How to fit rigid insulation in the footings cavity.

[epsilonGreedy]

I have finally given up on blown beads cavity insulation because it is not the done thing in mid Lincolnshire. When 9 out of 10 pro builders look perplexed at the mention of beads and say "isn't that what they retro fit into older properties", when the face of one's chosen SAP assessor turns grey at the mention of beads and when the building control inspector pops over for a social chat and assumes I will be using cavity batts, then the message is clear.

 

I will soon be bringing my external wall up to dpc but do not like the notion of cavity batts drooping down into the nether regions of my cavity. So I now need to consider how to fit some form of rigid insulation sheet in the cavity around the block and beam slab to prevent a cold bridge at the floor and also provide a base for the cavity batts above dpc.

 

A few questions spring to mind following this change of plan from beads to batts:

 

1. Should I fill the 100mm x 240mm high cavity below the beam and block floor?
2. Is the rigid sheet malleable enough to squash around the floor beams which intrude and inch or so into the cavity?
3. What type of sheet should I buy?

[PeterW]

So with 100mm cavity you will need extra internal insulation to meet regs. 

 

Your quickest approach is to use 75mm EPS right butted to the inside face blockwork and then fill the remaining gap with 6:1 sand cement to hold it in place. 

[epsilonGreedy]

18 minutes ago, PeterW said:

So with 100mm cavity you will need extra internal insulation to meet regs. 

 

 

Well yes a 100mm cavity is low-end these days but I think a top spec cavity batt out performs platinum beads? Hence this change should not adversely affect wall insulation.

 

I know you have strong views on a decent minimum spec for new build insulation however looking at the EPC of another new build with a similar design it got a mid Cat-C EPC certificate with Walls 0.28, Roof 0.12 and Floor 0.12. There were no other fancy features in the house to improve the EPC score.

[PeterW]

0.28 won’t get you through DER/TER 

[Jeremy Harris]

Good cavity batts have a λ of around 0.036 W/m·K to 0.037 W/m·K.  Decent bonded blown beads have a λ of around 0.033W/m·K, so out perform batts by around 10% or so, but there's not much in it.  As @PeterW says, your probably not going to even scrape through with batts in a 100mm cavity.  Not sure you would even if you used blown beads, either, TBH.  In real-world terms, batts probably perform worse than their spec indicates, as fitting them so they have no gaps is challenging, and any gaps will promote air movement from the warm to the cool side, reducing the insulation effectiveness.

[epsilonGreedy]

36 minutes ago, PeterW said:

0.28 won’t get you through DER/TER 

 

 

This suggests that a new build can get a middling EPC band C assessment but still fail the CO2 emissions part of the assessment.

[PeterW]

Yes correct. 

 

What are your DER and TER calculations ..? 

 

These are the model values that your DER would be assessed against (the TER) and you have to equal or better the score overall 

 

 

Your design with 0.12 on roof and floor cannot make up the 0.1 difference on walls

[bassanclan]

I can't say if they will come to Lincolnshire, but I used blown beads from Miller Pattison who are in Sheffield. They came to Warwickshire for that job.

[epsilonGreedy]

38 minutes ago, JSHarris said:

Good cavity batts have a λ of around 0.036 W/m·K to 0.037 W/m·K.

 

 

I have been looking at Drytherm-32 though not yet priced it up for the whole build. I am sure you can guess the U-Value at 100mm, if not here is the product page https://www.knaufinsulation.co.uk/products/glass-mineral-wool/earthwool-dritherm-32-ultimate

 

Medium blocks plus Drytherm-32 would get me to an overall wall U value of 0.26. I am still considering 30mm to 40mm of insulated plasterboard on the inner wall which should get me under 0.2 I hope..

[epsilonGreedy]

9 minutes ago, PeterW said:

Yes correct. 

 

What are your DER and TER calculations ..? 

 

This is on my winter to-do list.

[epsilonGreedy]

46 minutes ago, JSHarris said:

In real-world terms, batts probably perform worse than their spec indicates, as fitting them so they have no gaps is challenging, and any gaps will promote air movement from the warm to the cool side, reducing the insulation effectiveness.

 

 

For the same reason I have an intuitive suspicion that rigid PIR boards used for partial cavity fill are in practice an under performing drafty option.

[PeterW]

6 minutes ago, epsilonGreedy said:

 

This is on my winter to-do list.

 

Errr ... it’s part of your Building Regs submission ..?? Should have been done by the SAP assessor ..?

 

How do you know you’re going to meet regs if you don’t know what you are building to..?? 

[epsilonGreedy]

4 minutes ago, PeterW said:

Errr ... it’s part of your Building Regs submission ..?? Should have been done by the SAP assessor ..?

 

 

I left all that paperwork stuff to my private building control chap. I must be on an "as-built" pathway I assume.

 

6 minutes ago, PeterW said:

How do you know you’re going to meet regs if you don’t know what you are building to..?? 

 

 

If a 100mm cavity is not conformant with thermal building regs the whole industry is in trouble.

[PeterW]

1 minute ago, epsilonGreedy said:

100mm cavity is not conformant with thermal building regs the whole industry is in trouble.

 

It isn’t - they use internal insulated PB, or add points for things such as renewables to get through

 

 

[epsilonGreedy]

6 minutes ago, PeterW said:

It isn’t - they use internal insulated PB, or add points for things such as renewables to get through

 

 

During an informal chat with my future SAP assessor he said I will get a bonus point for my wood burning stove. The house design would accommodate two wood burners though not sure that equates to double points.

