Double-leaf timber frame building

[texutree]

Hi All

I am in the design stages of an outdoor music practice room project. I had a hard time finding any info on improving the sound transmission loss of timber-frame buildings until I stumbled across James' (badass_mcfunk) thread on this forum, from which I have gained a lot in insight.

I will be playing an upright piano in there, and the nearest neighbour is about 15m away. Ideally I would like to achieve a Transmission Loss of 30 or above. In terms of budget, I would like to complete the project for as close to £6000 GBP as possible.

I'll outline the plan here, and would be very grateful for any feedback.

The idea is to make a double wall construction.
At present, the site is a >20cm thick reinforced concrete slab (contains a damp-proof membrane), measuring 3.7x6.8m. The perimeter of this will be lined with concrete blocks and then an external frame contructed of 2x4” timber. The structure is to have a monopitch roof, and the back wall will be higher than the front. Roof beams (225mm) will span the width between the walls.

External FrameScaled.jpg

The exterior will be lined with a layer of 18mm OSB3 (moisture resistant) and a layer of 10mm cement board (mass: 13.5kg/m^2) attached to the OSB, onto which a Tyvek breather membrane will be placed, then the building will be clad, for aesthetic reasons... I am currently considering metal cladding for this.

The spaces between the studs will be filled with Rockwool Flexi insulation.

The building has a window and door in the front face; I originally wanted to install a normal double glazed opening window, but now believe thick, laminated glass makes more sense. I will have to source a heavy, solid door.

Roof

The exterior roof beams will be 225mm in depth, provisionally. I am going to get the dimensions of beams OK'd by a structural engineer – I may get away with less that what I stipulate here (or, I may not...) Above the beams will be a layer of 18mm OSB, 100mm of rigid insulation, another layer of OSB (screwed through the insulation into beams underneath), then underlayment and a metal roof.
Between the beams, I plan to pack rockwool flexi insulation.

(The reason for insulation above the roof deck is that I could find no way to ventilate the roof above insulation without compromising the sound isolation of the structure. After looking into the topic, I found some info on unventilated roofing – especially here: greenbuildingadvisor.com/blogs/dept/green-building-blog/podcast-how-insulate-unvented-roof.
The idea is that the above deck insulation keeps the roof deck and timbers above dew point in cold weather, thereby avoiding condensation.)

3Scaled.jpg

Internal Room

A series of 100mm joists will sit on the concrete slab, on neoprene pads.

floor joists.jpg

From this floor structure, an internal frame will be constructed from 2x3 timbers. Spaces between studs to be filled with rockwool.

internal frame.jpg

A vapour control layer will be attached to the inside of the studs. Next, a layer of 15mm MDF will be attached to the interior of the studs, followed by a double layer of gyproc soundbloc plasterboard (density 14.1kg/m^2), maybe with a layer of SBM5 mat (5kg/m^2) sandwiched between them. Internal ceiling joists (again, provisionally 225 x 47mm)will run from one side to the other, between exterior roof beams.

Ceiling

Due to external height constraints, the roof beams of the inner frame and outer frame will be staggered

staggered ceiling beams.jpg

Again, a layer of 15mm MDF will be attached to the underside of the ceiling joists, with a double layer of gyproc soundbloc attached to the underside of this.



Floor

For the floor, spaces between joists will be filled with rigid foam insulation, and a double layer of 22mm chipboard applied. Not sure if it would be a good idea to put a layer of old carpet tiles between the two layers of chipboard...


All electrical fittings and lights will be surface-mounted, as will the plastic trunking in which the cables run.

Room ventiliation will initially be by opening the door periodically. Over time I plan to install a proper system with baffles and a fan... I would like to check that retrofitting a ventilation system would be feasible: would it be OK to drill through both walls to install the duct?


I would be very grateful if anyone could advise me of any issues I have not taken into account, or provide any other suggestions.

My main specific questions at this stage are:

-Will the cladding-type affect the sound isolation of the structure?
-Is it feasible to retro-fit a ventilation system?

Regards
Jon

[Soundman2020]

Hi Jon, and Welcome!

