Cavity Wall Insulation- Should I Bother?

On Oct 11, 12:47=A0am, "js.b1" wrote: ////

That maybe right heat-wise but what is being forgotten is that the cavity wall is itself a vertical damp proof course. You bridge it at your peril. Most outer leaf building material (& mortar courses) is slightly porous and in a bare cavity what H2O that seeps through should run down the inside of the outer leaf & out at the drain holes just below the horizontal wall dpc.

You need to be absolutely certain that the filling (1) does not allow moisture to cross the gap (2) does not deteriorate in the presence of moisture (eg try damping rockwool/glass fibre). Only a small amount of moisture is involved. Deleterious effects (such as shaling of brickwork) take a long time to show up. Consequently you are looking for guarantees that will be valid 10-20 years out: who will honour the guarantee in 15 years?. Performance varies between filling materials, but IMHO beware of blown materials which are rockwool-type based.

Ideally there should be a 25mm (1 inch) air gap in the cavity against the outer leaf.

HTH

Reply to
jim
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That maybe right heat-wise but what is being forgotten is that the cavity wall is itself a vertical damp proof course. You bridge it at your peril. Most outer leaf building material (& mortar courses) is slightly porous and in a bare cavity what H2O that seeps through should run down the inside of the outer leaf & out at the drain holes just below the horizontal wall dpc.

You need to be absolutely certain that the filling (1) does not allow moisture to cross the gap (2) does not deteriorate in the presence of moisture (eg try damping rockwool/glass fibre). Only a small amount of moisture is involved. Deleterious effects (such as shaling of brickwork) take a long time to show up. Consequently you are looking for guarantees that will be valid 10-20 years out: who will honour the guarantee in 15 years?. Performance varies between filling materials, but IMHO beware of blown materials which are rockwool-type based.

Ideally there should be a 25mm (1 inch) air gap in the cavity against the outer leaf.

HTH

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Insulation is blown of mineral wool and from the bits left lying around when next door had theirs done is a bit like cotton wool (whitewool) although the company web site says they also use rockwool. The company is offering a 25 year CIGA guarantee.

What is your issue with Rockwool and does it also extend to whitewool?

Archie

Sorry about the indentation. For some reason OE doesn't auto indent this post.

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Reply to
Archie

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Just observing that cavity wall after fill is not without reported troubles, due SFAIUI to moisture seeping as outlined.

Would suggest you try leaving a fistful of rockwool somewhere in an exposed location. Web search for whitewool states it is a glass fibre type material treated for water resistance & designed for cavity after fill. I would be asking 'water resistant for how long?'.

As you may gather my experience of walls with 50mm cavities filled completely when built with 50mm Rockwool slabs leaves me cautious about blowing the stuff in as afterfill. YMMV.

Can only suggest you ask the supplier pertinent Qs and make sure you have a rock solid guarantee that outlasts your ownership of your home (& outlasts the guaranteeing firm).

Reply to
jim

ITYWF that is the definition of overload, load in excess of specified ratings. Last off the shelf mains transformer I bought only specified the continuous rating so any short term use outside that envelope is overload by definition.

Ah, so there's no risk of saturating a transformer by passing more than its design current through it and even if it did saturate it wouldn't matter?

Interesting theory.

Reply to
fred

But, but, but: Voltage in the UK is required to be 230V -5% +10% (or something) - same as in the rest of the EU. However, UK /actual/ voltage is 240V (and continental voltages are /actually/ 230V).

For calculating overheating etc, I'd be more interested in the current with an actual 240V.

Reply to
Martin Bonner

Manufacturers usually dont supply a chart of load versus time, just a steady state load figure. If you want to push the envelope you get to do the basic engineering design yourself.

You can debate definitions, but at the end of the day transformers are perfectly happy run well above continuous ratings for significant times. Almost every house in the country has just such operating day in day out in their kitchen.

Increasing load tends to take it out of saturation, not further in

NT

Reply to
NT

Well, that's true, but only nominally so.

You will find in practice that most people in the UK get more than 240V. Here, I am about 80m of cable away from the substation and (at 2.30 p.m. in the afternoon on a sunny day) have a voltage of 248V (I just measured it). I expect that later on (about 9.00 p.m.) when it's dark and much colder, the voltage will drop to 245V or so. I expect that people at the far-end of the cable 200-300m away from the substation with have figures more like 240V falling to 237V.

Don't forget that 90% of the regulations exist to act as a protection against the risk of fire and 10% against electric shock. A shower that pulls 42.8A instead of 37.9A doesn't make for a significantly increased fire risk and has no increased risk of causing electric shock (provided that the cable isn't grossly under-rated of course).

Only if you've actually got 240V...

It's only Ohm's Law (V=I*R) and the Power Law (P=V*I) and a bit of algebra. You don't need no stinkin' website calculator from TLC to work it out ;-)

Reply to
Dave Osborne

It might but here the volts drop during the working day when the load is really on the grid. We have our own transformer and about 15m of

240v cable to the meters. From about midnight the volts are up and pretty steady just under 240, then come 0600 they start to fall reaching a minimum about 0800 of 230ish. The rest of the day then generally bounces about with an upward trend back to 240v by midnight. The weekends have less variation.

