Garage Conversion- Ceiling Insulation

01-11-2022 12:29pm #1

I'm converting my garage at the moment. Standard semi-D, with the garage attached which is a little over 8 foot wide and 22 foot long. I'l be insulating floors & walls & ceilings. I'm doing the work myself because I don't have €20k or so lying around to pay someone else to do it for me. I want the space as warm & airtight as possible.

The roof slopes front to back, and I'll be framing a new ceiling beneath the existing roof. I obviously need to leave room for ventilation above this, which will restrict where I can put insulation. Also, I'll have to do everything from below, and the roof is already a little low at the back. I'm going to put insulation between the joists for the new ceiling obviously, but I was also looking at using insulated plasterboard attached to the ceiling joists beneath that.

Can people please tell me the problems (I'm sure there are lots) with this approach? And how to get around them?

02-11-2022 4:02pm

Anyone?

Gusser09 · 02-11-2022 4:07pm

I'd be looking for a professional to do it to be honest. 20k isn't of money these days and would be well worth it for a job done properly. It will only seem expensive the day you pay for it.

Ask a plaster who has done this before what his labour only price is. This is going to be on show as the finish so you want it done properly.

02-11-2022 4:19pm

https://www.boards.ie/discussion/comment/119824886#Comment_119824886

Q: "How do I do this particular part of this job myself"

A: "Pay someone else €20k to do the entire job for you. Sure €20k isn't nothing..."

That answer was actually worse than no answer at all. Do me a favour and hit the "ignore" button and feck off.

I posted this in the DIY forum for a reason. One of those reasons is that I've seen more than enough sh1tty work done by so-called "professionals" that at this stage I'd much rather make my own mistakes rather than pay someone else a lump of cash to make them for me. A significant part of the conversion will be redoing work which was messed up by the first AND second "professional" to have a go at it. I have no plans to give a third gouging fcukwit the opportunity to charge thousands to mess it up yet again.

Gusser09 · 02-11-2022 4:23pm

https://www.boards.ie/discussion/comment/119824936#Comment_119824936

Well go ahead and do it then. If you have to come onto a forum for a job like this looking for advice please post up pictures of the end result. Should make for a good laugh.

03-11-2022 9:29am

https://www.boards.ie/discussion/comment/119824945#Comment_119824945

Well thank you Mr Helpful. It must be nice to know everything there is to know and to have never made a mistake in your life as well being able to drop €20k. You are obviously a god to whom we should all bow, because all the people I've met can be divided into (A) those who know they make mistakes and don't know everything and (B) idiots who think they know everything but obviously don't and make mistakes all the time but never admit it. Also, WTF do you think people come onto a DIY forum for? And did you even notice what the question I actually asked was?

"I was also looking at using insulated plasterboard attached to the ceiling joists beneath that.

Can people please tell me the problems (I'm sure there are lots) with this approach? And how to get around them?"

Do any non-idiots have an answer to the actual question asked?

GreeBo · 03-11-2022 10:01am

Id start by sharing some pictured of what its like now OP as its hard for everyone else to imagine what you are talking about regarding the roof.

awec · 03-11-2022 10:43am

You should probably also specify the intended use of this converted garage as that may impact on the approach.

03-11-2022 11:14am

https://www.boards.ie/discussion/comment/119827602#Comment_119827602

The in-laws are from outside Ireland, so for a few weeks a year it will need to function as a granny flat. For the rest of the year it's likely to function as a home office. A downstairs bathroom is essential in the long-term (and there's nowhere else to put one) and ideally there'll be a kitchenette (for both the in-laws when visiting and for whoever is working there). Plan is for extractor ventilation from the bathroom for sure (it currently relies on a window which is directly over the oil burner for ventilation....), and from the kitchenette if there's going to be one there.

The main concern is maximising insulation while ensuring adequate ventilation to the roof timbers. The partially converted area is way too cold to work in, as there's a gale blowing through and no insulation (or tanking in the bathroom, or taping of plasterboard joints— see previous comments on "professionals"....).

Will do up a diagram or two & post. Photos are pretty useless TBH, it's impossible to get any angles which actually show anything relevant.

03-11-2022 11:25am

20221103_112243.jpg

Rough overhead view, not to scale.

03-11-2022 11:27am

If you are caught for head space under the rafters it would make sense just slab the ceiling and insulate in the attic space above, if you have the space for insulated slab you can do both to reduce heat loss.

