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inside cathedral ceiling insulation?


raelph's Avatar
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12-28-14, 04:40 PM   #1  
inside cathedral ceiling insulation?

My home has low angle beams , which are open. Much like vaulted cathedral ceilings. We live in a hot climate, and above this one particular room there is NO insulation between the interior cladding and the roof which is of fibre - cement sheeting and covered with solar panels.
This room gets hot in the summer. Very hot. Like a sauna... We have a 2 HP air conditioner in the room (35 sq meters/ or 385 sq ft.) and it just can't cool the entire room. So I thought to put insulation on the interior of the room between the open beams (180mm/ or 7 inches)
However, I am questioning my thoughts to use fibre insulation which would be much the easiest to install (still a pain). Will it keep the room cooler? Will I need some sort of ventilation? the room is basically all glass sliding doors on two sides (with 1/3 m / or 5ft eaves to keep the sun rays off the room) and when the air outside is hot, there is only the air conditioner and the ability to stop heat from radiating thru the underside of the cement fibre roof sheeting.
this ability to stop the radiating heat is what I really want to work on.
Any and all suggestions are welcomed.
Attached is picture of beam structure.
thanks
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12-28-14, 06:48 PM   #2  
Are we looking at the bottom of that fibre cement you describe?

If so, have you considered a foil faced rigid insulation?

Bud

 
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12-28-14, 07:46 PM   #3  
thanks for reply, Bud9051
there are two parts to the roof. Super 6 fibre cement roofing material (with asbestos...)
battons and then the white fibre board which you see. there is a gap between the two sheeting materials of about 2 to 3 1/2 inches due to the wavey form of the Super-6 roof.

Yes a friend of mine did suggest rigid insulation. He said it wouldn't have to be so thick...but would be very much more expensive...and difficult/tricky to install, expecially if beams are not exactly conforming in space between along length of beam. They are pretty well spaced along the length .but the distance between beams does vary thru out the room by maybe a cm or two.

thanks for you idea.

Raelph

 
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12-29-14, 12:31 AM   #4  
The humorous term we use for custom fitting that rigid insulation is "cut and cobble", slow but it works.

Is the space between the battons and the top roof material vented to the outside? If yes that would be helping to remove some of the excess heat. If not, I'll need to know more about the house and your climate, any cold weather?

Bud

 
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12-30-14, 12:57 AM   #5  
Hi Bud....there is a gap between the wavy roof sheeting and the eave fascia... the bottom part of the wave sits on the fascia and the wave itself lends to air movement......so there is some air that can get out....but no way for the air to get in other than the same way it gets out.
Slope of roof sheeting is 12degrees.
Temperature....today it was 33 degrees Celsius. Tomorrow the same....in winter it is maybe 23 and down to 13 at night (that is a cold night)
we live on the coast of Australia....about 800 meters to the water.
I think a hole thru the roof would be good for a venting fan....but it would take the air conditioned cool air out, which would be a waste.
Right now is getting just about right
It is 7 pm and the temp is 27 degrees and cooling .

thanks for your replies and thoughts.
Cut and Cobble. (not sure of the term cobble, but it sounds good. What does it mean? Put together???
And the rigid insulation ....? do you usually cut with a Stanley knife or a hot wire?
I've seen hot wires work, but don't have one.

thanks
Raelph

 
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12-30-14, 06:48 AM   #6  
Depends upon how thick and if foil faced or not. A saber saw with a guide will cut through most any rigid foam easily and safely. A circular saw can melt the foam and bind. The break-of style utility knives will cut deeper, but anything over 1" can be a struggle. I also have a funny looking fine tooth hand saw with a curved toothed blade on the end along with the straight toothed side. Whatever it was designed for not sure, but with the fine teeth it works well for small pieces and the curved end allows me to trim in place.

If you use a foil faced rigid foam, you need an air gap (1/2" or so) to block the radiant transfer. The foil surface also meets our fire codes and I think you can paint it.

Blocking solar heat starts with shading and then a surface that reflects the solar energy before it heats the surface. Trees or awnings can help on the shading approach and there are special paints that will reflect a portion of that heat. Once the roof surface gets hot, then that heat conducts, convects, and radiates the thermal energy down to the next layer.

With a low slope it is difficult to use a ridge vent and with high air temperatures the the cooling would be minimal.

Just thinking outside the box, has solar energy become popular down there? An array of panels would shade the roof and generate electricity, and they are becoming an acceptable addition, at least here in the US.

Cobble does mean to assemble and the phrase refers to assembling all of the little pieces needed to cover the desired surface, although yours might not need a lot of small pieces.

Bud

 
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12-30-14, 08:18 AM   #7  
thanks Bud,
yes I have solar panels and a few of them are over the room. But I cannot put more on the roof, due to electrical rules that would negate my selling electricity back to the grid. Shading is NOT an option as that was a horrible problem with trees shading the house and dropping leaves. It did not make using solar panels practical either. When the roots of those trees forced me to cut them down
(we loved them) then the panels could be used.
anyway this problem is not about panels.
....... polystyrene thermal panels. I would buy them and cut them and place them between the beams. Tighter the better.
But you say that an air gap of half an inch would help. So the foil would reflect the heat back up thru that 'air gap' to the sheeting board (inside) cladding.