Ideally I would like to achieve a Transmission Loss of 30 or above.

Do you mean a TL of 30 dB, or are you talking about STC-30? Since you didn't mention the units, it could be either, but those are rather different things.

The perimeter of this will be lined with concrete blocks

Why? What is the purpose of the blocks? What prevents you from just building your walls directly on the slab?

The structure is to have a monopitch roof,

Often also called a "shed roof".

Roof beams (225mm) will span the width between the walls.

What spacing? Did you do the calculations and check that your rafter dimensions and spacing are correct for the load that they will carry?

The exterior will be lined with a layer of 18mm OSB3 (moisture resistant) and a layer of 10mm cement board (mass: 13.5kg/m^2) attached to the OSB, onto which a Tyvek breather membrane will be placed,

Is that the normal sequence for constructing that type of wall in the UK? Where I live, the Tyvek would go between the OSB and the fiber-cement board. You could also skip the siding, and just use the fiber cement board as the final outer surface, to save money, since you are in such a tight budget. You'd need to seal and paint it somehow, of course, but it would still be cheaper than adding siding on top.

The spaces between the studs will be filled with Rockwool Flexi insulation.

What density is that? For MSM walls, you need something around 50 kg/m3 if you are using mineral wool insulation.

The building has a window and door in the front face; I originally wanted to install a normal double glazed opening window, but now believe thick, laminated glass makes more sense.

Right!

I will have to source a heavy, solid door.

Two. You need not one door, but two, set up back-to-back, with one door in each leaf. The same applies to your windows: two panes, one in each leaf.

Above the beams will be a layer of 18mm OSB, 100mm of rigid insulation, another layer of OSB (screwed through the insulation into beams underneath),

Hang on a sec... so how is the top layer of OSB attached? You say the lower layer is screwed to the rafters, then you have 100mm of insulation on top of that, then the second layer of OSB.... what is that attached to? It cannot just sit on top of the insulation! And I very much doubt that your plan of using 150mm long screws is going to pass inspection...

Also, that sounds like you are building a 3-leaf roof--- not a good idea for low frequency isolation.

then underlayment and a metal roof.

ummmmmm.....so on top of your flimsily attached upper layer of OSB, you plan to have another two layers of non-attached roofing materials???

The idea is that the above deck insulation keeps the roof deck and timbers above dew point in cold weather, thereby avoiding condensation.)

... Assuming that the room below is always warm enough to do that, day and night, constantly... will you be in there 24/7? If not, can you afford to leave the HVAC system running 24/7 when you are not there?

.... ans also assuming that enough heat rises into the insulation to keep it all warm, which is not so certain once you put the inner-leaf in place.... that's a lot of extra thermal mass and insulation you'll be adding... are you SURE that enough heat will make it up to the final roof surface?

Wouldn't it just be easier to build a conventional vented roof, with a two-leaf isolation ceiling below for acoustical purposes?

A series of 100mm joists will sit on the concrete slab, on neoprene pads.

WHY???? That is not the correct way to float a floor, and you do NOT even need to do that, despite the myths, marketing hype, and other silliness you might have seen on the internet... You probably should read this: http://www.johnlsayers.com/phpBB2/viewt ... f=2&t=8173

A single upright piano is nowhere near loud enough to need a floating floor, and even if it was, you already have a perfectly good slab-on-grade foundation, which is about the best possible acoustic floor you could hope for in a studio... why trash that by building a useless non-functioning non-floating light-weight resonant monstrosity over it, that will also take a large chunk out of your headroom?

From this floor structure, an internal frame will be constructed from 2x3 timbers. Spaces between studs to be filled with rockwool.

Nope. See above.

A vapour control layer will be attached to the inside of the studs.

Another bad idea. That would trap moisture in between the existing barrier under the slab, and the new barrier. I'm assuming that your slab was built properly, with a 6 mil plastic barrier under it? If not, then you have bigger issues that you'll need to deal with, which a vapor barrier will not solve...

You also seem to be confusing terminology here: floors don'0t have studs: they have joists. Studs stand upright, inside walls. Joists run horizontally inside floor and ceilings. Rafters run at angles under the roofing deck. Trusses might include both rafters and joists.

maybe with a layer of SBM5 mat (5kg/m^2) sandwiched between them.

