Are wide cavity walls worth the extra expense?

rockabaloo

Following on from a discussion toward the end of this thread -

http://www.boards.ie/vbulletin/showthread.php?t=2056511720&page=3

I just wanted to pose the question in the title.

I had kinda hijacked the other thread so thought it best to continue the discussion in this one that is dedicated to the question.

rockabaloo

The main posts were:

rockabaloo wrote: »

Is there really any economical incentive of building a house with a cavity this wide. I know there are definitely environmental incentives but purely from a fiscal point of view?

I did some rough calculations for a house the size you suggest. These figures are based on SAP calculations as used in the North.

The heat demand for a year for the house with a 100mm full filled cavity is roughly 14957 kWh/year. For the house if it were built with a 300mm cavity then the demand would be 12987 kWh/year.

This is a difference of 1970 kWh/year.

If mains gas costs 5 cents per kWh then the house with only 100mm cavity insulation would cost an extra 98.50 euros a year to run. If you take a 25 year span then the difference would be 2462.50 euros.

The difference in cost of insulation between the 100mm build and the 300mm build would be 2400 euros. Then add in the extra foundations, wall ties, workmanship, etc. And on top of that for most people the extra 2400 euros at the start of the build will be mortgage money. So it will cost significantly more than what would be saved in heating bills.

My calculations are based on only an average air tight house. So obviously big savings can be made if this were to be improved.

Does anybody have an opinion on this? Am I wrong completely? Are my calculations way off?

I don't mind being wrong so criticise away. I just wanted to ask the question.

dfader wrote: »

Well lads,

Finding this discussion very interesting so I did a calculation myself to find the cost of the heat lost through external walls per annum. I had to refer back to higher leaving cert Construction studies for this . It seems to be backing the claim that the savings are not there which is annoying me becuase I have my heart set on a wide cavity.

I used U value of 0.2. external wall area of 230m3 - based no my build. Heating using oil and on for 5 hours per day. I got a shocking low answer - see image below. I used the equation shown on the image. I can not stand over this calculation and can anyone find fault with this calculation as I was expecting a much higher answer.

http://screencast.com/t/HQEEF3DU

rockabaloo wrote: »

Hi gooner99, the main problem is the savings do not scale with the extra cost and work of the wider cavity. I'll try to summarise here a bit:-


Size of Cavity - U-value - Heat Demand - Savings/Year
100mm full filled - 0.31 - 14957 kWh/year - €00.00
150mm full filled - 0.22 - 13999 kWh/year - €47.90
200mm full filled - 0.175 - 13519 kWh/year - €71.90
250mm full filled - 0.145 - 13200 kWh/year - €87.85
300mm full filled - 0.125 - 12987 kWh/year - €98.50


These figures are based on the house size given earlier in this post - I think it was 3400 square feet. I guess the smaller the house then the savings would be worse still. The savings are also based on mains gas at a price of €0.05/kWh. The same sort of running costs you should be able to achieve with GSHP or ASHP.

What we are talking about here is the law of diminishing returns. There are decent savings to be made with 150mm instead if 100mm. But the wider you go then the savings become less significant.

The figures are based on average air tightness i.e. air tight enough to pass the building regs in the north. So obviously things could be improved there.

In fact, I think I would rather spend my money on air tightness than the extra cavity. I hate cold draughts in the house so I think I would 'feel' the benefit of this improvment more.



This worries me too. I would like to be wrong and would like the wide cavity to be the answer to all our problems. I'm disappointed if what I am saying is the case also.

I think the wide cavity as a solution only makes sense if we had prolonged periods of extreme temperatures. As in 3 to 6 months of sub-zero temperatures.

rockabaloo

If these figures don't add up then hopefully somebody can disprove.

If that's not the case then it seems that it may be more sensible to redirect money for the build to other areas.

beyondpassive

I'm just Looking at a house here with PHPP, According to the passivhaus method of heat loss calculation for walls excluding windows and doors.

