How fast does your house cool down ?

FreeBear

Deleted_User said:  Am I correct in thinking that without huge outlay energy loss is going to be fairly rapid in very cold weather?

Insulating your loft with 300mm of fibreglass and plugging as may of the could draughts need not cost much. Both will have a dramatic effect on heat loss.

Replacing windows/doors and insulating walls can get expensive though. Still worth doing just to improve quality of live even if you don't recover the cost through energy savings within a reasonable time frame.

jacko220

Deleted_User said:

jacko220 said:

Deleted_User said:

jacko220 said:

Depends upon two things, first of all the Construction and what is known as the U-value of the construction measured in Watts/,^2/Deg C, that is how many watts the construction will loose per square metre of area per Degree Centigrade of difference between inside and outside , the Deg K that is sometimes used is Degree Kelvin the difference in Deg K and Deg C is the same it starts from a difference place. The next is how leaky the house is due to air leaking from the house. I lectured in this for nearly 40 years, if anybody wants to ask questions please feel free. It also depends on the Admittance value of the structure, that is how much heat is stored by the insides of a building, If you wish to ask questions please feel free.

I'll take you up on that offer!!

OK - so even a thermos flask will lose heat quickly in cold weather. I have a high performance flask and in milder weather boiling water put in the night before can be used to make a cuppa in the morning. But in this cold snap the water is only warm by morning.

So without a heat source houses with state of the art insulation will lose heat overnight.

If I put a tea cozy over my flask (analogy with topping up loft insulation) it probably slows down heat loss in a measurable way, but I still have luke warm water in the morning.

So I guess my questions are focussed around diminishing return on investment. If you have a house with decent insulation (lets say 20 year old standards) how would YOU go about working out the point at which further insulation is going to cost you more than you'll save in it's lifetime? If 90% of the year your insulation is performing well how much is it worth spending to improve it for 10%? Often it will be much cheaper just to accept the higher energy costs in cold snaps.

And another question that is baffling me. My street has 14 x 20 year old bungalows with concrete tile roofs. Some people have added to the loft insulation to get it up to standard. Others haven't. ALL houses have thick frost on them when temps are below zero overnight and NONE thaw out faster than others. 

So observation 1 is that this little bit of extra insulation isn't making enough difference to change the rate at which ice on the roof thaws?

Here's the puzzling bit - next door neighbour had an extension build in the summer. He commented on how thick the insulation was in the loft - he was amazed at the difference between old and current standards. In all other ways roof construction is identical to the original houses - watched it being built. Yet in this bitterly cold weather, every house has a thick coating of frost on the roofs - except his new extension. It frosts up overnight but thaws quickly in the morning. How can that even be possible?

Think of it this way water will find its own level, the level of water is the temperature, as you say temperature drops more quickly in colder weather. No matter what you are going to do heat will flow from hot to cold or cold to hot that is a matter of semantics. 

I have been asked to write in paragraphs, so here goes. The U-value of construction is determined from the thermal conductivity or k or lambda value of  a material, generally the less dense a material the lower the thermal conductivity, so hence mineral wool is a better insulator than solid brick and "breeze", they are not breeze but aircrete blocks, like an aero bar are better than solid bricks, the density of bricks if about 1700 kg/m^3 and that of modern aircrete blocks 440 kg/m^3. 

The reciprocal is 1/k which gives the resistivity and multiplied by the thickness gives the Resistance, the U-value in turn is 1/ Sigma (Sum of ) Resistances, so you are correct it is diminishing returns, ie adding 50 mm to 100 mm is better than adding 50 mm to 150mm. 

I do hope this helps, please feel free to ask any questions, for optimum levels of insulation look at Building Research Establishment Energy Conservation Unit (BRECSU). Please do not block up the vents in your roofspace it will cause condensation, hence the Building Regulations require air gaps at eaves level or other means to create a cross flow of air which will remove the moisture, the air in the roof space is colder and the Saturated Vapour Pressure of the air the temp at which it condenses is lower, condensation and mould do to timber what it did to that little boy the other day.

Sorry for the ramble but I still have not dealt with heat loss due to ventilation and condensation, heating and condensation are interlinked.

Sorry to be a geek. 

You're not being a geek. But hopefully you understand if I try and translate into 'for dummies' language to make sure I understand!! LOL

Am I correct in thinking that without huge outlay energy loss is going to be fairly rapid in very cold weather?

So going back to the analogy of my flask - a good flask will keep fluids very hot for 24 hours; a moderate flask for 10 hours and a cheap flask for 4 hours. Which is all better than a cup which will cool in minutes.

So ultimately, if you withdraw the heat source it's only a matter of time before you have a house that's the same temperature as the outside air? No matter how state-of-the-art (and expensive) your insulation?

If your aim is to keep your house warmish overnight for most of the year, without heating,  the equivalent of the 'moderate flask' will do the trick and won't cost you too much. If you want your house to hold it's heat whilst you're at work - again, the 'moderate flask' will do. The expensive flask will save money on heating bills, but will probably cost you more than it takes to heat your entire house for a decade! In unusually cold snaps paying a bit extra for energy during those days is probably the most cost effective solution?

*****

And as for condensation - that's touching on something else I've been puzzling about. Somewhere in the past few days I read an article explaining that you should INCREASE the humidity in the air during very cold weather to keep warmer. That is counter-intuitive. I can't find anything to back this up and can't remember where I read it. Is it true?  I'm wondering if perhaps moist air holds more heat? I know warm air holds more moisture but is the converse true?

Energy loss will always happen it is a basic law of physics, heat loss = u x Area x TD, if the Temperature Difference is high then the heat loss will be high. The law of diminishing returns applies to insulation, 1/2 =0.5 1/4 = 0.25 difference of 0.25 1/6 = 0.17 difference of 0.08. U-values Single Glazing 5.7 Double Glazing 2.8 Treble Glazing 2.0, difference of 2.9 between Single and Double Glazing, difference of 0.8 between Double and Treble Glazing.

Turn the heating off and hot will flow to cold or cold to hot it is a matter of semantics, it will cool down. You are correct

The idea is to delay the heat loss so that you have to top up less heat , your analogy is very good.

Condensation the formula is Saturated Vapour Pressure = 0.6105 x e ((17.269 x t)/(239.3 +t)), e is exponential and t is temperature is Deg C. Cold air can hold more vapour than a straight extrapolation of warm air. Try looking at these

Psychrometric Chart - Bing images

Hope this helps if not please feel free to ask questions.

sandy700

I've got a Switchbot thermometer sensor and last night the temperature dropped from 21 deg C at 23:30 when the heating went off (the thermostat is set to 12 deg C during the night) to 15.7 deg C by 07:45 when the heating came on and by 09:00 the temperature was 20.1 deg C when it then slowly crept up to 21 deg C by 10:17.

The temperature then fluctuated between 21.1 deg C and 20.8 deg C until 23:30.

That used 53kWh of gas according to my IHD.

sandy700

brewerdave said:

How fast does it cool down ? Today's Q should be how long before it gets warm enuf !!!

It takes my boiler about an hour to get the room up to an adequate temperature

Andy499

My house cools down about 0.5-1 degree per hour.

Its pretty bad and very cold all the time. No point in using the heating as I cant afford it and it gets cold extremely fast so might as well just be cold all the time