Heat conduction from basement to earth/soil below

[CJT]

I disagree, but I'm willing to listen to your reasoning if you care to present it.

[nicksanspam]

Wrong again.

Nick

[CJT]

Then you need to develop an understanding of emmissivity.

[News]

I was just bottom lining what Nick was saying just above.

[News]

I'm sure Nick will elaborate on the wheres and whys of which way it faces for optimum performance - which is now the latest confused point on barriers.

[CJT]

There should be no confusion.

[News]

"Should" and "is" are two different things ;-)

[daestrom]

As I said before. Government and independent testing has shown that radiant barriers lose much of their effectiveness if they get a layer of dust over the foil side of them. The dust raises the emissivity to that of other non-metallic materials ( > 0.85).

In places like attics, the 'usual' installation of radiant barriers is not across the floor, but attached to the rafters overhead, facing downward. This avoids the dust buildup issue. Thus the radiant surface is 'aimed' downwards to the floor space of the attic. In climates that need a lot of A/C, this can work quite well. The solar heat gained by the roofing heats the sheathing and rafters, but the radiant barrier prevents it from radiating to the attic floor (ceiling of the living space). Testing for their efficacy in such installations has gone well. Radiant barriers in this sort of situation can be an inexpensive, easy to install way to reduce cooling energy needs.

Sadly, for climates needing a lot of heating, the situation doesn't work so well. Installing the radiant barrier on the rafters does little to reduce heat loss from the attic floor (living space ceiling). One reason for this is that attics in cold climates are deliberately ventilated to keep the attic cool. This prevents ice damage and ice dam formation on the eaves. Another reason for poor performance in heating climates is that with the heat flow upwards, natural convection of air from the attic floor to the radiant barrier far outweighs the radiant heat transfer component, so reducing the radiant heat transfer does little to reduce the overall heat transfer (most upward heat flow still happens from convection currents).

So, bottom line. If the direction of heat flow is upward, radiant barriers don't work well. Either convection outweighs the radiant component, or the surface gets contaminated with dust and requires cleaning, or both.

For downward heat flow, they can add to the overall insulation if installed correctly.

daestrom

[daestrom]

Let me see if I can clarify it a bit for you.

Radiant heat transfer involves two surfaces. The 'hotter' one radiants infrared energy, the 'cooler' one absorbs it. How well a particular surface emits infrared energy when heated is measured by its emissivity.

So to reduce radiant heat transfer, we can coat the cold surface with something that reflects infrared energy so it doesn't absorb as much. -OR-, we could coat the hot surface with something that doesn't radiate/emit infrared energy as well. Either one will reduce the amount of infrared energy that gets from the 'hot' surface to the cold surface.

Now, it just so happens, that with very few exceptions, surfaces that are poor at absorbing infrared are poor at emitting infrared. And surfaces that are good at emitting infrared are also good at absorbing infrared. Polished metal and metal foils are very poor at absorbing and emitting infrared. So radiant barriers have a metalized/foil surface. There emissivity is quite low (< 0.2, some as low as 0.05). Non metalic materials (wood, plaster, glass wool, etc...) are good absorbers/emitters (emissivity > 0.8, often >0.9).

So, in the case of under-floor radiant barriers, if we cover the &#39;hot&#39; surface with a material that is a poor absorber of infrared (and hence is a poor emitter of infrared), we get about the same overall affect as if we had covered the &#39;cold&#39; surface with it. We could cover either one and get about the same affect, at least in the short term.

But once the poor absorber/emitter is covered with dust, the heat can travel from the foil to the dust by conduction (a very good transfer mechanism). And household dust has a very high emissivity, so it absorbs/emitts infrared quite well. So the dust layer completely circumvents any savings of the radiant barrier. So we *really* want to keep the radiant barrier clean.

And by putting the poor absorber/emitter on the underside, we have it in a position (facing downward) where dust and dirt are less likely to settle on it.

daestrom

[daestrom]

I think you&#39;re confusing the placement of the "vapor barrier" with the placement of a "radiant barrier".

In construction, it is best to place the *vapor* barrier on the &#39;warm side&#39;. So in heating climates, the vapor barrier is place on the inside and in cooling climates on the outside. The logic behind this is you want to block the moisture that seeps through the wall *before* it cools down and has a chance to condense.

But the foil of a *radiant* barrier can be on either the hot or cold surface and the difference is not very much. The most important part about radiant barriers is that a) they be positioned/installed so the foil remains clean, b) the have an air gap facing the foil (direct contact with the next layer makes the foil useless), c) they are only effective if air convection against their surface is not an issue.

A 2" thick foam board with *no* radiant foil will add about R12 to the walls. If the foamboard has a closed-cell surface coating, it can double as a vapor/draft barrier as well. Sandwiching a foil layer between other materials with no air gap is a complete waste of money.

daestrom

[CJT]

I was disagreeing with "no good in the attic," and you seem to agree with me, at least in climates where AC costs predominate (like where I am -- Texas).

I thought the focus was on AC, it being summer and all, but you may have a point in a broader context.

[Robert Gammon]

A 2" thick foam board with a radiant barrier applied to the board with the board sitting behind a brick or concrete block wall has the required airspace in front of the radiant barrier. If HardiPlank or HardiPanel siding products or stucco are applied over the foam board, then paying for a radiant barrier is nonsense.

You assume that the cladding is applied directly to the foam board. Not always true.

[Jeff]

I&#39;ve been think about that as far as my staple up radiant goes. It looks to me that I want to cover most of the area (perhaps all) between joists with flashing thickness aluminum to spread the heat out. That&#39;s a lower operating temperature for the working surface. I don&#39;t think the emissivity is as low as foil, but probably around .08. It seems to me that part of the joist should also be covered in a radiant barrier.

It looks like you can gain a good bit of insulation value just from having a dead air space with an IR opaque boundary.

I see from my "Passive Solar Energy Book" That a horizontal foil surface with heatflow down has an R value of 4.55. That would seem to imply face nailing 1" polyiso foil covered, with the foil facing down onto the joists. Perhaps R17 total. Does that sound about right?

Jeff

[News]

Further to the above:

The UK authorities have pulled the plug on multifoils, when used on their own. The Multifoil Council) made pleaded to the Office of the Deputy Prime Minister when the last round of Part L (energy aspect) of the building regs was being assessed and had a reprieve to 01/01/2007 to give them time to show that multifoils worked as claimed. This repreive has been recinded early because of convincing evidence that the multifoil claims are exagerated. Multifoils will only be permissable if they can pass hot box tests, which they have never have. All the local authorities and the NHBC and such bodies have all been told to no longer accept multifoils.

Action may be taken against the 3rd party certifiers, principally BM Trada, who gave their stamp of approval to Actis and others.