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The stepping is due to the 2v or so resolution(*) of the APC Smart UPS that is providing the data I'm logging.

(*) WTF it reports to one decimal place when it only ever reports discreet 2v or so steps is beyond me. This gives a very false impression of the voltage measurement accuracy.

Reply to
Dave Liquorice

Back to the microwave again? Those transformers are specifically designed and specified to be used in that manner (heavy load, limited duty cycle) and so are not running outside their specifications. As you have said above, the only way the o/p could be sure that a solution using a transformer operated well outside it's specified ratings would work, would be to design the transformer himself or buy one and test it for compliance. That doesn't really sound like a practical solution.

I would say that the secondary voltage would reduce as saturation came on meaning that the voltage applied to the shower would be greater. That appears contrary to the object of using the transformer in the first place.

It goes back to: If you operate an off the shelf transformer at 100% more than it's specified rating then you don't know whether it will saturate or not. If it saturates it would not be effective in this application.

Reply to
fred

OK, Thanks for that. Do you measure the voltage at the point of your supply or somewhere along a final circuit? Does the load on the final circuit cable have a significant effect on the measured voltage or does the voltage vary this much at the point of supply?

I work from home and live in a moderate sized commuter town with a small light-industrial estate. Max demand is winter TV prime-time when the adverts come on and the kettle gets put on!

Anyway, if you have a shower at work, you would get a better shower if you get in early...

Reply to
Dave Osborne

It's on the (singular) ring circuit.

Not that I've noticed. Our peak load on the ring is only about 5kW, no the grill isn't on the ring, so make that 4kW less a bit for lights say 3.8kW If you look at some of my other photos on flickr you'll find power useage plots as well.

I'd say the general trends are those of the supply. Our base load goes from about 300W to just under 1kW with spikes from the kettle and broader but lower power from cooking. The profile of power use does not match that of the voltage variation.

Reply to
Dave Liquorice

The question of core saturation under load doesn't arise. As you load the secondary of a transformer the flux density in the core falls, reducing the 'back EMF' induced in the primary and allowing more current to flow from the supply. This fall in core flux is how the primary 'knows' that more current has to flow.

Saturation is not the issue. Heat dissipation from the I^2R copper losses is, and (lack of) voltage regulation might be too.

Reply to
Andy Wade

I am a little surprised that your voltage variation is so high. It does suggest that there are some issues with overloading/under-sized cables on the network in your area.

Cheers.

Reply to
Dave Osborne

It's *well* within spec, the variation is only 10v (240 to 230 ish). The spec is 218.5 to 253 or 34.5v (230v -5% +10%).

Reply to
Dave Liquorice

That sounds very logical but taken to the extreme, doesn't it mean that you could build a 700VA transformer using a 6" nail as the core?

Doesn't the core need to be capable of (magnetically) coupling the full load between primary and secondary?

Reply to
fred

They are designed with special features to meet the specific functions of microwaves, but the specialness in their design has nothing whatever to do with high load level and low duty cycle. Exactly the same principles apply re their load rating as with any other transformer.

That's not what I said.

If you're unable to do the simple engineering design involved in running a transformer safely above the manufacturer's continuous duty ratings, that only tells us of your abilities. Running a 400w transformer at 700w for 15-20 mins is so far inside its real ablities that it can be done by anyone without engineering skills, hence why I mentioned it.

Since it wouldnt saturate this is off the point.

On the contrary, I do.

NT

Reply to
NT

We've already covered the saturation questio being wrong. But also in this case, due to the circuit configuration, regulation in the transformer due to copper losses /increases/ the secondary terminal voltage, not reduces it, resulting in less voltage to the shower.

Actually this is one application where regulation doesn't matter a hoot, other than ideally you'd calculate for it altering the Vdrop on max shower power.

Where does that come from? I don't see how you'd achieve that.

sure, normally.

NT

Reply to
NT

Sooooo, at what point does the magnetic circuit fall over and what happens when it does.

ie: How much can you take out of a 400VA transformer before the magnetic circuit falls over and what happens when it does?

Reply to
fred

Don't you realize that is entirely contrary to the spirit of Usenet? How are we supposed to get a good argument going, if you go and spoil it with actual *data*? :-)

Seriously, thanks for that. Very interesting to get an idea of how high the /real/ variation is.

Reply to
Martin Bonner

The magnetic circuit doesn't 'fall over' on overload. You asked...

...to which the answer is something like this:

(i) because the total available core flux (flux density times core area) would be much too small. Lack of flux means more turns per volt would be required in the windings, implying thinner wire for any particular copper 'window' area, therefore higher winding resistance and more copper loss for a given load. With such a tiny core the copper circuit would be completely impractical in terms of its resistance and losses;

(ii) because a nail doesn't provide a closed magnetic circuit. Hence (a) the high reluctance through the air path would necessitate more magnetising ampere-turns in the primary (high copper loss again) and (b) poor magnetic coupling between the windings will further raise the impedance of the transformer (leakage reactance).

Only when the winding and core areas are in some sort of proportion does a power transformer design become practical. Power rating is then a function of the acceptable impedance (voltage regulation) and how heat is to be removed.

Reply to
Andy Wade

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