Insulate an attached garage

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Terrytpot

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Having never insulated anything before I’ve gained pretty much all I know on the subject from this forum although nearly all of the content on here refers to insulating detached timber framed workshops and not garages that form part of your property and as I’m (fingers crossed) close to moving to the bungalow shown, I’d be grateful for advice on the best way to attack it. On my current property I replaced the garage door with an insulated sectional door and intend to do the same at the bungalow along with having the rear access door changed to a composite one. I’ve not had an opportunity to gain access to the loft above the garage so am not sure what the layout is up there but would imagine that the wall separating the living room from the garage extends all the way up to the roof creating a separate loft just for the garage end of the building, and whether that wall has much in the way of insulation is also currently unknown. I would imagine that the walls that don’t adjoin the living room will be uninsulated, and possibly, also the loft area directly above the garage.
I don’t see many options available as far as the floor is concerned apart from some 25mm battens enclosing thin pir sheets and covered over with whatever robust flooring is economically viable, say 19mm osb?
For the loft, I’m not sure what I’ll do yet as I may open it up to create storage, in which case it would be pir between rafters but if it all looks too much of a chore it may just get put on the back burner and just have a load of itchy coo rolled out up there.
The wall’s I envisage doing something similar to the floor but bumping upto 100mm pir and obviously increasing the battens appropriately to suit although where moisture barriers and air gaps fit within all this is where I’m falling down on what goes where and if it’s needed or not.
 

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rogxwhit

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Since the urge to insulate spread through the land (apart from getting enshrined in building regs), a lot of builders had trouble wrapping their heads round the pitfalls. If you have internal wall insul as thick as 100mm! can you guarantee that no moisture from the heated internal space can get behind that insulation. Because if it can - by insulating, you've made what was the previous internal wall surface a colder surface than before, and any moisture that can get round the edges and onto that surface is likely to condense - and the condensate's got nowhere to go.

It's not comparable to a roof situation where you can ventilate above the insulation, whether it's at ceiling or rafter level.

There are solutions but they need some serious joined-up thinking - they can't be achieved casually.
 

Terrytpot

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…by insulating, you've made what was the previous internal wall surface a colder surface than before, and any moisture that can get round the edges and onto that surface is likely to condense - and the condensate's got nowhere to go.
There are solutions but they need some serious joined-up thinking - they can't be achieved casually.
I’m happy to bow to any wisdom imparted as , like I originally said, I’m totally uneducated on this subject and it’s probably proved by my struggling to comprehend how insulating a wall within the garage that has a warm room on the other side can become a colder surface regardless of which side of the wall you refer to. Surely an extra layer of insulation on the outside of any wall will make the space on the other side of it warmer, and , on the side the insulation is applied to , surely if it’s heated on it’s other side then insulating it on the colder side can only raise the temperature on that side too.

No doubt there are solutions and I’m hoping that they’ll become clearer before I commit to anything unwise 🤔
 

rogxwhit

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I thought you were trying to insulate the garage space? When you said walls similar to floor but bumped up, I thought you meant the (3?) external walls of the garage? And internal & external insul on external walls are completely different animals - external is worry-free, you just have to watch out for cold bridging but there's no condensation risk.

Now it sounds as if you're aiming to insulate the party wall & I'm puzzled why. Won't the garage be a heated space?
 

Seascaper

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Hello,
My advice would be to wait until you are in the house, then you can take at good look and evaluate it while you are there. Celotex is the common ceiling and roof insulation material, rock wool for the walls. It is important to allow ventilation in the form of space between whatever you use and the internal of external walls. To start your adventure make sure you install an electric heater on a timer to keep condensation away. You only need it on for a few minutes morning and evening but it will keep the air dry and prevent fine rust marks developing on any tools. You mention 19mm OSB, some of this nowadays is rubbish unless you get it from a reliable source, ie not the cheapest which can be full of voids. You may fine you can get much better 18 or 19mm exterior grade ply for very little more. Shop around, and don’t go for the cheapest is my advice, good luck with your project,
Regards
 

Terrytpot

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I thought you were trying to insulate the garage space? When you said walls similar to floor but bumped up, I thought you meant the (3?) external walls of the garage? And internal & external insul on external walls are completely different animals - external is worry-free, you just have to watch out for cold bridging but there's no condensation risk.

Now it sounds as if you're aiming to insulate the party wall & I'm puzzled why. Won't the garage be a heated
This all proves my lack of knowledge on the subject. I am aiming to insulate the garage space and was referring principally to the 3 external wall’s although the party wall was under consideration as swmbo won’t be overly pleased if the noise of any of my equipment is louder than the tv on the other side of the wall. Any form of exterior insulation is not being contemplated. As for heating, there’s currently a radiator just the other side of that wall under a rear window which we’re thinking of altering to bi-folds so it wouldn’t be much of a challenge to just resite it in the garage and leave it running throttled down on its trv.
 

rogxwhit

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Get an understanding of the terms temperature gradient, dew point, and cold bridge for starters. And consider the building holistically in a thermal sense.

You're talking about a floating floor - how do you stop moisture from vapour in the room air getting behind the boarding, at joints or round the room edges?

Condensation behind insulation, on its cold side, especially anywhere in the structure that isn't vented, is your enemy. Think of mould, rot, toadstools ... :-(

And I'm guessing that acoustic insulation might be a different kettle of fish to thermal insulation, except that sound travels through any gap just as heat does ...? But don't forget that you have to breathe ...
 

Hornbeam

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I assume you are trying to achieve a number of things
Make your workshop a bit warmer and more comfortable to work in
Reduce the risk of condensation and potential rust on your tools
Reduce noise transfer into the house

Noise transfer. You will need a dense acoustic rockwool or similar batt insulation. Lightweight cellotex wont have much effect. Also by insulating the garage/house wall you will effectively be making your workshop colder
General insulation. For the floor I would start with simple rubber tiles. Wikk make it feel warmer, nicer to walk on and kinder to any dropped chisels etc., Easiest to insulate the garage roof at ceiling level. Only issue is this creates cold roof space so not so good for storage of items. Use at least 50mm PIR or 100mm glass wool. For the wall use 50mm PIR with treated timber battens and OSB . There is a bit of a myth about cavities in this application. Brickwork is very slightly permeable and so will breathe out. OSB is almost airtight so there should mot be any issues
Your garage door will probably be the least insulated part of teh building and also have cold air leakage around teh edges
I assume most of your activities in teh workshop will be dry (youre not planning lots of seam bending etc)so a small amount of heat and/or a dehumidifier will reduce condensation risk. Keeping your tools in a closed cupboard will also help
 
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