Garden office plans, any advice?

[Timmophoto]

I am just about to embark on a new project converting a large detached prefab garage with corrugated roof into a garden office/room.

These are the plans I have made after researching other's projects and asking advice. I would be very grateful if anyone could just say what is right and wrong about my plans and any advice would be appreciated

1: clean out garage, liquid dpm over the joins between panels and where Walls meet floor. This is just extra precaution as to date there is no sign of any leaks.

Put 1200 dpm over concrete floor with excess up the bottom of walls.

50mm celotex on dpm, tape and lay t&g chipboard flooring down on top, glueing edges. (do I need vapour barrier too?)

Batton Walls with 47mm x 75mm carcassing, giving 47mm depth. Cut 40mm celotex to fit between, giving a small amount of airspace between wall and insulation. (the panels are also scalloped).

Put timbers between the metal supports for the roof, and insulate with 50mm celotex between.

I will also be building a stud wall where garage door was to change to single door and a similar thing for a rear door and window. This will be on a dpc and be insulated too.

Get electrics put in, (do they route cable through insulation or in the small void behind?)

12.5mm plasterboard on Walls, 9.5mm on ceiling. Get it skimmed and fit electrics. Put up stud wall to create rear workshop.

Lay laminate down, or engineered if we can afford underfloor heating.

Batton over the pebble dash type exterior and fit vertical t&g cladding to walls and new exterior stud to make it look smarter and consistent.

I hope I am fairly close with my plans. Really keen to get it done right the first time to avoid future problems. I will be sealing joins with extra silicone where needed and re flashing roof edges too.

Any advice on my plans would be gratefully appreciated, thanks in advance.

 

[mointainwalker]

garage with corrugated roof

Steel or fibrocrete ?

50mm celotex on dpm, tape and lay t&g chipboard flooring down on top, glueing edges. (do I need vapour barrier too?)

No.

Batton Walls with 47mm x 75mm carcassing, giving 47mm depth. Cut 40mm celotex to fit between, giving a small amount of airspace between wall and insulation. (the panels are also scalloped).

You don't need the airspace but since you can't reasonably avoid it, your proposal is right. You do however need a vapour-barrier on the warm side of tyhe insulation.

Put timbers between the metal supports for the roof, and insulate with 50mm celotex between

You don't give available height but I would always favour fibre-wool if possible because of the cost-benefit ratio. Once again vapour-barrier on the warm side of the insulation.

 

[Timmophoto]

Thanks for the advice.

It is cement fibre roofing. I was under the impression it was best not to use roll insulation so there would be a good amount of ventilation above.

Great advice on the vapour barrier, will order that today. Does the plasterboard sit directly on top of the vapour barrier?

I am very limited on height so don't want to come lower than the roof supports however they are around 100mm deep so plenty of space for insulation.

 

[freddiemercurystwin]

Personally, for the little extra cost (in the grand scheme of things) I would double your insulation thicknesses except maybe the floor which I would say go up to about 80mm.

You can get foil backed insulation which, once taped with foil tape is your vapour barrier. Or alternatively you can get foil backed plasterboard instead.

Unless its is practicable to have an air gap between the insulation and the roof finish and this can be vented, when fitting your roof insulation push it hard up against the roof finish to minimise the chances of condensation occurring. With a corrugated roof you will end up with air pockets though where condensation could occur, this can't be avoided unless you get into venting the void which is easier said than done. Corrugated roofs do not lend themselves very well to this kind of conversion as you will end up with neither a cold or warm roof system but somewhere in between but for a conversion it will probably be OK. You may get problems long term though.

 

[Timmophoto]

I can't really afford to loose much more of the room size, especially height so may be restricted with regards to doubling up the insulation.

As far as the roof goes, when you say push the insulation up against the roof sheets, do you mean rockwool or kingspan? I was under the impression that is I used kingspan that it would leave a void above which would act as ventilation, i.e cold roof system? Will air not surely be able to vent under the profile of the corrugations?

On the walls I could pack some rockwool behind the 40mm kingspan as there are 30mm recesses in each concrete wall panel. If I do this and the insulation is touching the wall, should I put a barrier on the wall first? Or is it best to just stick to the 40mm kingspan with a small void between?

