Insulation on Internal Walls of a Partial Garage Conversion?

[Hodgey]

Hi all, hope someone can help. I'm part way through a partial garage conversion (two new rooms inside an integral garage with a small part of the garage and the garage door retained). The Building Control officer is happy with the floor and she has given me instructions for ensuring the new internal walls are sufficiently fireproofed. The walls are up, plastered and need completing on the garage side. I was hoping to crack on with the walls this weekend but I've been unable to clarify what the insulation requirements are (I suspect she might have gone away for the Xmas break). I've used the Ubakus calculator and my wall layers are looking like they will have a U value of 0.34. So far I've failed to find any info on insulation requirements for these internal walls and I'm a bit concerned I might fall short of reg's given there is an unheated garage on the other side.

 

Does anyone know if there are minimum U values I must achieve on internal walls with an unheated garage on the other side? Also if anyone has any ideas to improve the U value I'd really appreciate your ideas on that.  

 

Location of new walls can be seen on attached Word doc and the construction from the inside through to the garage is as follows:

• @5mm plaster
• 12.5mm regular plasterboard
• 100mm Knauf Omnifit Roll 40
• 12.5mm pink fireproof plasterboard 
• 18mm OSB3 board

 

Cheers!!!

Hall_FarmNewWallsU_Value.pdf NewInternalWalls.docx

[andyscotland]

In Scotland, a wall is only "internal" if it is within the insulated envelope of the building.

 

So if (as I assume) the unheated garage is also uninsulated then the wall between the room and the garage is "external" as far as u-values etc are concerned.

 

If the garage is insulated (including the door) but just happens not to have a heater in it then there is no requirement to insulate the dividing wall.

 

I assume the rules will be the same in England & Wales.

 

In terms of improving insulation, options might include changing wool between studs to PIR (but that may be an issue for sound/fire), making the wall thicker to accommodate more wool, or lining the inside of the wall with a layer of PIR/insulated plasterboard over the studs.

 

 

Edited December 17, 2022 by andyscotland

[Hodgey]

2 hours ago, andyscotland said:

In Scotland, a wall is only "internal" if it is within the insulated envelope of the building.

So if (as I assume) the unheated garage is also uninsulated then the wall between the room and the garage is "external" as far as u-values etc are concerned.

If the garage is insulated (including the door) but just happens not to have a heater in it then there is no requirement to insulate the dividing wall.

I assume the rules will be the same in England & Wales.

In terms of improving insulation, options might include changing wool between studs to PIR (but that may be an issue for sound/fire), making the wall thicker to accommodate more wool, or lining the inside of the wall with a layer of PIR/insulated plasterboard over the studs.

 

 

Thanks Andy, the garage is the same construction as the rest of the house, a cavity wall with stone on the outside and blocks on the inside with that usual 1 inch of polystyrene insulation stuck on the blocks, so I think I might be OK. I think I'm going to add a layer of this stuff over the studs, it's cheap and has surprisingly good insulation values. I'll do the garage doors as well. 

 

I've added the polystyrene boards into the calculation and it's now showing excellent thermal values but really poor moisture proofing, not sure if I should be worried about that as it's not an external wall?

Hall_Farm_with_polyboards (2).pdf

Edited December 17, 2022 by Hodgey

[andyscotland]

I'm not sure what you mean by "that usual 1 inch of polystyrene stuck on the blocks", I've not often seen polystyrene used as internal wall insulation?

 

Does the garage have a garage door - is that insulated and airtight? I suspect you will need to demonstrate that either the outer or dividing wall meets an appropriate u-value for the regs including any openings.

 

Normally you ideally want the most water-vapour-permeable layers of a wall on the cold side, and the least permeable layers on the warm side. This reduces the risk of warm moist air from the room condensing into water as it cools on the way through the wall. So if you have wool between the studs you would put polystyrene/PIR on the room side, not the garage side as per your upload.

 

However I would definitely check with the BCO about using polystyrene at all in a wall that requires a fire resistance. Usually I think you would need to use a combination of materials that has a manufacturer's test certificate to show it will stand up to a fire for the required time.

 

 

 

 

[Temp]

Garages are normally treated as unheated so the separating wall between house and garage needs same U-values as an external wall. But I'm not sure what thet will be in your case.

 

I suspect for fire proofing you will need plasterboard on both sides? I know the NHBC isn't same as Building Regs but they suggest 30min fire rating is required..

 

https://www.nhbc.co.uk/binaries/content/assets/nhbc/tech-zone/nhbc-standards/tech-guidance/10.1/compartment-walls-between-garages-revised-march-2017-.pdf

 

The same will apply to any doors in the wall between cinverted rooms and garage.  

 

I think 12.5mm plasterboard each side gives 30min fire protection. 12.5mm fire resistant plasterboard board gets 60min.

 

 

 

 

[Hodgey]

On 18/12/2022 at 09:25, Temp said:

 

Thanks for the replies, the BC Officer phoned and informs me I need to achieve a U value of 0.30 in the wall so Kingspan board will need to be used. The 12.5mm pink plaster board will propvide the minimum 30 min fire protection.

[andyscotland]

That's good you have confirmation of what the BCO will require. What I would say is that 0.30 might be compliant but is really a very poor u-value. Energy costs are likely to be high for a long time to come - if it was my house I would be looking at whether I could do better than that. 

 

You might end up with a slightly thicker wall, but you may find there's not a huge difference in cost to upgrade since you're doing all the work anyway, and that you would get payback pretty quickly especially at current energy prices. You really don't want to find yourself ripping it out to improve it if you find your house is hard to heat in a few years.