5 minutes ago, bankruatsteve said:

Please reference your source for that definition.  A material that reflects energy does not absorb it so of course it would not radiate energy that it doesn't have.  

Look at @maxpowerposting

 

the hot plate is the same temperature. It is one unit the silver side is radiating much less than the black side. It is the same temperature.

 

TTFN

1 hour ago, sometimewoodworker said:

Look at @maxpowerposting

 

the hot plate is the same temperature. It is one unit the silver side is radiating much less than the black side. It is the same temperature.

 

TTFN

Your post received a 'Haha' you can now rest your case.

1 hour ago, bankruatsteve said:

Please reference your source for that definition.  A material that reflects energy does not absorb it so of course it would not radiate energy that it doesn't have.  

The three methods of heat transmission are radiation, convection and conduction.

A perfect black body absorbs all the radiation around it. Imperfect black bodies absorb a large percentage of the radiation. A reflective surface such as aluminium only absorbs a small fraction of the radiated heat.

If the principal method of transmission is conduction or convection, there is very little difference between the two materials.

Radiation from the sun falls on the top side of the roof. Underneath, the main method of transmission between the roof and an internal ceiling is convection. IMO this is where people get confused.

Aluminium foil is used on fibreglass insulation not only because it is a radiant barrier, but it also waterproofs and reduces convective transfer through the insulation.

3 minutes ago, Lacessit said:

Underneath, the main method of transmission between the roof and an internal ceiling is convection.

Yes and no.  The roof tiles absorb radiated sun energy.  That energy radiates into the attic which is absorbed by air molecules and those radiate in convection heat transfer to a ceiling and other structure.  A radiant heat barrier under the tiles significantly reduces this transfer.

19 minutes ago, bankruatsteve said:

A radiant heat barrier under the tiles significantly reduces this transfer.

Is barrier installed below the tiles some kind of insulating material or is it a reflective layer. If its a reflective layer which way will the shiny side face and why? 
 

5 minutes ago, maxpower said:

Is barrier installed below the tiles some kind of insulating material or is it a reflective layer. If its a reflective layer which way will the shiny side face and why? 
 

Reflecting radiant heat energy is different than insulating conducted/convected heat energy.  The reflective side faces the roof so that it reflects the radiated heat from the tiles.  If my explanations are confusing to some, then try Wikipedia:  https://en.wikipedia.org/wiki/Radiant_barrier

 

2 minutes ago, bankruatsteve said:

Reflecting radiant heat energy is different than insulating conducted/convected heat energy.  The reflective side faces the roof so that it reflects the radiated heat from the tiles.  If my explanations are confusing to some, then try Wikipedia:  https://en.wikipedia.org/wiki/Radiant_barrier

 

I see, so the shiny side faces the sun. Would it be useless if the shiny side faced into the roof space?

4 minutes ago, maxpower said:

I see, so the shiny side faces the sun. Would it be useless if the shiny side faced into the roof space?

The "shiny side" faces the point of radiation so to reflect it back.  And no, that doesn't make the tiles any hotter (another part of thermodynamic principles).

15 minutes ago, bankruatsteve said:

The "shiny side" faces the point of radiation so to reflect it back.  And no, that doesn't make the tiles any hotter (another part of thermodynamic principles).

Guess that means towards the sun with maybe a little ambiguity added for safety.

 

As recommended, a read of the Wiki page prefers the shiny side facing down method which I guess all relates to the emissivity theory along with a few dust limitations.

 

Rather than rely on Google variations, I define radiant barrier by looking up its noun 'barrier' which provides quite a wide scope to the overall definition.

 

If ever I have need to limit a roof space solar gain I will be open to the many barrier techniques rather than concentrate on the reflective trend Google searches return.  

As I understand it, insulating the underside of the roof is better for keeping the heat out, while insulating above the ceiling is better for keeping the heat in the home. 

 

I think the best bang for the buck in a climate like this will be ventilating the attic space.

 

6 minutes ago, maxpower said:

As recommended, a read of the Wiki page prefers the shiny side facing down method which I guess all relates to the emissivity theory along with a few dust limitations.

Where do you see that?  Reflectivity and emissivity are not the same thing.  Materials that are high reflective will have low emissivity.  IE: if Al reflects 95% of radiant heat that would leave 5% to be absorbed and emitted as radiant heat.

I give up again.  You all do it your way and I'll do it my way but I won't be complaining about having wasted money on efficacy of methods.

3 minutes ago, bankruatsteve said:

I give up again.

Dont give up, bring yourself up to speed and embrace the emissivity method.  Shiny side down and all that stuff the Wiki page rambles on about.

10 minutes ago, maxpower said:

Dont give up, bring yourself up to speed and embrace the emissivity method.

As I stated the fact previously, every object in the universe emits (radiates) thermal energy.  There is no "method" to embrace.

4 hours ago, maxpower said:

Is barrier installed below the tiles some kind of insulating material or is it a reflective layer. If its a reflective layer which way will the shiny side face and why? 
 

It goes shiny side down (if it’s single sided) so it will not radiate the heat into the space under.

 

“A radiant barrier reduces the amount of heat radiated across an air space that is adjacent to the radiant barrier. ”

3 hours ago, bankruatsteve said:

Where do you see that?  Reflectivity and emissivity are not the same thing.  Materials that are high reflective will have low emissivity.  IE: if Al reflects 95% of radiant heat that would leave 5% to be absorbed and emitted as radiant heat.

yes absolutely Reflectivity and emissivity are not the same thing.
 

”Materials that are high reflective will have low emissivity.” often but not always and the length of the radiation is a factor, a mirror is highly reflective of visible light but is almost equal to a black body for heat.

 

A misunderstanding or inadequate explanation. IE: if Al reflects 95% of radiant heat that would leave 5% to be absorbed and emitted as radiant heat.

emissivity:

A hot silver body will emit less heat by radiation than a similar hot black body

 

different case

 

Reflectivity:

A cool silver body will absorb less heat by radiation than a similar cool black body

 

the majority of the confusion arises from a lack of clarity used it talking about reflective and emissive radiant barriers and that the same item can be used in different locations to be either a reflective barrier or an emissive barrier.

38 minutes ago, sometimewoodworker said:

It goes shiny side down (if it’s single sided) so it will not radiate the heat into the space under.

I was actually trying to stimulate someone into thinking outside of the box. The methods and physics behind them I fully understand.

1 hour ago, maxpower said:

I was actually trying to stimulate someone into thinking outside of the box. The methods and physics behind them I fully understand.

The second sentence is clear from your posts.
 

The first seems to have either totally backfired or got a response that’s brilliantly satirical while giving complete miss information 

2 hours ago, sometimewoodworker said:

satirical while giving complete miss information

Correct.