Insulation in an almost flat roof

[ShaneE]

Hi all,

I am new to this forum and I did a search for an answer to my question but I was unable to find one.

I have a house that has an almost flat tin roof, with a pitch of about 5 degrees. The internal side of the roof matches the pitch.

The roof cavity is about 150mm and currently contains the foil type insulation. The problem is the insulation is practically ineffective, as it is regularly sub 5c in the early hours of the morning when I get up for work. As you can imagine, this makes it even harder to get out of the warm bed.

My question is, can the standard R3.5 insulation batts be installed into a roof that has a cavity space of ~150mm?

Thanks in advance for any replies.

 

[gecho1]

yes can be done

You don't mention where you live, i am assuming in one of our southern states.
You will need to lift the iron, and the sislation (Foil) lay wool batts down between the rafters. Replace the foil as this is more for condensation than insulation and replace the iron.
You will need fairly basic tools but a cordless screwdriver would be an advantage
Hope this helps somewhat

 

[ShaneE]

I live in Victoria, Near Geelong.

So put the foil on top of the wool batts?

I noted that R3.5 batts are 160mm thick, should they be put into a ceiling cavity of 150mm?

 

[moyjos]

what about polystyrene sheets??
The warehouse I work at has a large room that needs to be kept at 18 degrees +/- 2 degrees.

It is just clad in 50mm polystryene sheets and it just needs a reular small a/c unit to keep it at temp.

Cheers
Sue

 

[ShaneE]

what about polystyrene sheets??
The warehouse I work at has a large room that needs to be kept at 18 degrees +/- 2 degrees.

It is just clad in 50mm polystryene sheets and it just needs a reular small a/c unit to keep it at temp.

Cheers
Sue

Probably not all the suitable for a domestic home. It would get rather hot up in the roof cavity and I would suspect polystyrene would melt in those temperatures.

 

[depreciator]

Hi Shane,
I keep toying with the notion of putting insulation in my roof, but it never gets hot or cold enough in Sydney for me to make the effort. If I lived where you live, I would do it.
This is just what I've gleaned from talking to people:
1. Yep, you take the sheets off the roof and roll back the foil (it's called sarking).
2. Then you lay the batts on the ceiling. Easy.
3. Leave a bit of space around around the light fixture. (And if you have downlights, keep the transformers clear.
4. Batts come in two widths, so make sure you measure the space between the rafters before you buy the insulation.
5. Now, I've heard it's best to keep an air gap between the underside of the sarking and the batt to stop direct transfer of heat. So the 3.5 batts may be too thick.
6. Polystyrene wouldn't melt, but it would need to fit snugly against the roofing timbers so there would be lots of mucking around.

Insulation is cheap. You'll find it on Ebay, too.

If I didn't need to hire a long ladder to get onto my roof, I would have done it years ago.

Scott

 

[harb]

Probably not all the suitable for a domestic home. It would get rather hot up in the roof cavity and I would suspect polystyrene would melt in those temperatures.

Not unless your place was on fire.
The normal polystyrene is the cheapest and cleanest solution if you DIY, you can also look at fire resistant polystyrene, polyurethane or rockwool as an alternative to bats. Personally I'd go for the 50mm polystyrene which would have twice the R value of the fiberglass batts and a can or two of EPS foam for the fiddly bits around pipes and such.


cheers

 

[moyjos]

I don't think heat would be a problem.
Caravan annexes are made of polystyrene with metal cladding.
My inlaw's patio roof is also metal top-polystryene-white"stuff' underneath.

Cheers
Sue

 

[geoff stower]

Insulation

Hi
The requirements in QLd now is batts for walls and ceilings.
The certifier has requested us to install batts in roof (no.3 ) in our IP reno house.

More cost but I suppose it will work out in the end. The interest rates are on the way down .

regards
Geoff

 

[craigb]

The R value in all insulation stands for resistance, it shows the resistance to air/heat flow, all cavitys ie roof and walls should have some sort of air flow.
This will allow you to retain heat in winter, and resist heat entering in summer, if you fill the cavity with polystyrene, and eps foam the roof may collect moisture, and start dry rot, and one day you may look up in your warm bed and see things on your ceiling that just should not be poking out of the plaster.
If you require to do this on the cheap, go and remove the old batts from a demolition, people rearly remove, although very yucky job.
Remove the old nails/screws from the roof, and place the batts, replacing the screws as you go.
cheers craigb

 

[harb]

R is for the resistance to heat transfer. Air is an insulator. ;-)
A 50mm polystyrene sheet would still leave 100mm air gap between itself and the metal roof so even if somehow the cavity could be air sealed condensation/moisture would not be a problem unless the tin roof leaks inside. The advantage of polystyrene over batts is that it will not absorb and hold moisture/water. The EPS foam will seal around the fiddly bits so no warm air from inside the room could transfer into the cavity space and come into contact with the cold sheet on the roof. Without warm air hitting the cold tin roof there is no condensation forming under the roof. With high pitched roofs you can get away with condensation forming under the tin because it will run downhill and outside anyway. With flat and low pitched roofs it gets tricky as some of the condensation will just drip down onto the insulation. If you prefer using batts then better make them full foil faced and seal the joints with foil tape to stop the water accumulating inside the batts. Not only are they very slow to dry but also act like a sponge and will bring down the ceiling. Better still, use a foil lined roofing blanket as well as the normal batts.

cheers

 

[ShaneE]

Thanks for all your comments Guys

Getting mixed responses though, one idea I came up with was to put in a 'false roof', which will effectively make the internal roof flat. So build a frame under the existing roof to make it flat inside, and install the insulation on that.

What do people think of that idea?

 

[depreciator]

You mean a false ceiling?
Alot of mucking around.
Unless you don't like the look of the sloping ceiling inside.
You'll still need to put insulation in the new ceiling anyway.
I reckon taking off the roof sheets and bunging in some insulation would be the cheapest and easiest way to get the result you want i.e. keep the place warmer.

 

[ShaneE]

You mean a false ceiling?
Alot of mucking around.
Unless you don't like the look of the sloping ceiling inside.
You'll still need to put insulation in the new ceiling anyway.
I reckon taking off the roof sheets and bunging in some insulation would be the cheapest and easiest way to get the result you want i.e. keep the place warmer.

I was hoping it would be as simple as that, but from what I read above and from what other people have been saying, there might not be enough room in the ceiling cavity to put R3.5 batts in (150mm cavity and r3.5 batts are 160mm thick), also moisture will become an issue because of the lack of ventilation.

The sloping ceiling inside has potential, but the way it was built, it looks fairly ugly.

 

[depreciator]

I would have thought condensation would collect on the underside of the tin. The foil blanket (if it is in good condition) should capture this and take it away to the eaves. Though if there isn't much fall it may pool in places.
Go with harb's suggestion: a foil lined roofing blanket. Maybe then you can just use 2.5 batts.
Or given it's spring, maybe you can put it off till next year.

 

[ShaneE]

Well, summer is worse than winter in this place. NYE last year, it was 30c outside at midnight...35c in my bedroom. Not all that easy to sleep in.

 

[fatboy4]

Isn't there paint or some coating you can get for the roof that keeps the house cooler/warmer?

 

[depreciator]

Yeah there is. I think it's only for reflecting heat, not retaining it. I'm always sceptical about those coatings, but maybe Shane could be a guinea pig for us?