03-11-2022 11:42am

Side view, not to scale.

20221103_112333.jpg

The existing roof has joists going side to side every 1.2 metres or so from the main house to the garage part wall. At the lower end at the back, the height is 2.3 metres. At the front it's 2.5 metres. Width is about 2.5metres and length 6.5 metres. The new ceiling will be framed horizontally just below this to allow adequate ventilation. The step in the new ceiling is to give a bit more height towards the front.

Party wall is going to be 102.5mm plasterboard backed insulation. Wall with the house will be 62.5mm plasterboard backed insulation. Floor insulation will be 40mm plus 9mm OSB. New ceiling will use 70mm x 35mm timbers.

50mm on the floor plus 70mm for the timbers for the ceiling leave a max of 2.18 metres height at the back and 2.38 metres at the front.

70 mm insulation between the timbers of the new ceiling isn't going to do a whole lot, and the ceiling is going to be low no matter what I do. Which is where I came to my bright idea of "Hey, what are the problems with putting insulated plasterboard on the ceiling as well? Equally importantly, what's the best way to address those problems?"

monseiur · 03-11-2022 11:42am

Firstly let me say that I admire someone who has the gumption to take on DIY jobs and learn as the work proceeds. The key is to do as much research as possible at the start and get advice from lads in the trade who work at it day in day out. Don't worry about making the odd mistake - it's all part of the learning process.

The first thing to sort out is the electrics - location of sockets, lights, switches, data cables etc. etc.

Fix 50mm insulated plaster board to walls with appropriate mushroom fixings. You basically drill thru' plaster board & wall with right size masonry bit and tap in the fixings. The total thickness of plaster board will be 62.5mm - 12.5mm board + 50mm insulation which basically mean that your overall internal width will reduce by 5 inches. Skim joints and fixing heads.

Regarding ceiling you have two options 1. Fit standard attic insulation between joists, fix 12.5mm plaster board and skim joints or 2. Fix insulated plasterboard to joists using appropriate fixings I would go with option 1 and use Rockwool or similar. Re. existing floor - do you plan to dig it up and fit underfloor insulation under new floor ? If you leave in in situ and put new insulated floor over it you will obviously reduce the floor to ceiling height.

As the total length of garage is 22 feet consider putting a partition in the middle to give two separate rooms and fit plaster board directly on existing roof timber on the back room where is ceiling is lowest, the slope will hardly be noticeable. On the front room fit firring timbers on existing joists - for a 'level' ceiling and availing of maximum height available.

Although skimming of plaster board is not rocket science it's best to get a plasterer in for a day as dips, bumps and rough trowel work will show after painting.

03-11-2022 11:44am

https://www.boards.ie/discussion/comment/119827802#Comment_119827802

There's no attic space, that's the problem. I have to do everything from below.

03-11-2022 11:51am

You could put the insulation into the attic space prior to slabbing just use some tape and staples to support it until the slab goes on.

03-11-2022 11:54am

Monseiur, what would be appropriate fixings for fixing plasterboard backed insulation to the ceiling joists?

I see where you're coming from on fixing plasterboard direct to the existing joists, but the problem is that because they go side to side that would block the flow of air for ventilating them. Although possibly cross battening them with 2x4 and fixing plasterboard backed insulation to the battens could address that problem.

There is no possible way of doing this part of the job perfectly without ripping the existing roof off & rebuilding it. It just needs to be good enough!

I have every intention of getting a plasterer into skim the garage...eventually....

GreeBo · 03-11-2022 12:56pm

https://www.boards.ie/discussion/comment/119827943#Comment_119827943

Can you take a picture of whats there at the moment?

I'm not at all following your issue with existing "joists"....

The roof should be a cold roof with ventilation above the rafters, insulation between and below the rafters.

I would fit PIR boards to floor, walls and ceiling, with the walls battened out. Then vapour barrier over the lot with all seams taped. Then fit plasterboard to the battens/rafters as normal

If you just fit insulated warm boards directly to the wall you have potential problems for damp at every joint and mushroom.

I wouldnt bother trying for a level ceiling, just follow the rafters.

Is there an existing height change between the garage floor and the rest of the house?

A floating floor should be fine, so just PIR boards onto the concrete, then VB and then T&G OSB on top. You can carpet etc over this as required.

