The most thermally effective way to build an external wall?

[DIYedboy]

Everyone favours an insulated cavity, but I don't agree, the cavity is not air tight, cavity tray weep vents etc, the insulation only rests on each other and there is bound to be a few gaps.

The most effective way must be a 215mm block with thermal plasterboard on the inside cos that's also where the heat is!............what do you clever lot think?

 

[John D v2.0]

The insulation doesn't have to be where the heat is, it has to be between the heated envelope and outside. You could build a celotex tent around your house and out would work as insulation.
Usually people prefer to keep some thermal mass inside otherwise it will heat up and cool too quickly.
And if you think the air will blow around the cavity insulation, that is true if there are gaps or the air tightness is poor. However that applies to any build and if you internally insulate you're additionally at risk of interstitial condensation.
Our builders wrapped the cavity insulation like a Christmas present with tape and foam in all the gaps, but any insulation needs doing properly, even your winter coat will be useless if you have loads of gaps with the wind blowing inside it.

 

[^woody^]

Internal insulation uses non of the advantages of the thermal mass of the structure.

 

[motorbiking]

There are systems available today that will get you down to a U value of 0.15, at that point its generally considered that no central heating is required. Some will go to 0.1.

 

[DIYedboy]

But is thermal mass an advantage?............if the occupant isn't at home 24hrs a day because they're at work for 10hrs a day 6 days a week the thermal mass argument becomes greatly reduced. I don't want to heat a wall, I want only to heat the air and I want the wall to insulate my heat

 

[^woody^]

Any you also want to roast in summer by the sound of it

 

[DIYedboy]

Are you sure Wood?......................I've got all that lovely insulation on the inside keeping out the heat, plus remember that lovely thermalite block has an open structure so it won't heat up like a 9" brick wall would.

 

[^woody^]

It can be a relatively complex process involving the three main modes of heat transfer and the qualities of various materials interacting as a whole. So its not just a case of trying to keep heat in like putting on a wooly hat.

When you insulate the internal surface the room heats up quick but cools down quick too. You get no advantage of the wall material accepting, holding and then releasing heat. You get rapid temperature changes and more frequent need for the heating system to replace lost heat.

In summer, the insulation does not keep the heat out, but rather is holding the heat in. The heat comes in via the sunlight through the windows, the convected warm air and the warm structure. It comes in and it stays in for longer.

With a solid mass between the room and the insulation, this helps to moderate the internal temperature. Its wrong and simplistic to think of heating a wall. You are heating the room and the heat is then heating the wall which is then heating the room. With internal insulation, you are heating the room, and you are still heating the wall, but the wall does not then heat the room.

 

[DIYedboy]

That's an interesting idea Wood, solar gain can't get back out, mmmhhhh. that would not be good.

Regarding the thermal mass reducing frequency of heating cutting back in, this becomes important if you're at home all day, if you're at work and only home for the evening, I would think a faster heat up is preferable as it means you can have the heating come on later.