New build garage flat roof insulation options

[Adam2]

We're about to build the garage - hard to believe I actually typed that!!!

 

Originally I wasn't going to insulate it as it will never be used for much that needs keeping warm - it's attached to the side of the house with an access door in between. But, I figured maybe look at the cost options anyway. The roof is 225mm deep joists, dimensions about 6m x 6m. Final layer is waterproof single ply membrane.  Walls are mainly cavity which will have 70mm full fill glass fibre  insulation.

 

For the roof I figure I can go with:

 

Cheap glassfibre stuff maybe 100mm between the joists with a gap above and form some ventilation holes to exterior - not sure how many I'd need? Beneath this - would this be suitable for the VCL? Do I just staple this into the joists which will then support the insulation (well maybe they will friction fit) until I install plasterboard ceiling? Then above the joists just some ply and the single ply (installed by more competent others)

Knauf Earthwool 100mm (R=2.25)  £1.57/m * 36 = £56 (or I could go crazy and have 150mm)

 

 

Alternative of a warm roof - install my ply deck, install maybe 50mm PIR which needs to be tissue backed so not sure on cost (or could lay ply over foil backed PIR if cheaper) then single-ply membrane

Recticel PIR (foil) (R=2.25) £7.20/m *36 = £403

 

I know a warm roof is generally preferred but unless I messed up with the numbers it will save me a hell of a lot having a cold roof construction in the garage 

 

 

The main garage door will be insulated but fully appreciate the temp will not be amazing just trying to stop it getting mega cold. The heat source in there will be an MVHR - I was planning to make an insulated box for it but if the garage is insulated maybe will be beneficial to let the small amount of heat it generates go into the garage? Or maybe it's best being kept close to  the MVHR to aid heat recovery efficiency?

 

Also wondering now what to do with finishing the walls - S&C plaster or line with plasterboard with a VCL behind it.... too many things to think about and it's only the garage ? Outside walls will be rendered if that matters.

 

 

[Adam2]

Appreciate comments on the above to make sure I'm not overlooking anything

[Nickfromwales]

 

On 23/09/2021 at 20:48, Adam2 said:

The heat source in there will be an MVHR

Ermmmm, no it won't. Explain?

[Iceverge]

For what it is, you're never going to have an exceptionally well insulated garage. The garage door will not be anything near airtight so it'll never be warm. I assume you have no floor insulation?

 

I see your main aim as keeping condensation at bay and providing some level of frost protection. 

 

For the roof make sure you build something that won't rot. 

Warm roofs are generally safer in this regard but cold ones can work too provided you don't trap moisture in the structure. 

 

In your case I would build:

 

Roof final covering

Ply

Roof furring strips for slope

50mm min gap between joists above the insulation. Every single cavity well ventilated to the outside via the eves Ideally at opposite sides of the building. Include some insect mesh here to keep the creepys out. 

150mm earthwool push fitted between the joists .  It's very cheap and more is no harm as you'll loose some of the insulative value due to thermal bypass.  You can use netting or waste plastic strapping stapled to hold it in place.

 

Plasterboard taped and skimmed at the joints. Use a flexible sealant to join it to the wall and seal the few penetrations in the plasterboard well. This will be enough to prevent bulk air movement taking moisture into your rafters.

 

I would fit through the wall ventilation in the wall opposite the garage door to ensure the humidity didn't rise to unacceptable levels inside. Put them somewhere they won't get obstructed. 

 

As for the MVHR. Best practice is to have it within the thermal envelope of the house. If this is not an option I would build an insulated box for it to at least the same standard as the house.  Make it accessible for servicing. Don't forget about the condensate drain and consider that this will freeze if not protected. 

 

For the walls, I would lean toward S&C. More airtight, firmer to fix into, probably just as cheap as you're rendering externally anyway. 

 

If you use plasterboard I would omit the vapour barrier. It will cause more problems than solve them. It'll be full of holes anyway as you'll have to mechanically fix through it. 

Edited September 27, 2021 by Iceverge

[Adam2]

15 hours ago, Nickfromwales said:

 

Ermmmm, no it won't. Explain?

Just meaning no other planned appliances etc in there like a fridge that may generate heat. Figured the MVHR may, through the fans, create some small amount of heat that would be constant so may influence the environment a little - not thinking it will "heat" the garage so to speak 

[ETC]

Warm roof all the way. No need for ventilation.

[Mr Punter]

1 hour ago, Adam2 said:

Just meaning no other planned appliances etc in there like a fridge that may generate heat. Figured the MVHR may, through the fans, create some small amount of heat that would be constant so may influence the environment a little - not thinking it will "heat" the garage so to speak 

 

Forget the MVHR in the garage.  A complete waste of ducting and the system.

 

I had an insulated roof put on an unheated garage a couple of years ago and it stopped all condensation.  They came in large plastisol sandwich sheets with 40mm pir.  Single garage was £1600 plus VAT all in including removal and disposal of the old asbestos.  Mine came from these people http://www.icroofing.co.uk/

 

There must be someone similar down your way.

[Adam2]

Many thanks @Iceverge v helpful will be following that path then.

 

I had planed a VCL beneath the insulation - think you may have possibly missed that on the message above

 

14 hours ago, Iceverge said:

Every single cavity well ventilated

The joists are all posis with metal webs so that will help I think with allowing flow across the width of the garage.

 

I can put a full length soffit vent along both the front (above the main door and the top edge of the flay roof) and the rear or separate vent grills but the full length may look a bit better.

 

 

[Adam2]

20 minutes ago, Mr Punter said:

Forget the MVHR in the garage.  A complete waste of ducting and the system.

The ducting isn't a cost issue I have more than I will ever need thanks to a recent auction ? 

 

If the box is well insulated why is it a waste of time as you still get the air filtration though accept heat recovery may be less efficient?

[Mr Punter]

MVHR is all about ventilation - providing fresh air to occupants and extracting stale moist air.  I don't see that this is appropriate for a garage.  You will also need to run your unit harder to get it to cover the garage and if the garage is not heated you will need to have the ducting insulated.

[Adam2]

1 minute ago, Mr Punter said:

MVHR is all about ventilation - providing fresh air to occupants and extracting stale moist air.  I don't see that this is appropriate for a garage.  You will also need to run your unit harder to get it to cover the garage and if the garage is not heated you will need to have the ducting insulated.

Ahh I understand - no this is about placing the MVHR in the garage nit using it to provide fresh air to the garage - that should make more sense now. TBH - MVHR in this thread is really a red-herring as only mentioned in passing as the only thing in there that may generate any heat which will probably be lost through unwanted ventilation anyway

[Iceverge]

1 hour ago, Adam2 said:

 

I had planed a VCL beneath the insulation - think you may have possibly missed that on the message above

 

I considered it but decided you didn't need it. It's unlikely your VCL will be perfect in any case. Certainly not if you plan to fix plasterboard directly up underneath it as guys can play fast and loose with the screw gun. You're unlikely to have many penetrations in the plasterboard also. I'd save myself the hassle, sealant and paint will be fine.