Why? What purpose would that serve, acoustically?

Due to external height constraints, the roof beams of the inner frame and outer frame will be staggered

What you show there is more commonly called "interleaving", but I guess "staggering" is another possibility, as long as your contractor understands what you mean.

Again, a layer of 15mm MDF will be attached to the underside of the ceiling joists, with a double layer of gyproc soundbloc attached to the underside of this.

You say you want 30 dB of isolation, but you are building for about 60 dB: Something doesn't jibe here. 30 dB of isolation is about what you get from a typical simple stud wall in an average house, with 2x4 studs and a single layer of drywall on each side. You are going way beyond that, so either your initial goal of 30 dB is a typo, or you are greatly over-building if your goal really is 30 dB.

For the floor, spaces between joists will be filled with rigid foam insulation, and a double layer of 22mm chipboard applied. Not sure if it would be a good idea to put a layer of old carpet tiles between the two layers of chipboard..

It would be good idea to not build a thin, light-weight, resonant, non-isolating non-floating floor at all! Just stick with your plain old concrete slab as the floor. Polish it, or stain it, for a really nice looking finish. Or if you don't like the look of that, then just lay simple laminate flooring on top of it. That's another really good acoustic surface. You will also gain all those extra inches back as headroom, which helps to maximize room volume... and "maximum volume" is a very useful, very desirable acoustic goal.

All electrical fittings and lights will be surface-mounted, as will the plastic trunking in which the cables run.

Excellent

Room ventiliation will initially be by opening the door periodically.

Ummmm. Nope! That will not do anything at all to ventilate. Your studio will not be like a normal room,where there are numerous tiny leakage paths for air to move through when the door is open. Your room will be hermetically sealed, twice over, perfectly: No other paths for air will exist, so opening the door will accomplish nothing. Air only moves if there is a reason for it to move: a difference in pressure. Since opening the door will not produce any pressure differential at all, no air will move.

In other words, you NEED a ventilation system now, not later. Read over a few of the threads here on the forum where people have left out the HVAC system initially, and their final comments about how much they needed that at the end, and what they had to do to get it. A studio with no HVAC system is like a shower with no drain... not very useful.

Over time I plan to install a proper system with baffles and a fan...

... and how exactly would you do that, after all the walls and ceiling are already finished? You do realize that you'd have to chop out huge chunks of walls and/or ceiling in order to accomplish that? You might thing that you can just poke a little hole through the wall somewhere, but in reality you cannot. That will not work. It will require a major tear-down and re-build to do it afterwards.

I would like to check that retrofitting a ventilation system would be feasible:

It's only feasible if you don't mind trashing large parts of your walls/ceiling that you spend lots of time, money and effort to build...

My main specific questions at this stage are:

-Will the cladding-type affect the sound isolation of the structure?
-Is it feasible to retro-fit a ventilation system?

- You can skip the siding (cladding) if you seal and paint the fiber-cement board properly.
- It is feasible, yes, but it will require tearing apart stuff that you already built, then re-building it. It would be far better to do it now, since you will need it anyway. If you happen to enjoy breathing, then you do need HVAC in your room. It is not optional. People who thought it was have severely regretted doing that, and had to quickly retrofit... at large expense, and painfully... it's never a nice feeling to take a power saw to a beautifully finished wall, and chop it down....

- Stuart -

[texutree]

Many thanks indeed for your reply, Stuart - I really appreciate your taking the time to share your knowledge.

Do you mean a TL of 30 dB, or are you talking about STC-30?

I was aiming for at least a 30dB TL at 100Hz, but more would be better. I've measured the SPL when I play and it gets to about 85dB, when playing quite loud.

What is the purpose of the blocks? What prevents you from just building your walls directly on the slab?

The reason for constructing the external frame on a perimiter of block is to raise it away from surrounding ground level, which is about level with the top of the slab. The back of the slab is not easily accessible and tends to build up with leaves, etc so I wanted to make sure the frame won't get damp.

