Insulating garage roof

[iep]

I need to keep the temp in our garage above zero to prevent damage to some water pipes that we have installed during the summer.

To do this I'll be building a set of insulated garage doors and adding insulation to the roof. I'nm happy enough with the doors but but I don't know what insulation to use for the roof.

The roof is flat with joists positioned about 30cm apart. So, the simplest solution would just be to 'nae nails' some sheets of polystyrene to the roof between the joists. However, I can't help but worry that polystyrene is a bit of a fire risk and so might contravene some building regs etc.

Is there simple material that anyone might recomend for this job? I've noticed a product called celotex that seems to be used within roof spaces etc but it is twice the price of basic polystyrene sheets.

Cheers,

ip

 

[PrenticeBoyofDerry]

Polystyrene will have an effect on electrical cables, so should not be installed touching them.
Celotex and kingspan are much alike and or a very useful insulator are you boarding the ceiling and lagging the pipes.
Adequate ventilation is an essential part of insulation. A ventilation gap of 2 inches(50mm) between the insulation and the underside of the roof is required as standard. Ventilation can be provided either from side vents or roof vents. The roof vents should be sealed to prevent water ingress.

 

[iep]

There are a number of electrical cables running across the ceiling so I'll rule the polystyrene out on those grounds alone. I'm guessing that celotex and kingspan don't have this disadvantage?

Yes, all pipes will be lagged but I keep the washing machine in there too and already had the inlet valve fail during a cold night back in April. No choice really except the keep the room as warm as we can.

The plan is just to insulate the roof with boards and replace up and over aluminium door (ill fitting) with a set of insulated traditional doors.

From the sound of it though, I won't be able to attache the insulation boards straight onto the roofing boards. To meet the 2" air gap requirement I can add batons 2" below the roof and attach the boards to them? A number of side vents exist within the garage already (about 6 vented bricks). Do you think they would be adequate?

Thanks for the advice.

ip

 

[PrenticeBoyofDerry]

The vents will need to be above the roof insulation either side vents in your soffit boards or elevated roof vents, in the roof.
You can install the cellotex/kingspan etc by propping up by nails/screws, if needs be.
This type of insulation does not have the same effect on electric cables as polystyrene does, but with all insulation it does effect the current rating.
In lighting circuits it does not normal have an effect that would mean upgrading cable sizes, but it can in power circuits.
But providing the cables are not totally surround be insulation during any part of the route and ventilation is available, so heat can dissipate it should be okay.

 

[iep]

Great advice, thanks. No ring main cables will be close to the insulation so I'm not worried about the current rating beinmg affected. I will have to look into ventilation though as my vent brics are all at ground level.

Can you explain the requirement for the ventilation between the boards and roof? I don't doubt your advice, just keen to understand.

Cheers,

iep

 

[ajrobb]

The insulation that is safest for the flat roof structure is generally 'warm roof' insulation above the old roof. However, it is possible to add insulation below the roof:

Have a look at Kingspan K18 insulated plasterboard screwed to the bottom of the roof joists. The plasterboard should give 30 minutes fire protection. Prices vary, but maybe the 40mm insulation (52.5mm total thickness) works out cheapest/mm. All the joints should be taped and filled to give reasonable vapour proofing.

Another option is to put insulation between the joists but this restricts ventilation of the roof structure. As long as there is ventilation above the insulation, then a 50mm gap may be adequate above the insulation to keep the roof timbers dry. Mineral wool is much cheaper than Celotex or Kingspan. The mineral wool can be held in place between the joists with garden netting stapled to the joists. Plastic netting is OK if you then put up plasterboard.

If you use mineral wool, you should probably have a vapour barrier. This could be 2 coats of drywall sealer painted on the plasterboard after the joints have been taped and filled.

Whatever you plan to do, talk to your local building control (LBC).

 

[PrenticeBoyofDerry]

a minimum 50mm (2 inch) air gap is left between the insulation and the roof deck, with air vents installed
This gap will allow air-flow, so that warm, moist air, is allowed to escape, rather than coming in to contact with the 'cold deck' and condensing. Condensation can lead to; rotting decking and/or timbers, damp ceilings and damp insulation.

 

[iep]

Makes perfect sense. Thanks for explaining.

iep

 

[ajrobb]

There are a number of electrical cables running across the ceiling

Be careful adding thermal insulation around electric cables. It can reduce their maximum safe working current. Lighting cables are generally OK. Cooker and electric shower cables are a problem. Ring main cables might also be a problem.

 

[mointainwalker]

Since insulation doesn't create heat and will always lose heat ( at differing rates ), you may still need to think of heating if we have such prolonged cold spells as last winter.

Trace heating is a way to warm exposed pipes but don't know the cost very exactly ( guess 100 - 200 pnds ).

If you have no other need for an insulated garage, you may find not much cost difference between that and the insualtion, but with much less work and guaranteed not freezing even when very cold.

 

[ajrobb]

Since insulation doesn't create heat and will always lose heat ( at differing rates ), you may still need to think of heating if we have such prolonged cold spells as last winter.

Good point. I suppose it also depends on if there is a large uninsulated wall between the house and garage to provide heat.

 

[iep]

Good points. As it happens, there is a large single skin brick wall between the house and the garage (another reason to insulate) but I also have a greenhouse heater which I will switch on in winter (set to about 1 degree). This will use a bit of power but will still be cheaper than constantly repairing the washing machine etc.

Cheers,

iep