Your roof is probably not currently vented as its an exterior space, what are your plans here? You need an opening at the bottom (soffit?) and something higher up near the wall of the house (usually in the tiles) You dont need the depth in your drawings for ventilation, you just need airflow, hence my comment about just following the roofline with your celiing.

What are the plans for heating? With all your insulation you are isolating the garage from the heat of the main house, so it wont be warm by itself.

Is all the wiring and plumbing already in situ? If you are adding a toilet then tieing in the waste is not a small job. If you are adding a kitchen then you will need more than just a couple of sockets. You will also probably want some extractor fans.

Is the plan to have people sleeping in here?

03-11-2022 2:47pm

Thanks GreeBo, that's fairly comprehensive. And a good point about warmboard— I'm not a fan of insulation backed plasterboard myself, but have used it reluctantly on a few walls of my house where the space was tight enough that the only real alternative was no insulation.

Most of the wiring & plumbing is already there fortunately, so it'll be a case of re-routing rather than installing. The central heating pipes run through the garage and there's actually a couple of radiators already there but they're turned off (because why bother trying to heat an uninsulated space with a breeze blowing through it....the "professionals" again...). So once there's reasonable airtightness, I'll relocate them somewhere more sensible.

Yeah, extractor fans are a must. I'd like a micro MVHR in a perfect world but I'm not currently seeing where an intake could be located.

For ventilation of the roof timbers, the timbers themselves go side to side, but the airflow is front to back. The roofing sheets are directly onto the timbers. So fixing a sheet directly to the timbers would create multiple unvented spaces between the roof & the sheets bottom & top, walls side to side, and timbers front & back. There is a ventilation gap at the back thankfully, and I'm leaving one at the front also.

It'll probably become the guest bedroom (a cat cannot be swung in the current guest bedroom)— so at minimum it needs to be good enough for someone to use as an en-suite bedroom for a couple of weeks at a time: ie, warm and with fresh air but without drafts. Most of the time it's probably going to be the home-office for whichever of us wins the coin-toss.

03-11-2022 2:57pm

https://www.boards.ie/discussion/comment/119828216#Comment_119828216

Specific questions:

Does the foil on the PIR not act as a vapour barrier? (Taped over the mushroom fixings & the joints obviously). Is it recommended to put a vapour barrier on top anyway? I'm mindful that the first thing I would do after placing a separate vapour barrier it would be puncture it repeatedly to fix the battens to the wall. (I also though insulated plasterboard had inbuilt VB??).

Noted re PIR on ceiling etc (a good idea which hadn't occurred to me, thanks). As a matter of practicality, how would that be battened out for the plasterboard to be fixed to? Would it be possible to (say) batten the roof timbers to ensure a path for them to be ventilated, wedge the PIR in place, tape the PIR joints, and then use long plasterboard screws to fix the PB to the battens underneath (/above) the PIR on the ceiling?

Should I put a DPM beneath the PIR on the floor? (assuming yes & planning for this).

On the one hand, I want to maintain an unbroken VB, on the other hand I also want to put multiple holes in my VB so the plasterboard doesn't fall down....

Thanks.

awec · 03-11-2022 3:45pm

In your top down drawing, is the wall labeled "Party wall" at the top of the drawing the low point of the roof? And you are saying the rafters run parallel to, rather than perpendicular to this wall?

You also say the distance from "main house" to "party wall" is about 2.5 metres, with rafters every 1.2 metres (i.e. 3 rafters total) ?

Am I picking this up right?

03-11-2022 4:15pm

https://www.boards.ie/discussion/comment/119829167#Comment_119829167

No, the low point is to the left of that diagram (where the back door is). The roof slopes front to back. The party wall is with the neighbour's garage (they share a roof) and the roof timbers are horizontal, perpendicular to the party wall, on top of the party wall & set into the wall of the main house.

It was built in the early 1980s— building regulations didn't exist at that time, and there were no building bye laws in the county either.

awec · 03-11-2022 4:19pm

I'm not an expert but 1200m spacing sounds like a lot for roof rafters when you're going to be adding more weight to them, but I'm sure more knowledgable people on here will correct me if I'm wrong.

03-11-2022 6:30pm

https://www.boards.ie/discussion/comment/119829315#Comment_119829315

Fair point. I actually measured them (I'm very bad at estimating distances!) and they're actually at 15cm x 5cm at 90cm centres. For the weight that will be on them, they should be OK, but I'll use the battens on the walls to give them extra support as well I think.

standardg60 · 03-11-2022 8:14pm

I'd put in more rafters between the existing to start with, they really should be 400-450mm distances, plus it makes it easier to install standard rockwool between them. To hold it in place 2x1.5 battens between the rafters flush with the bottom of them at 1ft intervals.