With 250mm silver bead, the walls lose 2,480 kWh per annum plus 250kWh for the perimeter thermal bridge.

With 100mm silver bead the walls lose 5,780 kWh and 340 from the perimeter thermal bridge.

So the 100 cavity loses 3,390kWh per annum, thats €272 per annum.

Thats really not a good return on investment ( 12 yrs) when looked at in isolation, however we are designing the building as a system. We want to get away from intermittent heating and create warm internal surfaces and stable indoor temperature. That way we can make use of our obligatory renewable s which have an even worse return on investment. (14 yrs).

If I can be allowed to waffle a bit. When you also consider the reduced window. door and eaves thermal bridgeing heat loss from the well designed wide cavity, The return on investment drops to about 8 years.

The main advantage is that you now have a very small delta T, or heating system lift. You have higher interior comfort, because your surfaces are warm. You can now use a low grade heat such as your buffer tank, connected to a back boiler stove or solar array to meet a considerable proportion of your heat demand. This reduces the firing time of your boiler and also allows it to be used on low modulations to top up the hot water. In effect the heating system is asking you to provide at least 250mm of insulation. You are designing the building envelope around the comfort perameters rather than having the heating system continuously compensate for the heat wastage through the fabric.

rockabaloo

Thanks for replying beyondpassive. Your data makes for better reading with regard to potential energy savings.

Can I ask what formula you use for your calculations?

I was using (Area x U-value x Degree Days x 0.024). Is this right?

What figure did you use for your degree days?

beyondpassive

Rockabaloo

Transmission heat loss in kWh/an = Area of thermal envelope * U-value * Temperature-correction factor (1.0)* Heating degree hours

Heating Degree Hours: Gt = 63.9 kKh.a (based on:)

• Co Louth climate file from Meteonorm sat.
• Monthly method 89,000 degree hours per year 89kkh/a for Birr or 85kKh/a for Dublin not used
• In my calcs I used the annual method Birr 64.7kKh/a or Dublin 60.1kKh/a
• Deap aand Passivhaus use the annual method. Passivhaus also uses the Monthly method for certain types of building.
• Gt is based on the Sum of the temperature difference between inside and outside at 20 degree inside for the site location.
see slide 7 http://www.constructionstudies.ie/wt4006_03_notes.pdf
• Unit of measurement is kKh/a (kilo-Kelvin hours per year)
• The monthly method calculates the internally measured solar gains where as annual method calculates externally measured solar gains. Monthly is therefore more accurate and is used for high heat gain passivhaus buildings.
•If (a) the space heat demand is below 8kWh/m2 or (b) the ratio of free heat gains to heat losses is above 0.70 in annual method, then monthly method has to be applied.

rockabaloo

That makes interesting reading beyondpassive. Thanks for taking the time to explain.

What price were you using for a kWh of heating energy? I was using €0.05 for the price of mains gas.

Also, the figures are easier to justify for a larger house. If the house was only 150 square metres then the savings would be less. (Yes, the extra build costs would also be less but I don't think it would scale evenly).

beyondpassive

€0.08 or 8 cents per kWh. You have to factor in 92% efficiency, standing charges and VAT, annual service, electric circulation pumps.

Small houses can often be more inefficient than large ones. The ratio external envelope surface area versus usable floor area is called the form factor. Compact shapes are most efficient. Ask a herd of penguins, or whatever the collective term is. So the scaling will be linear if the form factor remains constant.

gooner99

Since wider cavities seem to be much more popular now and indeed probably the most common method for new build, has anyone done any further calculations or have any strong opinions of the real world energy savings of different full fill cavity widths. Is it really worth going from 150mm to 200mm for example given the need for structural sign off, more difficult detailing, along with the need for more wall ties which can introduce more thermal bridging and are more expensive. Does it make more sense in the real world to stay at 150mm full fill and spend the money saved on better floor insulation, better roof insulation, better windows along with good air-tightness.

