Hello from the Wild West(country)

[D_T]

Hello all,

 

I've been over on the forum (GBF) for some years now and thought it was now time to join this one too as we have a self build about to go live.

In brief it's a new-build bungalow (with mezzanine) on a rural site near Bath. Plot bought with full planning permission which I amended to get elevations more favourable to PHPP. Aiming for Passivhaus standards despite a tricky form heat loss factor (4.2) due to the aforementioned planning consent (pitched roof, L-shaped plan). So U-values of 0.07 to walls and roof, 0.08 to slab.  Construction is CLT frame on isoquick raft, rockwool insulation (vermin and arson concerns) between Larsen trusses, with timber rainscreen to walls and slates to roof.

 

All electric too as no gas nearby - I'm intending to use electric towel rails to provide space heating and two separate SunAmp units to provide localised DHW. I'll start a separate topic on this, but if anyone has any tips on how to model the SunAmps in PHPP I'd be really grateful!

 

Thanks in advance,

 

T

[Guest Alphonsox]

Welcome to the forum, Sounds like an interesting project.

Despite owning a couple of SunAmps I have no idea how the would be modelled in PHPP. Maybe as an instantaneous water heater alongside a small constant heat source ?.

[IanR]

Welcome D_T, looking forward to seeing details of your build to achieve those U Values.

 

You mention CLT and Larsen Truss, where is each of them being used? or is it a composite of the two systems? (I may not have the understanding of each that I thought I did)

[jack]

Welcome!  

 

Interesting build method.  I looked into CLT as a construction method when we were planning our build a couple of years ago, but the lack of local knowledge and difficulties with estimating costs made me a bit nervous.  

 

I'm assuming a CLT core with Larsen trusses outside?  It'd be really interesting to get some details of what you're doing and what you learn as you go along.  We may need to put in a CLT subforum! 

[Nickfromwales]

Hi and welcome. 

Good to see the SAPV's are starting to become a bit more popular. . 

Do you have a plumbing schematic yet?

 

[ProDave]

Hi and welcome.

 

I to am interested in the CLT frame (I think that CLT sub forum is needed PDQ)  Is the CLT because you want the wood to be the finished internal wall surface?  I saw one build like that, then clad in wood fibre board for insulation. To say the installation of the services was "challenging" would be an understatement.
 

[D_T]

Hello folks,

 

So many replies already!

 

CLT choice was a mixture of personal preference (aesthetic/reduced maintenance appeal of exposed inner surfaces) and the more practical robustness and airtightness qualities of the solid material. I subsequently found that our soil conditions and the isoquick raft approach meant that we would likely require a lightweight building anyway. Throw in some access issues (TPO'd trees, shared driveway) and the idea of offsite prefabrication was also appealing...

 

ProDave we have enough built-in storage ideas that I'm not too worried about hiding service distribution and we're even open to the idea of exposed galv conduit & MVHR ducts if we have to, although I may end up putting a ceiling in just to reduce the amount of exposed timber surfaces!

 

IanR, jack is correct, the CLT is the inner skin (equivalent of a v. thick OSB sheathing board). The Larsens (or likely Steico I-Joists) then just act as non-structural spacers to hold the cladding out far enough (450mm!) to allow cladding the CLT in dense rockwool. I've got some detailing challenges around the base of the trusses (to stop them wicking water) and I remain a little worried about freeze/thaw cycles on the outer chord since this is effectively uninsulated and exposed to moisture variation. Hoping to model this in Psi-Therm/WUFI to see if it's a serious issue but I don't know either package so will be feeling my way a bit... So far no-one appears to sell tannalised I-joist equivalents of tile battens.

 

Nickfromwales, I have a very scruffy hand drawn plumbing schematic which I hope to CAD up soon and share. In the meantime I've attached an older diagram that explains the basics. Essentially the bungalow format means all water distribution is on the ground floor and horizontal pipe length is the biggest issue. Originally I was looking at doing all this from a central point in the Utility room (as per the diagram) but realised it would probably be much better to just split the SunAmps and have the secondary one standalone in the en-suite.

