Timber Frame "to Passive or not to Passive" that is the question

[Red Kite]

Hi, we were just about to push the button on an MBC TF and because of the site constraints we were going to go with their Open Panel frame with extra insulation with an external wall of about 320 mm. Comprises of 140mm Knauf Eathwool and 80mm PIR.  This has a U=0.11 and air tightness of 3 AC/hr - not bad but given the choice we might have gone with their Passive wall at about 430mm U=0.12 and air tightness of 0.6 AC/hr - but it just wouldn't fit on site. Obviously we could have reduced the internal size to accommodate but the internal layout was pretty tight and we didn't want to give up the internal space so the Open Panel was a pretty reasonable compromise, and cheaper. Also we would not want to go for PH certification, and its pretty unlikely we would get it if we tried.

 

Well it would seem that we MAY be able to ease the site constraints and get the PH walls in on the site by growing the external footprint a little and keeping internals the same :- we have a shared sewer and when we started (over 4 years ago) the water co was very tight on 'thou shalt not build closer than 3m without our permission'. It now seems that the rules have relaxed and we could get to between 2m and 0.5m from the sewer as long as our foundations are below the sewer invert invert, which they will easily be. There might need to be some discussion but it looks like the water co. would agree. We might also need to talk to the Planners because the footprint will expand slightly, and given the rough and long ride we have had to get PP its not something I want to re-open without good reason.

 

So now the question - do we stick with the Open Panel or go to Passive? The extra cost of Passive TF is about 17% (another £12k). Not an insignificant sum but in the scheme of the entire build not a lot - given that we had already planned for 3G windows MVHR etc so no other obvious up-spec items. Or could I down-spec anything going for Passive walls? Anyone any thoughts about this? What would be 'good enough' and is it worth the pain and expense of the thicker and more expensive walls (which as they are cellulose have a better decrement delay - which thanks to this forum I have learnt is a 'good thing'). And any thoughts on the 'payback' for the better air tightness?

 

Thx.

P.S. anyone know why a 320mm wall has a U=0.11 is seen as 'worse' and cheaper and a Passive Wall of 430mm wall has U=0.12 and is 'better' and more expensive?

 

 

[Ian]

11 minutes ago, Red Kite said:

anyone know why a 320mm wall has a U=0.11 is seen as 'worse' and cheaper and a Passive Wall of 430mm wall has U=0.12 and is 'better' and more expensive?

 

 

the passive wall option will have lower heat losses overall due to better detailing for thermal bridging and a much better air-tightness. At a guess the difference between the two could mean approx a 30% to 50% saving in your heating bills.

 

Edit - it's worth doing the maths on your potential heating bill savings.

I had a similar decision on a small holiday home that I was building a few years ago on whether to go for a passive spec ie 0.1 'U' values and as air tight as I could make it or just go for meeting Building Regs which for my build was 0.15 average 'U' values and no special measures to achieve an air tight build..I achieved 3.7m3/h(h.m2). The final build is completely draught free and easy to heat.

The difference in heating bills between the 2 solutions using bulk LPG was less than £100/year.

Edited February 28, 2019 by Ian

[A_L]

Any explanation for the relatively poor 3ach? Can it not be upgraded? It would be your most rewarding upgrade.

 

Edit:- Assuming MVHR in both cases then reducing from 3ach to 0.6ach looks like about a 35% saving on total heating bill

Edited February 28, 2019 by A_L
add second line

[Jeremy Harris]

Another thing to consider is whether the longer decrement delay, plus the much better sound insulation, of the blown cellulose walls and roof would be an advantage to you.  For us, the longer decrement delay is a very definite advantage, as we're in a relatively sheltered spot that tends to be a degree or two warmer than the average for the area.  The longer decrement delay does improve the comfort level a fair bit, as it slows down the rate at which the temperature in the house changes in response to external temperature changes.  The last few days have tested this, with temperatures during the day rising to nearly 20°C, and the outer face of some walls and one face of the roof getting to well over 30°C in the bright sunshine, then cooling rapidly overnight to close to 0°C.  We've had no heating on for around 5 days now, as the house is just staying warm overnight from the long thermal time constant.

