spray foam roof insulation

[unheard]

6 minutes ago, nigelforbes said:

In your first post, you wrote:

 

"no insulation on the ceilings nor under the roof tiles". 

That wasn't me.

Looks like you've confused me with another poster.

[nigelforbes]

1 minute ago, unheard said:

That wasn't me.

Looks like you've confused me with another poster.

Yes I have, my apologies.....don't think it wasn't fun anyway!

[unheard]

5 minutes ago, nigelforbes said:

Yes I have, my apologies.....don't think it wasn't fun anyway!

Hey it's all good!

No apologies necessary.

[unheard]

18 minutes ago, Kwasaki said:

Just put 10 cm silver wrapped rockwool insulation over all ceiling areas, carefully placed without it going onto sofit areas.

Or even cheaper fiberglass rolls will work wonders in preventing attic heat transmission to ceilings.

[Kwasaki]

Just now, unheard said:

Or even cheaper fiberglass rolls will work wonders in preventing attic heat from heating up ceilings.

Yeah good stuff, forgot what's best these days for the visiting wild life rockwool or fibreglass.

 

That said I guess many houses built here now are pretty well seal to visitors. 

[sometimewoodworker]

2 hours ago, nigelforbes said:

The thin metal film is a radiant barrier, designed to stop heat from the tiles radiating onto the attic floor. In essence, that barrier converts radiated heat to convected heat, or hot air, which also must be dealt with by venting.

That is an interesting observation, false, but interesting. 
I am amazed by the interesting theories propounded that are not supported by physics.

 

At a given temperature a black body will radiate heat, a polished silver (white) body will radiate very little. The statement that “that barrier converts radiated heat to convected heat” would confuse any physics teacher who would be unable to explain it.

 

There would certainly be a modest degree of warming of the air immediately beside the barrier and certainly ventilation will assist. 
 

However the heat gain by convection is much less that would be radiated if the barrier was not present. It is not converted to convected heat, the majority is retained by the tiles becoming hotter.

[nigelforbes]

6 minutes ago, sometimewoodworker said:

That is an interesting observation, false, but interesting. 
I am amazed by the interesting theories propounded that are not supported by physics.

 

At a given temperature a black body will radiate heat, a polished silver (white) body will radiate very little. The statement that “that barrier converts radiated heat to convected heat” would confuse any physics teacher who would be unable to explain it.

 

There would certainly be a modest degree of warming of the air immediately beside the barrier and certainly ventilation will assist. 
 

However the heat gain by convection is much less that would be radiated if the barrier was not present. It is not converted to convected heat, the majority is retained by the tiles becoming hotter.

No, there's nothing false about what I wrote although if I was physics teacher or similar I probably would have written it differently. But the idea was to convey a process, from one lay man to another and that I believe was successful, unless of course you happen to be mr pedantic. 

 

"The thin metal film is a radiant barrier, designed to stop heat from the tiles radiating onto the attic floor". That's true and correct.

 

"In essence, (not in precision)  that barrier converts radiated heat to convected heat, or hot air, which also must be dealt with by venting". Does it convert? For the purpose of this explanation it does but the next time I come across a physics teacher I'll be certain to use different words. Goodbye.

[sometimewoodworker]

18 minutes ago, nigelforbes said:

The thin metal film is a radiant barrier, designed to stop heat from the tiles radiating onto the attic floor". That's true and correct.

Indeed it is. It’s the rest of the statement that is neither true nor consistent with fact.

[nigelforbes]

2 hours ago, sometimewoodworker said:

Indeed it is. It’s the rest of the statement that is neither true nor consistent with fact.

Oh dear, I do know how you appear to enjoy roaming these threads, looking to correct posters explanations of minor points in your officious ex-moderator style, but I wonder if you understand they appreciate it, far less than you think!

 

You want me to say that radiant barrier reflects heat back to whence it cometh. I don't know if a radiant barrier does reflect some part of the spectrum and if I did it wouldn't be worth mentioning because it doesn't add anything to a layman's description of the things we were discussing, in non-scientific terms. I realize it offends you when posters are not always 100% technically correct in these things but you'll just have to accept this is what they do sometimes and life's like that, sometimes.

 

The real benefit of the radiant barrier is that it interrupts the flow of heat from a stored heat source such as a blazing hot concrete roof tile. Since the radiant barrier is so thin it is not capable of storing heat and instead, having interrupted its flow, merely releases into the air.  If the radiant barrier wasn't' there, that flow of heat would continue until it hit a surface that was capable of storing the heat and the process of store and radiate would be repeated. I appreciate this is a stretch in scientific terms to say it converts radiated heat to converted heat but that is essentially what it does. And I do understand that scientists and physics teachers all over the world, along with a solitary you, would all stand up in protest if anyone tried to say such a thing, but we've already established we're not those things, we're just normal people explaining to each other, how stuff works.