 

[freddiemercurystwin]

You can only fit what insulation the space allows so up to you of course.

Regarding the roof a cold roof system has a ventilated void above the insulation but will only enable condensation to escape if the voids are vented to the outside (you need to have a continuous gap between the insulation and the roof finish not just the voids where the corrugations occur). Unless the design of the existing roof make up enables these voids to be ventilated already (which it may do?) you will have to introduce ventilation to the voids. Alternatively if retrospectively fitting ventilation at the eaves is too much effort (its only a rough conversion after all) it wouldn't be a bad idea to fit a thin layer (50mm) of rockwool up against the roof sheets and squash it up tight so that it fills the corrugations with your rigid insulation. This would kind of be a warm roof although not entirely as the joists will still allow a cold bridge to occur between the inside and the outside so a fairly minor possibly of condensation occurring on the joist positions under the plasterboard.

A void in the walls is not so much of an issue as hot air rises (obviously) so the roof is more prone to problems.

 

[Timmophoto]

Thanks for the advice.

The roof would end up vented at both ends as the end of the corrugated sheets are not sealed off. i.e you can currently see through to outside where the roof sheets sit on end walls. Would this therefore provide adequate venting? I put some sort of timber fascia on one end but would not seal it off completely. I also may need to mesh the ends to prevent pests setting up home in there.

On a insulation query, budget leaves me with 50 or 60mm kingspan for the ceiling. Will it be of any benefit to use 100 mm polystyrene instead or is that approx the same result anyway. Or would it be worth putting a 40mm of polystyrene on top of the kingspan?

 

[mointainwalker]

To minimise cost and use max insulating space, I would fit polystyrene sheet against the roof so the the corrugations remain open and fully vented and then fill the space beneath with fibre-wool followed by v-b.

100 mm of this will give the same insulating efficiency as 60 mm Kingspan and be a lot cheaper.

 

[freddiemercurystwin]

OK that's good, so leave a continuous ventilation gap between the insulation and the roof sheet of say 40mm which will leave you with a 60mm gap for the Kingspan. If budget and space allowed I would fit a continuous layer of your polystyrene between the underside of the joists and the plasterboard ensuring the cold bridges are minimised.

 

[Timmophoto]

That sounds fairly straightforward and any money saving is always appreciated! Do I need to put vapour barrier between fibre wool and plasterboard?

How about above polystyrene? (this would be tricky as there are the roof supports and fixings to work around)

 

[mointainwalker]

You must have a vapour-barrier unless you actually desire to promote condensation and mould. It can be a plastic sheet or foil-backed p-b with foil-taped joints.

 

[Timmophoto]

Sorry for all the questions!

Is the vapour barrier going between insulation and plasterboard or right at the top by the roof sheets?

 

[Deluks]

I would leave an airflow gap beneath the corrugated sheets, and vent the corrugations at the eaves, albeit with insect mesh over the gaps. Or build a false soffit with proprietary vents fitted.

The cement/asbestos type boards are somewhat breathable, I didn't used to have a problem with condensation in my garage until I painted the underside with bitumen paint. Doh!

The bigger problem as I see it is applying cladding to the external walls, you then reduce the roof overhang which will increase the amount of rainwater that runs down the exterior.

I would run leccy internally in conduit/trunking.

 

[mointainwalker]

Is the vapour barrier going between insulation and plasterboard or right at the top by the roof sheets?

Once again vapour-barrier on the warm side of the insulation.

 

[Timmophoto]

The cladding would be as flush to the walls as possible using thin battens. I have found some L shape capping which will overlap the edge of the roof and the top few inches of cladding, this would run the full length of the roof.

As far as the condensation, we never have any problems at the moment and from what I read they do not suffer as badly as the metal or onduline ones.

Not being too clues up on terminology, what is conduit/trunking? Does this allow it to pass through the 40 mm kingspan from the behind through to the plasterboard?

I take it down lighters are out of the question? Was hoping that perhaps low voltage ones might be ok if there was a big enough gap of insulation around them.