Decrement delay of vacuum panels

[Dreadnaught]

From @JSHarris and others I have learnt about the importance of decrement delay as a characteristic of the fabric of a house in providing internal comfort.

 

Background. My build may well be a single-floored, flat-roofed building of contemporary design. As a rear-garden plot surrounded by other dwellings, it has precious few sight lines and instead will have a profusion of roof lights to let in light.  As a (near) Passive House, the roof will be thick and there is a concern that the roof lights will give the impression from inside of a house deep underground. One suggestion to address this is to thin the roof, to use high-prefromance insulation such as vacuum panels in the roof to improve the aesthetic look instead of cellulose filled I-beams. 

 

So far so good but I worry that a roof with vacuum panels will have a low decrement-delay factor.

 

So to the subject decrement delay. As I have learnt, a cellulose-filled roof would have a welcome decrement factor. But what about vacuum panels? I have done a little google-ing and came across the following summarised from here:

 

Quote

• • Insulation materials offering a high decrement ‘factor’ include:
    • Wood fibre insulation board (11.3 hr);
    • Cellulose fibre (7.3 hr),
  • low
    • Low-density mineral fibre (3.7 hr)
    • Low density foamed/extruded/expanded plastics polyurethane/ polystyrene (2 hr).
  • but lowest of all
    • Vacuum Insulated Panels (VIPs) (0 hr)

 

Thus to my question: how can Vacuum Insulated Panels have a decrement delay of 0 hr?

 

I understand that the decrement-delay factor is product of λ (lambda), which is very low for vacuum panels. But is also related to specific heat capacity and the density of the material concerned and I do not understand how to consider these two for a vacuum insulated panel.

 

I wonder, can anyone enlighten me?

 

(For those interested in learning more about decrement delay factor, I found this explanation a help: http://www.greenspec.co.uk/building-design/decrement-delay/)

 

 

Edited May 20, 2018 by Dreadnaught

[SteamyTea]

Because a vacuum has no mass.

 

Not strictly true in the case of a vacuum panel, but they are of such low mass and low SHC (the kJ/kg.K or kJ/m³.K) that they can be treated as zero mass.

 

They insulate by stopping conduction, which is usually the dominant transfer of power (the W/m.K).

 

(this is often known as a 2 1/2 D problem)

 

 

Edited May 20, 2018 by SteamyTea

[Dreadnaught]

Thanks @SteamyTea.

 

Would it be more meaningful to say vacuum panels have a (near) infinite decrement delay factor, not zero as in the linked information?

 

And, even more relevantly, in your view should I not be concerned about decrement delay in a thin roof with vacuum panels?

[SteamyTea]

If you take away mass, which is just the stuff something is made from, energy is transferred by radiation only.  So it depends what colour and finish the outside of your roof is.

Shiny silver, don't worry, matt black, worry.

 

I think a pure vacuum's decrement delay is only limited by the speed of light.  So no.  But that is before virtual particles comes into play.

Edited May 20, 2018 by SteamyTea

[PeterW]

I’d be worried for the “caravan roof effect” with a very thin roof as you will hear everything .!!!

 

Splays on the roof lights will help a lot.  

[ToughButterCup]

What you need, son, is summat  with a high thermals mass. 300mm of concrete should do it .

 

It's OK, I'm outta here - no need to pick up me coat. ?

[pdf27]

1 hour ago, SteamyTea said:

If you take away mass, which is just the stuff something is made from, energy is transferred by radiation only.  So it depends what colour and finish the outside of your roof is.

Shiny silver, don't worry, matt black, worry.

 

I think a pure vacuum's decrement delay is only limited by the speed of light.  So no.  But that is before virtual particles comes into play.

Only if the roof itself is a vacuum chamber - otherwise it's the surface of the inside of the panels you need to worry about. There is also conduction through the silica foam used to provide the structure of the panel. So essentially you have a very small number divided by virtually zero, which means what response there is will be very rapid - but the insulation should be good enough that the actual power transmitted will be quite low.

This means that the relevant thermal mass is more that of the whole building than of the roof insulation itself. In a well insulated, high decrement factor wall the heat transmission across it will be low and it will take a long time to measure any change in external temperature. For a well insulated, low decrement factor wall the heat transmission across it will be low but it will take very little time to measure changes in external temperature. If heat transmission is low, however, this isn't such a big deal - the roof temperature might for example drop by half a degree over the course of 20 minutes rather than 12 hours. Provided there is a reasonable amount of thermal inertia elsewhere in the system (a concrete floor for instance) it should be no big deal - if the concrete is at close to equilibrium with the air, even small changes are self-correcting as the rate of heat transfer will rapidly increase as the air temperature drops. The key is keeping power demand small - temperature swing will be directly proportional to power.

 

Personally, I wouldn't ever have an unpumped/unpumpable vacuum system at home - I spent the first 10 years of my career as a vacuum engineer, much of which was finding new ways to find obscure leaks. The idea of the insulation relying on nobody puncturing a foil bag on a building site just causes my mind to boggle a bit - I'm used to systems being assembled in clean rooms and a single scratch or fingerprint being enough to trash the system. Admittedly I was working on ultra-high vacuum systems, but still...

[Gone West]

I seem to remember that there are large trees surrounding your proposed build so the roof will probably be shaded significantly. I'm not sure how much of an effect decrement delay will have. If decrement delay was a significant factor then I would be thinking along the lines of a combination of a concrete slab and a layer of aerogel.

[TerryE]

In plain speak, just like N-glazed windows, the decrement delay is tiny -- effectively zero.  That means what whilst the aggregate U value might be small, there is little or no lag in that thermal leak passing through the roof.  If you want to integrate up / average out the heat then the simple way is to add some mass above the roof, but rather than concrete, why not go for a green roof?  This will also give you shade and some evaporative cooling as well as looking a lot nicer than most other finishing surfaces.

Edited May 22, 2018 by TerryE

[Dreadnaught]

Thank you all. Excellent points. Plenty of new thoughts to mull. Notably…

• splays
• decrement delay of vacuum panels may be low but actual heat flow will likely be tiny
• the risk of puncturing vacuum panels
• alternatives to vacuum panels, e.g. aerogel
• a green roof giving evaporative cooling and being an additional thermal buffer. (Neighbour would like the look of it too)

I am now minded to avoid vacuum panels and be armed with all these new ideas instead. Thank you.

 

 

 

Edited May 22, 2018 by Dreadnaught

[TerryE]

Yup, a green roof looks so much better than other coverings if viewed from above.  Keeping the neighbours on your side really helps with planning very approvals, IMO -- not so much formally but the odd letter of support helps with comfort factors for the LPA. It can also help during construction, because the build process does inconvenience neighbours and they are far more likely to grin and bare it if they like the outcome.

Edited May 23, 2018 by TerryE