Damp room with "Warm Roof" and no ventilation

I am about to turn my garage flat roof into a "Warm Roof" (I have a seperate thread discussing this task in the Roofing section) with the primary goal of having no ventilation whatsoever, as I will be building a decoupled room inside this sealed garage shell to use as a music/recording studio - so every single hole becomes a compromise for sound isolation.

The garage does, however, have a damp problem (I also have a seperate thread for this here in this section, but it is long winded) and solid advice from members here so far has been to do "external work" to eradicate damp by getting outside paving slabs 150mm below DPC - I have a few problems that may make this almost impossible though.

Have now had three seperate external companies suggesting "dimpled membrane" to cover floor and walls (with weep holes in corner of garage etc) and then build decoupled room with no worries.

I don't have running water that would require an oulet (I understand the concept of having the weep holes in case there was standing water that needed to escape) --- but what will happen to all of the moisture in the form of a damp floor that will be trapped behind and underneath the dimpled membranes if there is no ventilation up in the roof and no air bricks at ground level etc?

With a warm roof in place there should be no immediate condensation taking place, but surely that damp will cause other havoc --- is there a solution?

If I cannot eradicate the damp 100% - then would it be better if I did NOT create a warm roof and rather go for a ventilated cold roof so that this trapped moisture has a way to get out eventually --- or should I stick with the warm roof but install some sort of extractor fan in the ceiling void coupled to a vent that can be switched on if needed after rains etc?

Thanks for any help guys - terribly confused now.

Steve

Same here. Inner walls will be studwork on single course of engi bricks, dpc over the floor lapped over the bricks.

Pitched roof, with rockwool on ceiling (same as a loft in a house) cold plyboarded roof, no passive ventilation but boxed and soundproof ducted fans bringing air through the roof void.

Flat roof will be trickier, unless you leave a good few inches between the top of the ceiling joists and bottom of the roof joists for airflow with a fan, this will mean a particularly high roof though so you might as well go with a shallow pitch on it, more space to fit a soundproof fan box that way.

Aha - now that might be someting to investigate.
So rather than having "natural" breeze/airflow ventilation, you will be forcing the flow with a fan?
Hmmmm - surely a massive saving on the £1500 (materials alone) for my warm roof plan.
Where would these fans be pulling air from though - airbricks??
Are you also faced with an existing damp problem, or are you accounting for unforseen circumstance?
Are you going to be custom building these fan/duct systems and if so, what is the price of admission to your world of research so far
Thanks
Steve

Airbrick/louvre vent on the exterior wall/gable, flexi ducting attached to this travelling a few feet into the roofspace, into a rockwool lined box made from mdf lined with a couple layers of plasterboard, this box also incorporates the fan.

The ducting itself should be wrapped in insulation and boxed in along it's length.

The fan box should have a couple of baffles incorporated, basically a couple of 'internal walls' that interrupt the direct air flow, making it 'go round a corner'.

These boxes will be fairly big, 12-18 inches in width.


Same at the other end of the roof but the fan is right next to the wall vent chucking out the 'stale' air.

costings? Sheet of 18mm mdf, couple of sheets of plasterboard, flexi ducting plastic or foil. For the fans, I'm thinking of using S&P TD160 fans.
1 at each end of the roofspace, and a couple for changing air in the room itself, as it will be airtight and get stuffy inside very quickly. So 4 in total.

This will be a bit more but ready to go:

http://www.justfans.co.uk/small-acoustic-fan-p-537.html

turnitup said:

what will happen to all of the moisture in the form of a damp floor that will be trapped behind and underneath the dimpled membranes if there is no ventilation up in the roof and no air bricks at ground level etc?

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How much spare height do you have at floor level? I Assume your garage floor is below the house's DPC and slopes slightly down towards the door for drainage.

Why not have a suspended floor? Have joists running front to back on resilient supports with air bricks front and back. That way you could put medium density mineral wool between the joists under the floor; mostly to absorb noise but with some heat insulation.

You could use 22mm tongued and grooved cement filled particle board for the floor with joints glued. These floorboards are unaffected by and pretty impervious to moisture (can even be immersed). They offer good sound insulation and a firm surface. Underneath you might use Tanalised wood joists, which are treated with arsenic as a preserver.

1 at each end of the roofspace

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Right - so air drawing across ceiling void only? Is there no need to try to induce air to draw from ground/floor level?
Or is this covered by the theory of the damp on the floor eventually evaporating due to the warmth of the insulation in the void between the original walls and the inner plasterboard walls?

How much spare height do you have at floor level?

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This would depend upon the minimum air gap I would need to have in the floor void in order for the ventilation to work down there.
I currently have joists running at very shallow pitch across width of garage and once the new ceiling is fitted for the INNER decoupled room, I would probably end up with around 2.3m on the house side and 2.2m on the opposite side.

Thanks for opinions and advice guys.

Steve

My theory relies on, any moisture that comes through at the bottom of the wall cavity will rise up into the roofspace, being taken up by natural convection (heat rising up) The other possibility is that heat from within will drive out moisture back through the exterior masonry walls.
Of course I haven't taken into account the fact the the room will be minimally heated and all walls insulated with 100mm rockwool. But the spec dictates that this cannot be a traditionally vented build so fingers crossed!

In any case I will leave a min 50mm gap between insulation and exterior walls, possibly even 100mm.


I also intend to have a timber floor, but unvented! Cannot find any concrete info on this but will lay treated batten onto dpc, rockwool in between and then 38mm chipboard over this. May use another dpc or breather membrane over the inso.

In your case I would avoid anything that raises floor height as you'll end up with with no headroom.