MicktheMan

gooner99 wrote: »

Does it make more sense in the real world to stay at 150mm full fill and spend the money saved on better floor insulation, better roof insulation, better windows along with good air-tightness.

Or better thermal bridging detailing, ventilation design, etc.

Imo, the only way to competently / confidently work this out is to model the build in phpp (ideally using site specific climate data) and "play" with the different scenarios to see which is most cost effective (while keeping an eye on DEAP for possible part L compliance issues).

gooner99

MicktheMan wrote: »

Or better thermal bridging detailing, ventilation design, etc.

Imo, the only way to competently / confidently work this out is to model the build in phpp (ideally using site specific climate data) and "play" with the different scenarios to see which is most cost effective (while keeping an eye on DEAP for possible part L compliance issues).

Yes I guess there are many factors at play.But in general terms what are the advantages of 200+ over 150?

BryanF

gooner99 wrote: »

Yes I guess there are many factors at play.But in general terms what are the advantages of 200+ over 150?

A Better u-value? Are you looking for someone to tell you it's ok to build a 150cavity?

4Sticks

Generally speaking if comparing 150 cavity + 50 internal dry lining with 200 cavity and no dry lining

Better whole wall U value due to better Psi values
high thermal mass
better internal fixing racking strength

But take on board Mickthemans excellent advice above

bifl

I'm going ufh so want to keep the inner leaf insulation free so I need all the insulation in the cavity

Based on my readings here, I had started out considering a 250mm pumped bead giving a u-value of 0.12.

However as I can get a u-value of 0.13 on a 150mm cavity with a good quality insulation board it was then a case then of weighing up the pros and cons of each and the 150mm cavity won out.

- more traditional 150mm cavity, builders possibly more familiar with it
- need to buy less blocks for the inner leaf in terms of structural support
- less wall ties
- less difficult detailing around windows

4Sticks

Full fill board bifl?

bifl

yes full fill (think the board leaves a 5mm gap )

4Sticks

Be aware that in order to get the calculated U value in real life that you will need very conceincous block-layers

all butt joints to be tight



all joints to be clean



boards to be tight to inner leaf

bifl

Thanks for those 4Sticks these were some of the reasons I was going with pumped cavity initially as I was afraid of the above happening. However when I looked further into pumped cavity there is opportunity for it to be done badly as well and is harder to spot & fix after it's being done. Came across things like not enough glue being used, beads not being put in under enough pressure, under windows etc not being filled.

So on both systems they there is a right way and a wrong way !!
At least with the board I can be there to watch out for the issues you have described when its being installed.

4Sticks

and don't forget - additional dpcs at corners see here page 11

4Sticks

bifl wrote: »

So on both systems they there is a right way and a wrong way !!

True of all aspects of construction. Workmanship can realise or ruin the best of designs.

hexosan

bifl wrote: »

At least with the board I can be there to watch out for the issues you have described when its being installed.

When people state that they will be on site to watch the boards been install & catch any mistakes I find it laughable, do you seriously think that you are going to have the time to stand beside the block layer while he installs board after board after board. Is the block going to want to do the job while your peering over his shoulder for 3/4weeks.

You only have to look away or take a phone call to miss a board going hence why I believe full fill is the way to go. At least if the bead isn't pumped in correctly it can be seen on thermal imaging and extra can be pumped into the void. Explain how you plan on fixing thermal looping when the boards aren't installed correctly.

gooner99

BryanF wrote: »

A Better u-value? Are you looking for someone to tell you it's ok to build a 150cavity?[/QUO

I don't think I mentioned that.What I was asking was at what point does it make more sense to stop widening the cavity and balance it out with more floor, roof insulation along with better windows. There must be a point where you won't save much year on year in terms of heat demand.

4Sticks

Going in circles now. Micktheman answered you several posts back.