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Garage Conversion - Insulation

  • 15-08-2022 11:22am
    Registered Users Posts: 992 ✭✭✭


    thinking of converting detached garage built in 2007 (<40 sq/m) to a habitable space with kitchen/bedroom for one of the older kids.

    fairly competent at most DIY once I’ve identified correct materials.

    my question relates to insulation methods. I can figure out U value stuff just not aware of correct method to prevent condensation etc.

    Garage is built with blocks below. Concrete floor is non insulated with radon barrier.

    Wall Insulation? (Insulated kingsman boards I guess but how to fix?)

    Roof rafter insulation? (Do you still need 50mm air gap these days if I fill rafters with modern semi rigid rolls?)

    Floor insulation? (Thinking of having non insulations just engineers floor boards).



  • Posts: 0 [Deleted User]

    I'd suggest 100mm insulated plasterboard on the external walls, and 40mm (or more if you can fit it in) floor insulation with plywood on top for the floor. Google "mushroom insulation fixing" for the mechanical fixings for the insulated plasterboard— just drill an 8mm hole into the concrete block & hammer them in. Put the plastic cap on, then it's just a matter of taping & jointing.

    If your garage is big enough, it would in my opinion be worth having 100mm (or more) rigid insulation on the walls, then batten it out for services behind plasterboard on the battens.

    I wouldn't skip the floor insulation— it's definitely worth it for comfort.

    Also, if you're getting new windows & doors in, make sure they have frame extenders as this will make it easier to put a significant amount of insulation on the reveals.

    For my garage conversion I'll be putting a new ceiling below the existing roof, with rigid insulation between the joists & insulated plasterboard below that. It'll result in a low ceiling, but it will also maintain the ventilation gap for the roof of the garage.

  • Registered Users Posts: 992 ✭✭✭Peppa Cig

    Thanks for your detailed response - appreciated.

    For external walls, should the walls be battened out before applying insulated plasterboard? (considering wall block type).

    Is a membrane (vapour membrane) required on the wall (before application of insulated plasterboard)?

    Isover G3 appears to allow full fill of rafters (as it is breathable) as an alternative to cutting rigid insulation and leaving 50mm air gap.

  • Registered Users Posts: 751 ✭✭✭C. Eastwood

    Do you have any photos of the inside of the garage.

    Your external walls are constructed with 225 mm Cavity Block Walls.

    Are the internal face of the Cavity Blocks clean or painted or plastered.

    You mentioned fitting insulation between the rafters. Are there any ceiling joists, and if yes, then fit the insulation between the Ceiling Joists and not between the rafters.

  • Posts: 0 [Deleted User]

    Assuming the wall is flat, insulated plasterboard can just be applied directly. It should have a vapour control layer in the plasterboard I think. The vapour barrier should always be on the warm side, to stop moist air getting to the cold side & condensing causing mould etc. A vapour barrier on the cold side traps moisture in exactly the place you don't want moisture.....

    Moisture should generally, insofar as possible, be kept out of roof spaces. So a vapour barrier (foil backed plasterboard or polythene etc) should be used regardless of insulation type. The roof space should also be ventilated.

    I'm not sure of the roof type & where the rafters are. If you're talking about insulating the rafters directly beneath the slates/tiles with no ventilation gap I'd consider that a bad idea. If it's the rafters which would be the celing (ie, the floor of the attic) it's probably fine.

  • Posts: 0 [Deleted User]

    If space is very tight then it's also worth noting that soft insulation has about 50% the insulation value as rigid foam insulation— so it would need to be twice as thick to retain the heat to the same degree.

    By your opening post you're converting your garage to a studio/one-bed for your kid. I'm guessing it'll be electrically heated, considering the giant pain of extending another form of heating to a separate building. Unless you don't like that particular offspring overmuch and/or you have no issue with a massive increase in your electricity bill, I'd suggest insulating the sh1t out of the place so they don't freeze in winter— in my own house, 100mm of rigid insulation on most of the walls (some spaces just couldn't accomodate that) plus 40mm floor insulation plus draughtproofing cut my oil consumption by over 50% while massively increasing my comfort levels. (Attic was already pretty well insulated)

    And don't forget air-tightness. A draft in the wrong place will undo at least half of the benefit of insulation in that space.

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  • Registered Users Posts: 992 ✭✭✭Peppa Cig

    Thanks for replies!

    Garage is actually c.7m x 9m.

    Photos following as requested

    Internal face of 225mm Cavity Blocks are clean.

    8” “ceiling” joists.

    upstairs roof rafters are 6”


  • Registered Users Posts: 992 ✭✭✭Peppa Cig

    Yep. Will need to be electric heating. Your right - ++++ on insulation 👍

    not sure in need to insulate “ceiling joists” given that “upstairs” will be habitable? (other than for soundproofing).

  • Posts: 0 [Deleted User]

    Unless I'm totally misinterpreting your photos there's a downstairs & an upstairs and you want to convert both? And the downstairs ceiling is regular height, ie 2.4 metres/ 8 foot? Height upstairs is obviously very limited.

    I personally would slap (at least) 100m rigid insulation around all of the downstairs walls and (at least) 50mm rigid insulation with 18mm OSB on top of all of the floors downstairs. I would then batten out the walls for services (mostly cabling I'd imagine) and build whatever studwork for new walls on top of the OSB (probably not best practice, but perfect is the enemy of good).

    As you have good access, I would maintain insulation coverage against the walls between the floor/ceiling joists, and bring that insulation across to meet the insulation upstairs.

    I think the dwarf wall upstairs would probably need more studs to function as the wall of a room. As there's loads of space behind for services, I'd go with 100mm plasterboard on the studs (making sure there's ventilation for the rafters) and have an (insulated) access door for either side.

    Hard to tell from the photos of the inside, but it looks like the roof might be sheet of some description? Possibly GRP? And therefore not itself breathable? I'd be even more nervous about obstructing the ventilation of the rafters if so, and would consider battening out the whole external walls of that space to make sure the rafters could breathe. (I think there's caps— essentially large washers— available for fixing the insulated plasterboard directly to studwork).

    My 2 cents for whatever it may be worth: insulate the crap out of everything, and make sure the timbers can breath.

    My garage is, sadly, significantly smaller than yours...

  • Registered Users Posts: 992 ✭✭✭Peppa Cig

    Thanks again for your input.

    Yes downstairs and upstairs with conversion of both. Upstairs has a plywood floor.

    Roof is slate with Tyvex membrane.

    Downstairs is a good 8ft to joists.

    ”upstairs” would be a bedroom etc. Height is c.7-8ft at highest down to 4-5ft at “dwarf wall” which will need studding out (looks lower in photo above).

    So 100mm foil backed insulation direct to external walls. Mechanically fixed. 50mm Batton on top fixed to block behind 100mm insulation for cabling.