Any drums in there or all computer based/acoustic guitar/mongolian throat singing?

turnitup said:

Or is this covered by the theory of the damp on the floor eventually evaporating due to the warmth of the insulation in the void between the original walls and the inner plasterboard walls?

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I doubt it. Unless the void under the floor is vented, damp from the sub floor will be added to by moisture from the room and cause mould and rot. With the underfloor void connected to the cold cavities, you could get condensation getting past the VPL in the wall through the floor and condensing on the cold walls. I think the only way to avoid this is through plenty of ventilation (air bricks or forced draught) of the underfloor void. Don't consider a VPL in the floor, it prevents spills from draining and drying.

Deluks said:

Any drums in there or all computer based/acoustic guitar/mongolian throat singing?

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Don't forget the dedicated Didgeridoo isolation booth!

Nah - I actually have a vast collection of older school kit, although I do record to hard disk --- but stubbornly refusing to submit to the world of plugins, so use PC (Cubase SX-3 with 16 analogue in 32 analogue out soundcards) purely as a virtual tape machine.

I work in IT, so have deeply scarred reasons to stay AWAY from the PC as much as I possibly can whilst making music!

2 metre wide Soundtracs Inline console, 5 x 14U wooden slanted outboard racks, 2 x 12U wooden racks for samplers, PC, soundcards, converters etc, 3 x 16U racks for patchbays, recorders, amps and power supplies, 3 x 6U synth module racks, 8 x synthesizers, V-drums, various guitar amps, 8 x guitars --- PLUS hoping to get a small storage cupboard, 2 x beanbags and a lava lamp in there somewhere!!

Reality is that I need to have as big a space as I can possibly squeeze just to be able to fit everything in --- treatment and ratio's will have to be dealt with after the fact as opposed to being part of the design.

Strangely enough, I can get it all into my final space which will be around 5.4m x 3.0m --- the only snag I have with my planned placements will be the doorway to get in that will be stumping me --- if I could do a sliding door for the inside door it would all fit!!

Steve

Deluks - whereabouts in Surrey are you?

turnitup said:

I work in IT, so have deeply scarred reasons to stay AWAY from the PC as much as I possibly can whilst making music!
Steve

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But they make it all sound soooo easy in the magazines!!!!

-

Croydonia - the home of Maydie hospital!

Well, well, well ... a chip and a putt from me then --- I am in Banstead.
Wouldn't mind giving you a ring at some point to chat about the constant compromises and crossed fingers if you have a 20 minute window sometime?
I am all over the place at the moment with this garage - cold roof vs warm roof, ventilation vs sound leakage, raised floor vs ceiling height, damp vs "just build it" etc etc
Excuse the ignorance, but does this forum have a PM function?
Steve

Even better than that I'll pop over and have a look. Mainly because I'm nosey


Click 'friends' at the top of the page.

Hi!

So! Silly question to ask, but I will! How do you intend to get into this sound booth? I assume that you will be blocking up behind the garage door and have a secondary means of getting into the garage - or how about a picture or two or three!

One point to consider, its not the moisture from the existing floor that you really need to be concerned about, its more to do with the vapour generated by the occupancy of the room i.e. your breathing!

One further point to consider; If you have a well insulated shell/structure the energy costs to keep the room temperature above condensation point should be reasonably affordable - even in winter.

And another point - You need to steer clear of blown fibre insulation as it has a very poor vapour resistance.

While your on the subject of ventilation, you may wish to consider an in-line heat recovery system, some systems claim around an 80% efficiency, you could also add on an in-line heater which could be tied into a temperature/humidistat sensor which should solve all your problems!

Obviously, not as simple as it sounds, more a case of some food for thought!

Regards

Hi there - sorry for the delay in responding, off feet at work yesterday!

Yes, will be blocking up behind the main doors as well as the single door into the backyard --- but there is a door leading into the kitchen which will left in place (beefed up with 4 x layers 18mm MDF and a layer or 5/8" plasterboard!!!).

Here is a TERRIBLE sketch done in MSPaint (haven't found the time to master Sketchup yet) of the garage in its current state.

Maroon walls are house brick walls.
Black walls are garage block walls.
Blue lines are existing doors.
Yellow areas are existing external slabs which are about an inch above DPC (as per my other threads) - with the manhole positions marked --- these are part of the reason it may prove awkward to chop down at external slabs to sort damp etc.


And here is another HORRIBLE sketch showing the basic inner room concept with pink lines depicting drywalling.


Will need to angle wall in corner at kitchen door to allow access to Gas meter - plus we do need to get our washing machine in there, right at the door (yes, it does fit - there is no scale in these sketches).

Excuse my ignorance, but what is an Inline Heat Recovery System?

Re: vapour - there will be a split unit air conditioning system in place servicing the internal room only --- but this will, of course, only be switched on when the room is in use.

Re: insulation - there will be 3" insulation in the narrow voids between the original walls and the plasterboard leaf, including in the ceiling.
Apparently, 3" in the ceiling will allow a MINIMUM of 100mm Kingspan above roof sub deck for a warm roof - and I need to keep this to 100mm in order to avoid cutting a new flashing channel into the pebbledashed house wall.
The only plan I have not covered yet is insulating the floor at all.

Most recent build plan is to get a "warm roof" on to reduce condensation risk at roof point, zero ventilation in the ceiling void to not compromise sound isolation, dimpled membrane covering floor and all walls to protect inner room structure from direct moisture (shallow channel around edges leading to basic weep hole at floor level in case of "flooding") and a dehumidifier in the little corner where the washing machine will be situated --- that triangular space is a part of the entire void between external/internal wall leafs and ceiling/roof leafs - so hoping to be able to monitor for moisture in that space and dehumidify as needed.

Although a cumbersome approach in some ways, does it sound flawed in any way?

Steve