 

[sometimewoodworker]

10 hours ago, nigelforbes said:

You want me to say that radiant barrier reflects heat back to whence it cometh.

No I don’t, as if it did that it would not be acting as a radiant barrier but as a reflective barrier.

 

the fact that the more expensive film sheet is silvered on both sides and if placed with an air gap between the roof material and the film allows it to be both a reflective and a radiant barrier allows for even more confusion.

 

I have an empirical test of the cheaper paper backed film giving a drop in temperature of about 22 degrees C so while the paper could have got to the higher temperature (59C I didn’t check) the surface of the radiant barrier silver face was at 37 C or ambient air temperature 

10 hours ago, nigelforbes said:

The real benefit of the radiant barrier is that it interrupts the flow of heat from a stored heat source such as a blazing hot concrete roof tile.

Absolutely, no dispute there.

 

10 hours ago, nigelforbes said:

Since the radiant barrier is so thin it is not capable of storing heat

Agreed 

10 hours ago, nigelforbes said:

merely releases into the air

That is where the problem arises, since the barrier does not (in my test) raise its temperature enough to allow that please let me know how exactly that is achieved?

[Kwasaki]

What's obvious with many roof constructions the designs can be very different and for those inexperienced about building basically : - 

Vent the loft area by whatever means is available for the type of roof you have. 

In the main insulate the ceiling areas job done.

[nigelforbes]

7 hours ago, sometimewoodworker said:

No I don’t, as if it did that it would not be acting as a radiant barrier but as a reflective barrier.

 

the fact that the more expensive film sheet is silvered on both sides and if placed with an air gap between the roof material and the film allows it to be both a reflective and a radiant barrier allows for even more confusion.

 

I have an empirical test of the cheaper paper backed film giving a drop in temperature of about 22 degrees C so while the paper could have got to the higher temperature (59C I didn’t check) the surface of the radiant barrier silver face was at 37 C or ambient air temperature 

Absolutely, no dispute there.

 

Agreed 

That is where the problem arises, since the barrier does not (in my test) raise its temperature enough to allow that please let me know how exactly that is achieved?

If the heat (energy) is not directed back at the roof tiles and it is not transferred into the air, I don't see a third alternative hence it must be one or the other. But honestly, for the purposes of this discussion, does it really matter? If we were designing Enterprises heat shield I can agree it would, but....!

[Kwasaki]

3 minutes ago, nigelforbes said:

If the heat (energy) is not directed back at the roof tiles and it is not transferred into the air, I don't see a third alternative hence it must be one or the other. But honestly, for the purposes of this discussion, does it really matter? If we were designing Enterprises heat shield I can agree it would, but....!

Yeah we are discussing roof construction in Thailand not the heat shield on a space shuttle. ???????? ???? ???? 

[sometimewoodworker]

1 hour ago, nigelforbes said:

If the heat (energy) is not directed back at the roof tiles and it is not transferred into the air, I don't see a third alternative hence it must be one or the other.

The heat is either retained by the roof or reflected back to the roof

 

in Diagram 2 the hot air is usually exhausted from the roof space as the barrier is never airtight so the roof air will also heat up

[nigelforbes]

22 minutes ago, sometimewoodworker said:

The heat is either retained by the roof or reflected back to the roof

 

in Diagram 2 the hot air is usually exhausted from the roof space as the barrier is never airtight so the roof air will also heat up

I really do not know why I'm bothering to debate this issue. Your post has done nothing more than reaffirm to me at least that the heat is transferred to air which is something I think we all knew at the outset but you said was "neither true nor consistent with fact"! Of course the heat doesn't go back into the tile, it can't.

 

Anyone who has been inside a roof void or attic that has a radiant barrier installed will know it is a very very hot place, mine is frequently in the 40's, if not higher. If I didn't have  four very large gable end vents and an equal amount of air intake at the eaves, the temperature would be hotter and the floor insulation would be defeated in a few hours. As things stand the bottom one foot of air in the attic is cold because it reflects the incoming cooler air through the eaves boards. Hot air doesn't just escape from the roof void through spaces between the tiles, not unless there is an equally sized  source of incoming air at a lower level to allow that to happen - air out equals air in.

 

Anyway, I'm done here, you can beat this up some more if you want but the sad fact is that we've gone full circle, back to the what I said at the outset before you popped up. 

[sometimewoodworker]

1 minute ago, nigelforbes said:

Of course the heat doesn't go back into the tile, it can't.

Diagram 1 disagrees with your statement.

[nigelforbes]

8 minutes ago, sometimewoodworker said:

Diagram 1 disagrees with your statement.

I don't care. Bye.