Ideally, I would like wooden cladding on the exterior for aesthetic reasons; if it comes down to it, though, the cement board may not look too bad once sealed and painted.

What density is that? For MSM walls, you need something around 50 kg/m3 if you are using mineral wool insulation.

The insulation I was going to use in the walls between studs, rockwool flexi, has a mass of 45kg/m^3. It's marketed as a dual-purpose acoustic and thermal insulation product so I assume that it will do the job.

Why? What purpose would that [SBM5 mat] serve, acoustically?

I was thinking of adding the layer of SMB5 mat between the two layers of soundbloc plasterboard simply to add more mass and improve the sound isolation (it weights 5kg/m^2 and is only 2mm thick).


In view of what you have said, I need to rethink the ceiling/roof. I can't say whether the plan to insulate above deck would keep the timbers warm enough or not to avoid ventilation - especially as there's considerable thermal mass and a separate structure underneath. I simply don't have the headroom to make a proper 2 leaf isolation ceiling.

Perhaps the safest option is to use RSIC clips and hat channel for the ceiling. There's a brand called Genie Clip sold in the UK. I have made a rough diagram of the way I am thinking of approaching this. I have shown a double layer of MDF attached to the underside of the rafters (to make another leaf); the genie clips/hat channel sit below this MDF. There would be an air gap of around 50mm between the plasterboard and MDF. This is the only way I can see of creating a 2 leaf, decoupled ceiling, given the height constraints.

Genie Clip Ceiling DiagResized.jpg

It would be good idea to not build a thin, light-weight, resonant, non-isolating non-floating floor at all! Just stick with your plain old concrete slab as the floor.

One of the reasons I was going to install a floating floor was that it would allow me to put some thermal insulation underneath. I will alter the plan here. i)build the internal frame off the slab. ii)directly attach floor joists to slab (no attempts at floating) and fill the spaces between with thermal insulation. Rather than 100mm joists, I may go with 50mm, in order to maximise headroom. I know a solid floor would be preferable, but I would like to put as much thermal insulation in the structure as I can...

you NEED a ventilation system now, not later.

Point taken about HVAC! I need to start looking into it properly, then.

Anyway, many thanks again for taking the time to give feedback. It's got me thinking through the design again and reviewing the pitfalls.

[Soundman2020]

The insulation I was going to use in the walls between studs, rockwool flexi, has a mass of 45kg/m^3. It's marketed as a dual-purpose acoustic and thermal insulation product so I assume that it will do the job.

That should work fine.

I was thinking of adding the layer of SMB5 mat between the two layers of soundbloc plasterboard simply to add more mass and improve the sound isolation (it weights 5kg/m^2 and is only 2mm thick).

... and costs a fortune?

I would do the math and see if you need more mass, or more air space, or not.

In view of what you have said, I need to rethink the ceiling/roof. I can't say whether the plan to insulate above deck would keep the timbers warm enough or not to avoid ventilation - especially as there's considerable thermal mass and a separate structure underneath.

I would check with local experts in your area, who have experience with your climate, and also with your local building code.

I have made a rough diagram of the way I am thinking of approaching this. I have shown a double layer of MDF attached to the underside of the rafters (to make another leaf); the genie clips/hat channel sit below this MDF.

That would be a mistake. The air gap is far too thin. The resonant frequency would be be too high, and there would be poor isolation in the low end. The general rule of thumb is that you need at least 100mm of air gap to get the frequency down low enough.

I will alter the plan here. i)build the internal frame off the slab. ii)directly attach floor joists to slab (no attempts at floating) and fill the spaces between with thermal insulation.

The floor would still be resonant like that. It wouldn't float, correct, you you are missing the point: you still have a resonant system down there that will trash your acoustics and also your isolation. The floor should have no air gaps at all in it. You probably need to read this: http://www.johnlsayers.com/phpBB2/viewt ... f=2&t=8173

Rather than 100mm joists, I may go with 50mm, in order to maximise headroom.

... thus doubling the resonant frequency, and reducing even more of your low-end isolation...

Anyway, many thanks again for taking the time to give feedback. It's got me thinking through the design again and reviewing the pitfalls.

- Stuart -