 

I expressed surprise at this given the prevailing sentiment about how bad wood burning is.

 

Anyhow thanks for the advice, I will bump SAP calcs up my priority list though my hunch is that 350mm in the attic, 150mm PIR in the floor, 100mm drytherm-32 toppped off with 30mm of PIR lining the inner walls will get me up to a scrapping EPC band B pass.

Edited September 5, 2019 by epsilonGreedy

[the_r_sole]

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Edited September 26, 2019 by the_r_sole

[epsilonGreedy]

2 minutes ago, the_r_sole said:

get a sap calc done as soon as possible with your different insulation values, a quick paperwork exercise at this stage is much cheaper than not passing once construction is complete

 

 

Just found his card on my desk where he wrote "SAP £80 + vat". As you say money well spent.

[Jeremy Harris]

I'm a bit surprised you've not already got a design SAP, TBH.  BC asked for my as-designed EPC as a part of the full plans submission.  The design stage assessment will have picked up if there's a DER/TER issue, so as long as the build meets or exceeds the spec used for that then it should be OK.

 

I have to question quite why anyone self-building would deliberately choose to build to the minimum allowable spec, basically the same as a crap volume build house, though.  The standard in building regs isn't high, and it's pretty easy to exceed it, and get a more comfortable and easy to heat house, for barely any increase in build cost.

[joe90]

I used dri therm 32 in my 200mm cavity, my builder was very fussy (at my insistence, tho in fairness he was very fussy anyway).

[epsilonGreedy]

6 hours ago, JSHarris said:

I have to question quite why anyone self-building would deliberately choose to build to the minimum allowable spec, basically the same as a crap volume build house, though.

 

 

The sequence of events was. I purchased a plot with planning permission in the middle of a highly regarded village subject to conservation area design restrictions. The heritage design approved for my plot was the outcome of a bitter local planning process and there is no realistic chance of any exterior changes, even the front door colour is a planning condition. Given the relatively small'ish house size of 1500 sq ft, a ridge height limit, Georgian sash windows and zero chance of PV there is little opportunity for a substantial revision to the thermal design.

 

6 hours ago, JSHarris said:

The standard in building regs isn't high, and it's pretty easy to exceed it, and get a more comfortable and easy to heat house, for barely any increase in build cost.

 

 

Although much maligned on this forum, a faithful cheat-free implementation of minimum 2013 building regs equates to a home that performs better than 80% or 90% of British homes. The annual heating cost differential for a minimum 2013 regs build and the Passiv standard for my size of home is about £500. I hope to close the gap between 2013 regs and Passiv by:

1. Setting myself an air change target of 2.0.
2. MVHR
3. Exceeding a typical 2013 reg specifications house by 10% for roof, walls and floor.

 

[PeterW]

But 2013 is the wrong version of the regs unless you got applied for Building Regs approval prior to 10th April 2016. After that, irrespective of the design or planning approval,  you have to meet the 2016 regulations. 

 

This isn’t a choice ..! It’s a requirement ..!!

[ProDave]

13 minutes ago, epsilonGreedy said:

Although much maligned on this forum, a faithful cheat-free implementation of minimum 2013 building regs equates to a home that performs better than 80% or 90% of British homes. The annual heating cost differential for a minimum 2013 regs build and the Passiv standard for my size of home is about £500. 

What is the annual predicted cost of heating this house?  

 

Planning conditions can stipulate what a house looks like but not how the walls are built and how much insulation is fitted.  Having to have sash windows will impact air tightness, but that is about all.

[epsilonGreedy]

24 minutes ago, PeterW said:

But 2013 is the wrong version of the regs unless you got applied for Building Regs approval prior to 10th April 2016. After that, irrespective of the design or planning approval,  you have to meet the 2016 regulations. 

 

This isn’t a choice ..! It’s a requirement ..!!

 

 

This is incredible. Those new target fabric figures represent a near doubling of insulation in the all the major components of a house.

 

In comparison the jump from 2001 to 2013 regs was a raising of insulation targets by just 6%. Your table is showing targets rising by 80%. Something does not add up here because your table would show that with a stroke of the legislative pen the Government mandated Passiv new homes for all.

Edited September 5, 2019 by epsilonGreedy

[PeterW]

It was called a massive wake up that heating houses was causing significant energy based emissions and huge bills for homeowners. 

 

The 1990s regs saw 50mm of fibre in a wall as a “good” standard - now, 150mm is a bare pass. But in terms of carbon efficiency, the savings on both CO2 emissions and cost of heating is significant. 

[Jeremy Harris]

8 hours ago, epsilonGreedy said:

 

 The annual heating cost differential for a minimum 2013 regs build and the Passiv standard for my size of home is about £500. I hope to close the gap between 2013 regs and Passiv by:

 

 

 

That £500 is more than our total energy bill for a year, including charging my car...

 

 

 

8 hours ago, epsilonGreedy said:

 

This is incredible. Those new target fabric figures represent a near doubling of insulation in the all the major components of a house.

 

In comparison the jump from 2001 to 2013 regs was a raising of insulation targets by just 6%. Your table is showing targets rising by 80%. Something does not add up here because your table would show that with a stroke of the legislative pen the Government mandated Passiv new homes for all.

 

 

Exactly.  This is a very long overdue change that brings UK housing part way up to the standards used in places like Ireland and other parts of Western Europe, although it's still not exactly great.