Room under garage insulation problem

[Malkie]

My house is split level and I have a room built under the garage floor. The floor of the garage is concrete screed on bison beams. The ceiling in the room below is plasterboard on 50mm wooden struts and there is no evidence of any insulation but also no damp. This is a cold room and I was planning to use PIR boards to improve the ceiling insulation but have been advised that is not a good idea without air flow through the gap below the concrete and there is none. Any ideas?

[Gus Potter]

What a good question.

 

11 minutes ago, Malkie said:

My house is split level and I have a room built under the garage floor.

 

My starting point here is to look at the direction of travel of the moist air (call this air that contains water gas) and where this gas will condense into a liquid (water which causes the problem) that is called the dew point. The garage is colder than the upper floor thus if water gas gets past the existing ceiling and migrates up it won't condense as the temperature above is higher as you move up the floor depth.. you move towards the habitable heated space... warm air can carry more water gas without it condensing out...hitting the dew point.

 

Most intergral garage ceilings I have seen have two layers of plasterboard for fire protection. Before you do anything you need to make sure you don't compromise the fire protection.

 

Now if you were to add an extra layer of insulated plasterboard (which is often has it's own vapour membrane)  below the fire protected ceiling then as the garage is cold the water gas still won't encounter a lower temperature zone as it move upwards so you should not hit the dew point.

 

In theory you could have a very moist bedroom and the water gas migrates downwards and condenses.. but that will happen anyway as it hits / nears the cold side of the existing garage ceiling. But in real life elevated levels of mositure will go up towards the bedroom ceiling. If the bedroom even approaches that level of moisture you must have another problem that would make it close to uninhabitable.

 

 

 

 

[Redbeard]

@Gus Potter, I think you have read this as I first did, that there is a bedroom *over* the garage. However there is no reference to a room over the garage (though there may be one), but to a room *under* the garage, so for that room I think the 'normal' concerns re vapour control are relevant, as the (presumably heated) 'under-room' has a (presumably cold) garage over.

 

OP: Haven't had the thoughts yet as to how to address it, but first call would be a condensation risk analysis (CRA) (preferably dynamic) and an assessment of ventilation. If we pretend for a moment that the ceiling is actually a solid wall, then we would not automatically be looking for a ventilated void if doing internal insulation of an external wall (IWI) (though it would make life easier and would increase the amount of insulation you could use). If IWI 'hard to the wall' is OK, then *some* insulation 'hard to the ceiling (or under the battens) should be OK. The CRA will help you to assess how much.

Edited October 1, 2023 by Redbeard
Typo

[ProDave]

Can we assume that this room under the garage is also a semi basement?  At least the front bit of the room (front in terms of where the garage door is) must be underground.

 

So presumably everything about the room is cold? not just the ceiling.  So you are probably looking at insulating the walls as well.

 

Assuming there really is no insulation (age of building?) then anything will be a vast improvement, even 50mm pf PIR.  How much can you afford to lower the ceiling height by or reduce the room size by with wall insulation?

[Malkie]

So yes this is a family room under the garage. The garage is integrated into the house and there is no room above it only the roof which is insulated. 
The front of the room  is underground and there is really only one external wall, one internal wall and the other backs onto a stair that provides access to the upper floor. The room is useable and heated as required by the central heating. 
The issue is it cools quickly and I want to improve the comfort levels if possible. I am pretty certain there is no insulation in the existing ceiling. Just plasterboard and a 50mm gap to the underside of the garage floor.  No obvious way to provide ventilation

[Malkie]

House was built in mid eighties by the way. Timber frame construction with brick / roughcast finish

[ProDave]

What you really need it a thermal camera, assuming you can't stretch to one of those, an IR thermometer would tell you if it's the ceiling or walls that are coldest hence losing the most heat.

 

Probably no floor insulation either. 

[Gus Potter]

On 01/10/2023 at 11:48, Redbeard said:

I think you have read this as I first did, that there is a bedroom *over* the garage.

Indeed.. my mistake. that changes the game!

[Gus Potter]

On 01/10/2023 at 11:48, Redbeard said:

Haven't had the thoughts yet as to how to address it,

What about going for the simple stupid.. an insulated garage door and then you could treat the garage floor as a quasi party wall.. just horizontal?

 

Next would be to look at how often would condensation really occur? In the coldest weather you may want to introduce a little background heating any way in the garage.. it would be a few quid over a week or so?

 

During normal use a bit of PIR would help. So long as it's not a kitchen or bathroom your level of water gas generation will probably be less onerous. Ok so you shift the dew point down.. but the premise here is that the concrete is breathable.. which it is not really.. if the garage floor has a painted surface then shifting the dew point down ain't going to make much difference?

 

 

[Malkie]

Had an insulated garage door fitted last year and that makes quite a difference and also the floor is painted. Even on the coldest day it never is freezing in there and the heating in the room below is turned down if not in use.