Rooms above garage - Thermal envelope?

[SuperJohnG]

Not sure if this is the right section, but house construction didn't seem to cover it. so please move to appropriate place if required mods, thanks.  

 

After numerous iterations we have nearly finalised on our layout. (See below). It made sense for us to move the master bedroom above the garage to aid the flow of the house and we really like the layout.  I had however previously avoided this because I assumed we would have issues with being inside the thermal envelope and air tightness and so forth. It will be a timber frame or SIPS build and I am hoping to achieve high levels of air tightness, is there anything I should be looking at or concerned about with our master being above the garage and obviously the garage will be outside the thermal envelope and not very air tight at all. Will the floor construction need particular attention? . any other thoughts. 

 

 

 

[Moonshine]

6 minutes ago, SuperJohnG said:

Will the floor construction need particular attention?

 

yes the floor will need to meet building regs for fire and thermal insulation. Also the door to the mud room will likely need to be a 30min fire door with smoke seals.

 

The bedroom has some long spans over the double garage, so you will need to think about getting the spec of the timber joists right so you don't get too much deflection / bounce. This may limit the floor to ceiling height in the garage, but not the end of the world.

 

Careful detailing will be needed when the garage wall to the house meets the floor for thermal performance.

[scottishjohn]

pretty sure you will need alot of thickness+ fire proofing in ceiling of garage and fume proof if over a garage in case of car going on fire

also garage floor will need to be lower than house so burning fuel cannot run in

 as for long runs of bedroom floor joist --close them up and fit more of them ?

 your architect will know

 

[ProDave]

This is nothing unusual at all.

 

Standard detail for garage ceiling with habitable rooms above is two layers of 15mm fireline plasterboard, the pink one 15mm thick, with the joints taped and filled, and the joints of the two layers staggered.  You will have thick joists for the spans you have so plenty of room for a decent amount of insulation.  build in a taped air tight membrane under the floor and taped up to meet the wall air tight layers and that's your details sorted out.

[scottishjohn]

the drawing shows garage floor same height as house --no step shown --that will not do -must be lower

[SuperJohnG]

7 minutes ago, scottishjohn said:

the drawing shows garage floor same height as house --no step shown --that will not do -must be lower

 

Is this a building regs thing?

[scottishjohn]

Just now, SuperJohnG said:

 

Is this a building regs thing?

it was when my house was built 

[scottishjohn]

and air bricks  as well

[ProDave]

A step down from the house is mandatory (in Scotland)

 

Ventilation of a garage is only mandatory above I think it's 30 square metres.  Our single garage does not require any.  Building regs do say in that case not to try to seal the building, so I did not fit air tight membrane for instance.

[Vijay]

AFAIK it's either a step down or a slope towards the external garage door

[SuperJohnG]

This kind of plays with my plans a little. I am hoping for an insulated passive insulated raft - so woudl I put the garage floor in the raft or not - or can I have two levels in a raft?

 

just for clarity - this is in Scotland. 

Edited February 18, 2020 by SuperJohnG

[Moonshine]

26 minutes ago, Vijay said:

AFAIK it's either a step down or a slope towards the external garage door

 

you are correct as below (best check the regs to make sure its still valid)

 

 

[Rishard]

On 18/02/2020 at 14:54, ProDave said:

This is nothing unusual at all.

 

Standard detail for garage ceiling with habitable rooms above is two layers of 15mm fireline plasterboard, the pink one 15mm thick, with the joints taped and filled, and the joints of the two layers staggered.  You will have thick joists for the spans you have so plenty of room for a decent amount of insulation.  build in a taped air tight membrane under the floor and taped up to meet the wall air tight layers and that's your details sorted out.

Morning, I’m just going through a similar process with a separate double garage with room above, timber frame. I have the option to carry the airtight membrane under the joists which form the garage ceiling. I have a pitched roof with 2x gables. Would I be best joining the membrane to the external membrane on the timber frame gables as you say? Also, I can detail the rafter eaves so the membrane can carry on up past my joist ends to meet my roof membrane, would this be best rather than turning it in on the joists like a Tony tray? My roof pitch is 45* and the wall plate is on top of my joist ends over load bearing walls. I intend to split my rafters at the joist end point so I can have some ‘dummy’ oak rafter feet as the below garage is oak frame. I have done this detail on a few oak buildings but the airtightness hasn’t always been a priority. This being my own garage, it is, alongside good insulation ect. The point where I split the rafters would be where I can connect up ceiling to roof membrane.