Then run same size battens front to back underneath the rafters to create a ventilation run, attach vapour barrier and standard plasterboard, i wouldn't bother with insulated to maintain the ceiling height.

Murph85 · 03-11-2022 8:20pm

What way are you heating the room?

whizbang · 03-11-2022 10:33pm

I dont get the issue with PIR backed Plasterboard. Its far superior in every way to most anything else. You dont need to batten in your case either.

You mentioned 70mm in the ceiling - thats plenty enough for PIR. you need 300mm rockwool (without any airflow over it) to beat that. And still need a vapour barrier.

Stay away from the existing roofing. Modifying the existing roof in any way is digging a big hole in a minefield.

Frame ceiling in 2x4, Only fit PIR between the joists if you are confident you can do it airtight. Tape under along the joists, then 37.5mm insulated plasterboard under. Very handy way to do it; precisly cut each 1200 slab down the middle, thats your 600mm spacing between joists. 2400 length should cover the width of the ceiling with 1 noggin probably enough if the PIR is tight.

With 40mmPIR on the walls, a good fart will keep the place warm. But the detailing will be the making of it. ;)

good luck and enjoy.

GreeBo · 04-11-2022 9:43am

https://www.boards.ie/discussion/comment/119830875#Comment_119830875

The problem is that you dont have an unbroken vapour retarder, its broken at both side, top and bottom of each board. The mushrooms dont help either!

Also, if using foil backed your plasterboard installation needs to be very neat and tidy, otherwise you have thermal and vapour breaks all over the place.

If you separate them layers out (insulation, VB, Plasterboard) then you can give the required attention to detail for each:

the insulation can have joins all over the place as long as they are close. The VB can be taped to remove any joints, and the plasterboard is purely for aesthetics and can be filled/skimmed to make good.

GreeBo · 04-11-2022 9:55am

https://www.boards.ie/discussion/comment/119828944#Comment_119828944

Problem is you cant use a barrier tape, you will be relying on plasterboard joint tape/compound.

The battens go onto the wall first, then insulate between, then VB over the lot, then fix the plasterboard to the battens.

You can use a thinner insulated board here if you want to avoid some of the thermal bridging from the battens.

The VB should be continuous, so floor taped to all walls and all walls taped to the ceiling.

For your floor I would be using 18mm OSB rather than 9mm, forgot to say that earlier.

If your roof timbers are 900mm apart then I would consider cross battening at 400mm so you can fit the plasterboard. however this will leave a void unless you also fill this with PIR boards (and having them perpendicular to the layer thats between the rafters is a nice detail anyway)

You have options regarding the ventilation of the roof, something as simple as using a hole saw to create a continuous air passage would also work, youd probably want a hole every 100mm or so. Good example of this in this video: https://youtu.be/LL2-8RXSYKc?t=216

You can get self sealing tapes that are used over each batten that effectively seal around each plasterboard screw, but tbh its probably overkill.

Also put it this way, if you are worried about screw holes, how many screw holes would make up the each edge of a warmboard panel.... :)

04-11-2022 10:55am

Thanks. GreeBo.

Current updated plan is to cross-batten ceiling & place PIR & vapour barrier against the battens (need the void for ventilation) and most likely use long plasterboard screws through the PIR to the battens to fix the PB.

GreeBo · 04-11-2022 12:05pm

If it was me I wouldnt lose that much ceiling by using 2x4 battens, you don't need that size timber to hold plasterboard, its going to have 30+ screws in it.

I'd drill ventilation holes in the existing rafters to allow cross ventilation and then use roof tile battens for your plasterboard fixings (1x2 or similar).

However you need to fill this void (at least at the ends) so I would recommend 25mm PIR.

See here for guides on whats permissible when drilling joists.

https://www.labc.co.uk/news/how-get-it-right-notches-holes-solid-timber-joists

You don't need a millions holes, your suspended wooden floor only has a couple of vents at each side.

If you are still going to insulate under the existing timbers, then insulate first, then batten, then VB and then plasterboard. You dont need to batten between the PIR, just go over it.

Garage Conversion- Ceiling Insulation

Comments