Insulating room above freezing garage ?

[mike42]

Its always cold in winter and hot in summer. Apart from knocking it all down and starting again are there any quickish wins to be had here? I used my infrared thermometer and all it really told me was that the top of the room is warmer than the bottom.....

The rest of the house doesn't have spectacular insulation although does have loft insulation to about 270mm and cavity wall blow in (100mm cavity I think) but it is way, way warmer in Winter. The room above the garage is so bad its not really liveable in winter or summer (gets too hot).

But the odd thing is the quality of insulation in this room isn't _that_ much worse I think. It has:

loft above garage room: 100mm fibre glass insulation (easy to top that up but its only a small area - see pic)
rafter insulation - accounts for about half the wall area of the room. Currently has 75mm rockwool batts (see other pic - top batt removed for photo)
"dwarf" wall - cavity wall with cavity I think only 75mm stuffed with rockwool
floor - stuffed with you guessed it rockwool

I can access the rafters from the loft space above the room and I could also fairly easily access the rafters and the wall from outside by removing roof tiles. So could probably get kingspan type insulation between the rafters but thats only a half of the wall area and tiny part of the area all together

But am I looking in the wrong place? Given there is an unheated garage below is the floor the problem? Or should I concentrate more on the room as heat goes up?

 

[DAZB]

The floor would be a huge source of cold coming into the room and that is where you need to focus on. It should have insulation inbetween the joists and also double plasterboarded with staggered joints and intumescent sealant to the perimeter or any pipe and cable entries. You will notice a difference if that is up to standard. Also the insulation between the rafters looks badly fitted and is really not doing a great job so worth replacing with PIR or at least fitting the rockwool much tighter so it actually sits snug between the timbers. Another factor would also be the garage door and any windows you have in there as most garage doors offer very little draughtproofing so usually very cold near them.

 

[^woody^]

Insulate above if you can, but below would be the priority.

PIR and quilt in the floor if you can get access, PIR uppermost. Otherwise line the garage ceiling.

Fill gaps and voids to stop draughts in the floor void as they will cause the most heatloss.

 

[mike42]

Thanks a lot for great practical replies.

And just to ask a daft question, "normally" I understanding doing the floor to be least important as heat rises. i.e. when looking at a full house you'd look at the loft first then cavity wall - I think.

Here there is a garage below so to my daft brain it makes sense but given the other side of the walls/rafters are the open air (so even colder than the garage) and they are higher than the floor, why wouldn't they be the priority ? (Notwithstanding what you both kindly said about insulating them too)

 

[DAZB]

It's more about stopping the cold entering as opposed to preventing the heat leaving and if you tackle the sourse of the problem, i.e the place where the cold air enters, then it reduces or eliminates the problem in the first place. Of course adding more, or better, insulation to the roof doubles up the effort and will retain the warm air for longer too.

 

[mike42]

Right so I'm 98% swayed to follow the advice and do the floor properly first. I dug out some old photos when the new floor went down and back then I stupidly thought using spare loft insulation would be good enough. Given this room was retrofitted years after the garage was built is it quite possible that those small ceiling joists could come down from below (along with the existing plasterboard ceiling) and still meet spanning table reqs? (I'll check the numbers anyway but an educated guess would be useful anyhow).

That would mean I'd have much more room between the floor joists to stuff proper PIR in (100mm if I can afford it) and fill any further void with the glass fibre stuff - any excess glass fibre I could use elsewhere. Then attach the new plasterboard to the floor joists.

The other option I can see (actually mentioned by Woody) would be to board out the garage ceiling but the ceiling is already low at 2.2M in a double garage so probably wouldn't want to get anymore than 50mm PIR with another layer of plasterboard below that to hold it.