Yes? thanks again. keeps my mind rolling .

 
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12-30-14, 10:57 AM   #8  
Yes, a radiant barrier works in both directions, high reflectivity and low emissivity. In your application it is the reflective property you will benefit from. The net result will be a small increase in top material temperature with more heat radiated back into space and more blown away by the wind.

Bud

 
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12-30-14, 11:54 AM   #9  
that reads very well Bud.

So you feel that there might be some circulating air beneath the roof cladding?
And that somehow the wind manages to drag some of that heat away?

Would be interesting to have a thermometer in there some time Haha..maybe I will just do that and shove one in the gap today.

And the thickness of the panels?

these panels with foil on them are very expensive. (as are most things in Australia...petrol/gasoline is still $1.25 per liter (minimum)
here despite the drop in oil price, While I read in Canada it has dropped to 94cts)
So my question is , if I used panels without the foil how much difference would that make, ....???...
I guess this is where the R rating properties are supposed to help.
Here in Australia I am not sure if the R ratings are the same as in North America.

A polystyrene panel (without foil) has an R rating 5 .....at 1 inch thick here..... and the codes suggest that 4 is necessary for my area in ceilings. that is extruded polystyrene...
I suppose the non - extruded is that stuff that is steamed and pressed together out of polystyrene balls????
And not so good.

 
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12-30-14, 02:00 PM   #10  
Sounds like your r-ratings are similar to ours, but our gas prices haven't reached that low as yet, $2.35 here in Maine.
However, insulation mfg's and energy professionals differ on how much credit they will give to a radiant barrier. IMO, it helps enough to go the extra cost. In our deep southern zones they will use plywood for the roof that has a layer of foil on the bottom. Where you only need it on one side, perhaps a 1/2" (or thinnest you can get) layer and then those extra 4" of rigid, which isn't going to be cheap.

Given a choice between 2" of r-5 per inch and one inch of foil faced I would call it a draw. But the foil faced may qualify as the fire barrier if double sided and one is required or desirable.

One energy principles is that the first layer of insulation gives you the most for your money. As you add more, it is a process of diminishing returns. Adding r-5 to an uninsulated ceiling saves more energy than adding r-5 to a well insulated ceiling.

Just as a note, if you glued (like wall paper) a layer of reflective material to the ceiling between those rafters and then provided a gap and covered the rest with whatever rigid foam you had in mind, the radiant benefits would be the same. I know they sell sheet material with a reflective surface, but I have never worked with it here in cold country.

Bud

 
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12-30-14, 03:22 PM   #11  
thanks heaps Bud.

I think you missed the part of my post on gas prices / petrol they call it here..... where it says per liter...that is like per quart
so it is 5$ a gallon here.

that 1/2 inch gap....???....how necessary is it?

Just asking....because if I glued foil to the present cladding...then a gap wouldn't be needed at all, ....so then I question its necessity . Just a question.

 
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12-30-14, 03:28 PM   #12  
How much are you paying for electricity?

 
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12-30-14, 03:51 PM   #13  
A gap is necessary to eliminate the conductive transfer of energy. Id a RB faces an open room, then that qualifies. If the gap is as small as 1/4" it still works. If any material is installed directly in contact with the RB, then the RB would be of no benefit.

Yes, I missed the liter. Ach! We are in such a period of transition that it is difficult to know what will be powering the world in 30 or 50 years. 50 years ago as I was entering college the only computers were the size of a house and less powerful than a kids toy of today. I suspect virtually free energy is not that far away, but I have no idea what the repercussions of that will be. We can only hope for good.

"Just asking....because if I glued foil to the present cladding...then a gap wouldn't be needed at all, ....so then I question its necessity . Just a question."
Exposed to circulating air currents a simple layer of foil (a guess) would be in the range of R-2. Regardless of what the foil provides, you do need some old fashioned R-value and so do the building codes.

How is the peak of that roof constructed? Do the channels from one side match up with channels on the other side? If so, then the wind could be pushing the hot air all the way through. Here in the US it is common to open that peak and add a ridge vent, but with your low slope that might not be a good idea. but if the two sides could be made continuous there would be some venting.

Bud

 
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01-07-15, 11:32 AM   #14  
hello Bud,
Today will go to hardware. I have found a company that sells the product. they have a 15mm foil - backed Styrofoam (and I want to find / see if extruded or compressed balls or what) The 15mm product is rated at R2.5 while the 20mm is rated at R2.85
In this part of the world the building code suggests that R3 is sufficient. So I am also going to check the prices.
The Styrofoam is also has another backing on the opposite side to keep the pieces together. It is a good idea....no particles falling later.
But the price....let's find out the price....in Australia we get ripped off everywhere.
Can you imagine paying for a house and land package in an ordinary location for 390,000$ and you dont' get a basement, you don't get double glazed windows, you don't get a ducted heating system, and until recently you didn't even get insulation in your walls....? Ripped off.

 
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