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Building a room withing a room, and damp

Triggaaar

Distinguished Member
A common option to improve sound isolation is to build a room within a room, and I'm hoping to do the same. It would mean building new walls and ceiling (with insulation) in an existing room, leaving a gap between the new structure and old.

In general building, you wouldn't leave an air gap that's not ventilated, because moist air from the room will get into the space, cool down (because it's now past the insulation) and condense, creating damp/water.

Has anyone go experience of this happening/not happening, or know how to reduce the risk of it happening?
 

Lensman5d

Active Member
I had a barn converted to a mancave / cinema. The floor and all walls were insulated with 50mm expanded foam - kingspan- and then 7" of fibreglass rockwell in the ceiling. The only air gap is the ceiling to the roof. The roof had insulated panels fitted too.
No damp, no issues, this was around three years ago, so a few miles on the clock :)
Not sure that would qualify as a decoupled room but it's all I got. Hope it helps !
 

Atomicus

Active Member
Doing the copious amount of research I did into this myself, I never came across or read about an incidence of this occurring. I have just finished such a room, so too early to comment on personal experience.
 

Triggaaar

Distinguished Member
Doing the copious amount of research I did into this myself, I never came across or read about an incidence of this occurring.
Why do you think that is? It's not a made up issue, I've seen it in buildings that weren't built properly. Now it might be that the other side of the new walls don't get cold enough for the air to condense, or it might just get a small amount of mold etc, and not enough for damp to enter the room, but I can't see why it won't be an issue at all.
 

Atomicus

Active Member
Why do you think that is? It's not a made up issue, I've seen it in buildings that weren't built properly. Now it might be that the other side of the new walls don't get cold enough for the air to condense, or it might just get a small amount of mold etc, and not enough for damp to enter the room, but I can't see why it won't be an issue at all.

I'm sure it's possible, but an extremely rare occurrence is all, and in most places, weather/temp depending, very unlikely. I would suggest you ask at John Sayers forums. There are some proper clever chaps over there.
 

ufo550

Distinguished Member
I suppose this could only be an issue on external walls, timber frames stud walls etc are used throughout construction. It would only be an issue IMO, if the moisture couldn’t get out of the framework, but block work is breathable.

A vapour barrier could be placed on the inside, and a breathable one on the outside.

But it’s also no different to a dot & dab plasterboard on a concrete block wall. Timber framed houses have vapour barriers, but the don’t have any internal block walls.

Guess you would have to ask an expert.
 

Triggaaar

Distinguished Member
I suppose this could only be an issue on external walls
Yes, sorry, I should have said that. It shouldn't be happening on internal walls.

timber frames stud walls etc are used throughout construction. It would only be an issue IMO, if the moisture couldn’t get out of the framework, but block work is breathable.
I'll be putting mine in an existing room, with plastered and painted walls, which therefore won't be breathable.

A vapour barrier could be placed on the inside, and a breathable one on the outside.
I was considering putting a vapour barrier on the inside, but I don't know whether that would just be on the external wall, or if I'd need them across the other walls too, which would be a pain. I don't think there's any use on putting anything on the outside. Usually you fix the issue by stopping the vapour getting there in the first place, or ventilating the space.

But it’s also no different to a dot & dab plasterboard on a concrete block wall.
That's an interesting way of looking at it. I wonder if vapour there goes into the blockwork, as you suggested.

Thanks for all the thoughts.
 

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