In case you were still unclear on the thermal bridging - unfortunately it's not as simple as just amending the R value by the % area - the heat tends to want to flow through the path of least resistance, so the software essentially has to calculate all the individual thermal paths then work out how much heat will throw through each route. The good news is changeplan will do this for you - you just have to add each material type with the relevant % of the wall area. E.g. as above with the 85% mineral wool / 15% timber battens.

 

Couple of other things to consider:

• Do you have a source for 12mm PIR? I have a hunch that 25mm is the thinnest generally available - I think you can get slightly thinner mounted on insulated plasterboard but that won't work with your service void detail.
• I'm not certain but I would suspect a 50mm stud (especially on 600mm centres) might be too wobbly in practice. You could turn thicker timbers sideways, or reduce the centres, or fit extra noggins - but all of that would increase the timber fraction of that layer and reduce the u-value.

If you're planning to use PIR as part of the buildup, you might be worth contacting Ecotherm technical. They were super helpful when I was trying to optimise wall thickness compared to u-value and ran off various different options for me (for free). They'd need to know your target u-value and any other constraints. They will potentially also use a higher resistance for the cavity due to the low-emissivity foil coating on the boards, and possibly also a better allowance for the unheated space next door.

 

Also, not sure about England but in Scotland you are allowed to have a component that is worse than the target so long as you compensate for it by increasing the performance of something else. So e.g. your garage wall could have a slightly higher u-value if you "over-insulate" the roof. Or indeed if you just specify a window / door that exceeds minimum building regs values. Obviously the calcs for that are a bit more involved (not too bad - basically multiply each u-value by the area and add them all up) and you don't want to take it to extremes for your own comfort and energy usage. But might be worth considering if you're really struggling to get that wall thick enough.

 

 

 

 

[Gooman]

Ah, I hadn't realised that Changeplan handled the studwork and thermal bridging implications. Neat! 

 

Celotex PB4012 is 12mm. Not priced it up yet ... that may change my plans again.

 

I'm in England, so unless someone else can comment positively on that route it's not going to be an option.