 

[scottishjohn]

1 hour ago, Ian said:

anyone know why a 320mm wall has a U=0.11 is seen as 'worse' and cheaper and a Passive Wall of 430mm wall has U=0.12 and is 'better' and more expensive?

i would ask your supplier to explain this .lower the number the better it should be

how ever both those numbers are good

 

does not seem correct .

[ProDave]

I am puzzled why the more basic option has such a very much worse air tightness "specification"?

 

Surely that is down to detailing, and if nothing else YOU can detail that by taping where apropriate, or even adding extra air tightness membrane where necessary.

 

At the moment it sends the message "we don't take so much care" with the more basic product.

[Jeremy Harris]

17 minutes ago, scottishjohn said:

i would ask your supplier to explain this .lower the number the better it should be

how ever both those numbers are good

 

does not seem correct .

 

The U value difference is trivial, and has very little overall impact, far less than that of the improved airtightness.  It's purely down to the difference in λ between the various insulation types, and λ isn't the only significant property of insulation, decrement delay is at least, if not more important.

 

13 minutes ago, ProDave said:

I am puzzled why the more basic option has such a very much worse air tightness "specification"?

 

Surely that is down to detailing, and if nothing else YOU can detail that by taping where apropriate, or even adding extra air tightness membrane where necessary.

 

At the moment it sends the message "we don't take so much care" with the more basic product.

 

The answer to that is because the blown cellulose does a pretty good job of making the house airtight, so reduces the need to do lots of time-consuming sealing up, all that's needed is to secure the ready-fitted membranes around the joists, tape all the internal joints and tape the door and window frames, which is a pretty quick job.

[bissoejosh]

I'd save the £12,000 and spend a small amount upgrading (if it's not already in the spec) the internal OSB to an airtight version + some tape. This should make DIY airtightness as straightforward as possible.

[Oz07]

Fwiw I had a similar situation with planners and they let me use a non material amendment to explain walls were growing but internal volume remaining the same

[ProDave]

3 minutes ago, bissoejosh said:

I'd save the £12,000 and spend a small amount upgrading (if it's not already in the spec) the internal OSB to an airtight version + some tape. This should make DIY airtightness as straightforward as possible.

Most of it is easy, I used the Protect Barriair air tight membrane and Tescon Vana tape.

 

However it was thanks to my builders that some of the details were made easy.  It was they that suggested installing what has been referred to on here as a "tony Tray"  That is basically a length of air tight membrane that goes from the inside of the wall downstairs, around the ends of all the joists, and back to the inside of the wall upstairs.  That was easy to install as they put the joists in and made the process or sealing the building easy and effective.

 

If the MBC budget package omits things like that, then you are going to be left with a whole load of joist ends to tape up individually.

[lizzie]

I have the 'other' walls  U value 0.11 my air test was 0.58  ACH@50pa (designed at 0.60 ) ......my walls are a bit thicker than the original spec as the new supplier had to provide a different thickness internal insulation board to get to the u value.  MBC guaranteed an air tightness of 0.60 in my contract, they almost got there, guess the small margin is acceptable. 

 

As Jeremy says there was a lot of sealing up for us to do, for the airtight test the testers just stuck tape where it was leaky and we were told to seal up properly afterwards.  Took my men on site ages and a lot of sealant etc to actually get it properly airtight after the test. It certainly feels airtight now, no draughts anywhere, and this has been commented on by visitors.

 

I was not aware of price difference of one wall system to the other.  I got a quote on what I was told was the best one for the job..........I had seen an MBC house some time before and the very deep window reveals with small windows put me right off, was very dark, the slimmer wall profile was chosen for that reason as far as I can remember.

 

I had never heard the term decrement delay until this forum which came well after the frame was ordered.  For all that my house is quiet and warm, our heating has not kicked in for days, the MVHR is holding it at about 22 degrees which is ok, humidity way too low though.  Enthalpy unit being fitted in two weeks so hopefully that will solve that issue.

