Payback on insulation

newbuild2013

Hi

I'm trying to work out the payback period of improved insulation in cavity walls. I'm in the process of a selfbuild. We're at foundation level and starting to make decisions on insulation. We have 150mm cavity, plan to use thermal mass and do not plan on putting insulated slab on inside leaf (many other posts on boards about not going that option)

So for wall insulation we have a few prices but to keep this simple I'm going to only provide 2. The first is €8,500 which will achieve .12 (150mm full fill PIR), the second is €4,500 which will achieve .17 (110mm PIR). So that is 4,000 euro to achieve .05 better u-value in the walls.

I put both values into the DEAP software tool (along with all other figures from the provisional BER) and the 0.12 wall gives us 47.38 kWh/m2/yr where as the 0.17 gives 49.39 kWh/m2/yr.

Now this is where I need help validating my calculations. We are going with Oil for heating and on the SEAI website says oils costs 9 cent/KwH

So I'm taking the 9 cent, multiplying by the delta in the KwH/m2/yr from DEAP (2) and multiplying that by the square meters of the house (280) to determine the extra cost in oil per year by going with the 0.17 uvalue.

So by my workings that an total annual cost of : 0.09 x 2 x 280 = €50.4

Considering the extra capital cost of the insulation is €4,000, factoring in a very high annual inflation of 10% on price of oil, it would take 22 years to payback the initial extra spend on insulation. This seems very high to me.

So my questions are
1. Are my calculations above correct, I'm not confident the total annual cost is correct
2. Are there other considerations such as cosiness of house etc to be considered in this decision or should be it be just about payback period
3. Would I be better putting the 4k into windows (average u-value of 1.26) or elsewhere in the house

We're on a tight budget so 4k is quiet a lot of money however as its something that can't be addresses afterwards I would put it in if it was justified.

(Other details are, we are targeting airtightness of less than 2 and using MHRV)

Many thanks in advance for help. This is not my field of expertise so excuse any mistakes above. I do plan to go back to BER assessor and engineer but wanted to get my head around it first.

rockabaloo

I asked a similar question before: http://www.boards.ie/vbulletin/showthread.php?t=2056811453

I think it's clear there is gonna be a point where the financial viability of extra insulation isn't there. Unless you're planning for 50+ years in the future.

BarneyMc

newbuild2013 wrote: »

The first is €8,500 which will achieve .12 (150mm full fill PIR), the second is €4,500 which will achieve .17 (110mm PIR).

I assume these PIR boards have some sort of 'protrusion' to keep the surface of it from the outter leaf, i.e. 150mm board in a 150mm cavity doesn't leave much room for ventilation!

fclauson

Why do you want to ventilate the cavity ?

On the issue of pay back as per the referenced thread which beyondpassive wrote its much more complicated than just the uvalue of a surface x- cold bridges can account for a massive amount f the loss - think how many linear meters there are of wall to floor and wall to roof through which heat can escape if its not done right

The calcs show a 200 or 250 full fill cavity is about the best with very well detailed junctions

Get it right and you could end up with a very low running cost

Would be interested in how you will comply with part l ?

BarneyMc

I shouldn't have said 'ventilate'. Surely there should be a gap left to prevent moisture coming from the outside skin and getting at the insulation?

fclauson

You don't get that with bead so why with solid board

More of an issue with board is to make sure no moisture can "flow" across badly formed junctions and joins

BryanF

fclauson wrote: »

You don't get that with bead so why with solid board

More of an issue with board is to make sure no moisture can "flow" across badly formed junctions and joins

there is a popular product that has 10mm 'fins' on the cold side of the insulation ie towards the outer leaf, so that an moisture may drip down the cavity.

BarneyMc

BryanF wrote: »

there is a popular product that has 10mm 'fins' on the cold side of the insulation ie towards the outer leaf, so that an moisture may drip down the cavity.

Yes, that's the product I'm thinking of.

fclauson

I looked at this - the challenge is getting every miter junction perfect - if there is a gap you get air and a cold spot which in turn will lead to issues etc

So if perfect then it's ok - if not then .......