Rate of room temperature change related to U values of composites

[sophiecentaur]

TL;DR Summary

Effect of the position of an insulating layer on cooling and heating

I am contemplating installing external insulation on your hundred year old fashioned house with solid walls. This house and those of my neighbours are described as 'cold houses' by the neighbours. I need to do something if I can and eternal insulation is a possible solution.

I'm trying to predict the likely improvement of some external cladding vs a cavity wall. the usual calculation used in planning CH systems. The U values of the sort of insulation I could use is in the right order of the U value improvement going from solid to cavity walls. (a number of tables online) But the thermal capacity of the bricks must be relevant to the timescale of following the outside temperature. It's the equivalent of a chain of resistors with capacitors at the nodes. The long - term steady state will just follow a K2 law and I realise it will be swings and roundabouts, with the thermal capacity dictating the 'feel' of the walls over the short term. style.

I could spend / waste a lot of time re-inventing this wheel. Are any PF members familiar with the problem and is there a method, well known to the world with the right units and values for domestic walls

 

[jrmichler]

How deep do you want to get? A full answer to your post would require writing a book. I suggest doing some background reading at https://buildingscience.com/. I suggest starting with their Articles & Papers tab, and just start reading.

The U-value is only a small part of a wall design. Moisture control and air leaks are even more important. Thermal capacity is important for comfort in a poorly insulated building, and of little importance in a well insulated building.

 

[sophiecentaur]

How deep do you want to get? A full answer to your post would require writing a book. I suggest doing some background reading at https://buildingscience.com/. I suggest starting with their Articles & Papers tab, and just start reading.

The U-value is only a small part of a wall design. Moisture control and air leaks are even more important. Thermal capacity is important for comfort in a poorly insulated building, and of little importance in a well insulated building.

Thanks for your response. Yes, I know there are many factors involved. I already made some gaffs more than 40 years ago when these things didn't matter so much. My reason for the query was that I would like to clear up another unknown. I wanted more knowledge about a single component (U) of the problem. You say that heat capacity is more relevant in a badly insulated building (good understandable point). Well, the old solid walls certainly come in that category. A cavity is good value for both moisture and heat loss and I don't have that option but some external cladding will definitely make a difference to the internal wall surfaces - which is significant. Moreso when we replace the old aluminium windows and door with new UPVC. Drafts should not be an issue any more, either.

In your link, the presentation:' Exterior Wall Insulation: Don’t Eat Your Sweater' really spoiled my day and that refers to new constructions. How much worse for us? The only thing is that we have a relatively mild climate so the ΔT will be a lot less and I don't worry too much about condensation problems. Never had mould or rot in any of our five houses except the very poor quality single glazed windows in one house; the timber was very poor and everyone in the street (identical self-builds) had the same problem.

I think chasing humidity around the house and occasionally using a de-humidifier would be one way of dealing but it doesn't seem to be discussed. I may do some data logging from sensors around the house; a nice nerdy pastime. But we haven't lived here for a full single year even so 'before and after' measurements are not possible. I would only have the neighbours' comments to inform me about success of any mods I do. The roofers have removed our only chimney stack and replaced all the ridge tiles and many other spots. That should cut out much of the damp, caused direct from rain.

 

[russ_watters]

I guess I'm not completely clear on what you are asking. Are you wanting to compare different insulation schemes? Can you draw diagrams of the wall layers for each scheme? You can basically just add the R (inverse of U) values together for each layer to get the total.

Modeling actual performance gets difficult, but for basic design purposes total R value should be enough.

 

[Lnewqban]

Unless you can achieve a perfectly sealed existing wall sealed-cladding cavity, cold and humid air will reach your exterior wall, as well as before the cladding installation.
That is not much improvement for heat loss from that solid wall, but interior insulation upgrade can be.

 

[gmax137]

I have had a couple of "energy assessments" over the years. A guy comes out and does an inspection of the house and then makes suggestions for improvements. They always recommend attic insulation, then window/door improvements, and finally exterior wall insulation.

 

[jrmichler]

I think chasing humidity around the house and occasionally using a de-humidifier would be one way of dealing

Well insulated homes have humidity problems in warm humid weather. The AC cannot run long enough to get the indoor humidity down. Some people deal with this by setting the thermostat to a lower temperature, but that leads to a cold house and does not always get the indoor humidity down to a comfortable level.

We have a cheap digital temperature humidity meter in the living room. When the indoor humidity gets over 50%, the dehumidifier is turned on. It's a portable unit in the utility room. The door to that room is left open when the dehumidifier is running. I am willing to let the humidity get up to 60%, but my wife likes to keep it below 50%.

I never piped up the condensate drain because we found that we only needed to empty the bucket once a day at most. Our house is well air sealed (measured at 0.85 ACH50), so the primary source of humid outside air is the heat recovery ventilator. A leaky house is not only drafty, but has a lot of humid air coming in. Our 50 pint unit only runs a few hours on very humid days. If a 50 pint dehumidifier runs most of the day to keep the humidity down, the house is extremely leaky. In that case, air sealing is more important than adding insulation.

 

[sophiecentaur]

I have had a couple of "energy assessments" over the years. A guy comes out and does an inspection of the house and then makes suggestions for improvements. They always recommend attic insulation, then window/door improvements, and finally exterior wall insulation.

I've will have new the windows in a few weeks, am tackling "attic insulation" in stages and the walls need attention to the brickwork and the rendering. It would cost the man-hours to re-render every m²of the walls so now is the time to add some insulation, I think.

The AC cannot run long enough to get the indoor humidity down.

That's a fearsome cost and AC is not that common in UK (yet). Not really justifiable in a country where Net Zero is encouraged. Our indoor humidity is never far above 60C and often significantly lower. Eliminating drafts can improve the feel of a house a lot. Without a chimney and with new doors and windows it's just got to be more cosy. The house survey pointed to several places with rain-based damp and that will have been taken care of within a few months.

The UK doesn't seem to have grasped the Energy nettle firmly and politicians are deliberately fanciful, ignoring the insulation problem in relation to the air source heat pump idea, forgetting that the heat output of their quoted examples is hardly adequate for anything but an eco-home. But it's the sexiest thing around at the moment and that's what they are all going for.

the primary source of humid outside air is the heat recovery ventilator

Not many of those around in my neck of the woods. Comments? Should I buy one?

 

[sophiecentaur]

I guess I'm not completely clear on what you are asking.

As an example, how long before the room walls will get warmer before the heat energy makes it to the outside? They have to warm up to pass the energy outwards - it's just the same process as a chain of (series)R(shunt)C circuits but the values and calculations are not as straightforward.

 

[jrmichler]

Not many of those around in my neck of the woods. Comments? Should I buy one?

When the outside air is better than the inside air, you need to add ventilation. A leaky house gets the ventilation from holes and cracks, all of which cause drafts. A drafty house has excess air flowing in, getting heated, and flowing out. The heating system is heating the outside.

Sealing holes and cracks stops drafts. If you seal the house tight enough, it will get stuffy inside. When that happens, you have a choice between opening windows and creating drafts, or putting in a heat recovery ventilator. Heat recovery ventilators have two blowers. One blower pushes air out the house, and the other one pulls air in. The two air streams pass each other in a heat exchanger that transfers heat from the outgoing air to the incoming air.

 

[sophiecentaur]

The two air streams pass each other in a heat exchanger that transfers heat from the outgoing air to the incoming air.

Yeas, I know the principle and the theoretical advantages but how much room does the system take up and is it scarily expensive?
I guess it all depends where the house is sited.