U - values

[Keith Dunbar]

Further to previous posts about calculating radiator sizes, my 17th century timberframed house has been renovated as far as I can tell by replacing all the old wall materials with rendered breeze blocks. The walls seem to vary in width in different places, but the minimum seems to be 9 inches, presumably solid breeze block. My attempts to google for a u value haven't been very successful. Can anyone suggest a reasonable u value for this wall construction that I can use in my calculations?

TIA

Keith

[The Natural Philosopher]

well I have K values (divide by width in meters for U Value)

aerated concteret 0.16 lightweigght block 0.19 med block 0.51 heavy concrete 1.63 screed 0.38 Brick 0.64

9" is about 125mm or 1/8th of a meter, which gives possible U values depending on density, from 1.28 up to 12 or 13 or so.

I would suspect around 4-5. Similar to brick.

Ultimately it doesn't matter cos you want to line it all anyway don't you?

Or do you need to expose timbers?

[Tony Bryer]

If they are medium density blocks (1400kg/m3), then about 1.5. Aerated concrete (Thermalite/Celcon, 500kg/m3) about 0.6.

[Keith Dunbar]

I hadn't planned to alter the walls in any way - I'm just trying to check on radiator sizes. Are you sure about these values? Values I have vary from

0.65 (11" insulated brick cavity) to 2.44 (9" solid brick). Trouble is I have no easy way to find out what type of block was used. The only clue is that the work was done in 1967.

Keith

[Keith Dunbar]

perhaps 1.5?

Keith

[Andy Hall]

The other thing is that as this is for a heating sizing exercise, take a look on the spreadsheet or program that you use at how much heat loss is involved in watts once the areas are taken into account.

For example, you may find that a wall that you find is difficult to determine is not a major contributor to heat loss because of a relatively small area. There's no point in fretting about +/- 20% on 500W on one wall if there's 2kW going out of another wall that you can determine easily.

[Timothy Murphy]

All these figures seem absurdly high to me. I cannot imagine a room which would not be warmed up by a 2kW fire, unless there is an actual opening to the outside.

[Andy Hall]

I just used the figures to illustrate the point about focussing on the components that actually matter. They are deliberately high.

I could have equally said 20% on 50W and 500W through the other wall.

[Newshound]

Sample drilling will tell you whether it's lighweight blocks or not. Just an old carbon steel drill: will go straight through thermalite, won't touch concrete

[mail]

My guess is between 2 and 3 Chris I would unhesitatingly line it intermally if possible Let me know if you want a spec for this as I am doing this myself and have developed quite a good system even tho i say it myself! chris

[Keith Dunbar]

There are loads of beams visible inside, so lining isn't really an option without losing a lot of the character of the place. But thanks anyway for the offer.

Keith

[Keith Dunbar]

Ah, I think I may already know the answer then. But I shall do a drill test later. If it is concrete, what u value would we be talking about for 9".

Keith

[The Natural Philosopher]

\ If you are simply sizing radiators, you will want to oversize anyway for fast warm up times.

Assume 9" solid brick.

[The Natural Philosopher]

I can.

I used to live i an uninuslated 9" solid brick cottage with leaky sahs windows.

Around 2Kw ona small room.

Now I get away with around 500W/room or less.

[The Natural Philosopher]

Even a thin layer of something insulating makes a considerable difference in a solid brick wall.

Do no ignore the pssibility of external insulation, either.. If its rendered outside, that is a distinct possibiliy.