 

 

 

 

 

[Jeremy Harris]

Just now, lizzie said:

I have the 'other' walls  U value 0.11 my air test was 0.58  ACH@50pa (designed at 0.60 ) ......my walls are a bit thicker than the original spec as the new supplier had to provide a different thickness internal insulation board to get to the u value.  MBC guaranteed an air tightness of 0.60 in my contract, they almost got there, guess the small margin is acceptable.

 

 

Sounds like they exceeded the spec to me, not "almost got there".  0.58 ACH is better than 0.6 ACH.

[lizzie]

2 minutes ago, JSHarris said:

 

 

Sounds like they exceeded the spec to me, not "almost got there".  0.58 ACH is better than 0.6 ACH.

is It? LOL I know I am thick but no-one told me that........thats why they seemed pleased!!

 

Scuttles off to corner to put on hat with large D!

[Dreadnaught]

@Red Kite, I have been discussing the various wall systems with MBC for my forthcoming build.

 

You may be interested to hear that I have been offered a guaranteed 1.0 ACH/hr at 50 Pa for the 0.11 W/m².k, 140 mm open-panel wall. I asked if they would guarantee 0.6 ACH and they said no, that's only with their 300 mm passive-standard frame.

 

Which roof make-up are you thinking of choosing? I am going for the pumped-cellulose filled roof option, 0.10 W/m.²k, which given that mine is a bungalow should go a long way of giving me much of the benefit of low decrement delay.

Edited February 28, 2019 by Dreadnaught
Corrected a roof typo.

[lizzie]

@Dreadnaught I have the 0.10 pumped cellulose roof and the  0.11 walls and they gave me the guaranteed 0.60 air tightness....I wonder why they have now changed that?

[Dreadnaught]

2 minutes ago, lizzie said:

@Dreadnaught I have the 0.10 pumped cellulose roof and the  0.11 walls and they gave me the guaranteed 0.60 air tightness....I wonder why they have now changed that?

 

Good point. I might well revisit this subject with them before I sign on the dotted line.

[newhome]

3 minutes ago, lizzie said:

I wonder why they have now changed that?

 

Possibly because it’s easy to miss 0.6 so gives them a lot more leeway? Doesn’t mean it won’t meet 0.6 though, just means they are only guaranteeing 1.0. 

[Jeremy Harris]

It shouldn't be hard to get down to the PH standard of 0.6 ACH with the blown cellulose roof, as the roof is the part that really does benefit the most from having the benefits of blown cellulose, IMHO, both because warm roof airtightness can be an area that's a bit harder to ensure and because much of the advantage in using a longer decrement delay insulation will be felt in the roof, as there's little heat capacity in the roof covering.

[Gone West]

13 hours ago, Dreadnaught said:

You may be interested to hear that I have been offered a guaranteed 1.0 ACH/hr at 50 Pa for the 0.11 W/m².k, 140 mm open-panel wall. I asked if they would guarantee 0.6 ACH and they said no, that's only with their 300 mm passive-standard frame.

I ran my house design through PHPP with varying air tightness values from IIRC 0.2 to 2.0 and the difference in heat loss between 0.6 ACH and 1.0 ACH was very small.

[Red Kite]

Thanks for the thoughtful replies. The MBC Open Panel does include blown cellulose in the roof so thats a bonus. With the walls its a complex cost/benefit debate and my gut feel is that for the house we plan to live in the the PH Walls would be preferable. It is also a debate that this house has a basement and that the levels of insulation below ground need to be similar to those above ground - so how much insulation (and airtighness) is achievable for both areas. And having been in an MCH house with the PH walls (thx Vivien) the level of calm and solidity was palpable - regardless of the insulation / airtightness it just 'felt' right.

 

In terms of implementation  I talked to my Planning Consultant (who has been brilliant BTW - PM me if you want her details) and her advice was: First talk to Wessex Water and see if they would be happy and then talk to the Planning Officer and see what they think. If that seems OK. Then apply for a Variance of Conditions to change the passed plans - likely to a bigger change  than the scope of non-material amendments and better than a new Full App which would open the door to a complete reconsideration - and lots of contention.

 

Will keep you posted - all I can say is that 'its complicated!'