 

Alphonsox, thank you for the suggestion of modelling the SAPV's as an instantaneous heater. This is what I've done so far, but it ignores the issue of PV generation to supplement the electricity use. As such, the carbon factor of the heater is fairly detrimental from a PHPP aspect (i.e. it assumes all grid provided) and probably worse than it would be in reality.

 

Thanks for all the responses so far.

 

[ProDave]

When I talk about "services" I mean electrical wiring and plumbing?  You have no "service void" for cables. As the CLT is your finished surface, you have to VERY early on decide where EVERY switch and socket is going and cut the holes.  Then where do you run the cables? not in the insulated wall otherwise derating will apply and a lot of over sized cables. You could (this is what the house I saw did) run them out in conduit to (almost) the outside skin of the house to avoid derating (questionable as the bit to get through the wall probably still needs derating)  Then what if you change your mind, want an extra socket, want something moved? an almost "unmodifiable" installation.

 

That needs some serious thinking about (unless you really want the whole house wired in surface galv conduit)

 

 

Don't under estimate the weight. The house I saw was a modular off site build and they were scratching their heads to find a hiab truck that could lift the sections. It was much heavier than any other construction method they had used before.
 

[jack]

There's an interesting Grand Designs using CLT (which I'm sure @D_T has seen):

 

 

 

I seem to recall a bit where he decides to go it alone on the panel erection after splitting with his project manager.  There's a really hairy bit where he and a mate are trying to put up a roof panel on a rainy dark night - terrifying stuff!

[Jeremy Harris]

Welcome, sounds like an interesting build. 

 

The prevention of interstitial condensation can be a bit of a challenge, as the standard models tend to only cope well with steady-state, or at best slowly varying, conditions, and not with the sort of rapid variation in external temperature and humidity that we can get quite often.

 

I looked long and hard at this, as our frame uses modified Larsen truss construction, but with cellulose insulation.  The two key things seem to be to ensure that the vapour permeability outwards through the construction layers is always good (i.e. most vapour permeable layer on the outside, least on the inside) and that the most likely locus for condensation, the sole plate, is insulated well from the ground beneath and has a low heat capacity, so less energy is needed to warm it up and drive vapour back out. 

 

Not having an outer skin with a high heat capacity helps, too, as the worst case seems to be a cold, damp, night, followed by a warm sunny morning where the relatively high external skin temperature can create high local humidity next to the structure and this will then tend to move towards any cold areas and might condense out.  Once liquid water is present, it takes quite a bit of energy (heat) to drive it back out again.

 

It seems to have taken some of the suppliers of things like SIPs panels a while to spot this, as they were still giving out an approved detail for the sole plate to foundation junction, that made interstitial condensation under rapidly changing conditions a near-certainty, until relatively recently.

 

[D_T]

ProDave, thanks for the point on services. I should have been clearer - we went to see a similar CLT passivhaus in Hackney whilst we still lived nearby and decided that galv conduit surface mounted on the walls is not the end of the world as long as we locate it carefully. Our layout (below) means we generally have a 'servant space' (utility, ensuite etc.) adjacent to a furniture wall in a living space (bedrooms etc.) so I'm not that worried about locating drops etc. at this stage

 

JSH, thanks for the pointers on vapour movement etc. I think our insulation envelope is fairly consistent around the building, although I'm struggling slightly with the base of the Larsen trusses and whether they can just sit on rigid insulation as the detail below. I have been assuming XPS here due to the higher risk of saturation at this level but this doesn't really sit well with your point about condensing outwards etc. Self-adhesive DPM on the outer face of the CLT should keep this dry and separated from any external water (plus it forms a backup airtightness layer at junctions.

 

jack, thanks for the weekend viewing. No intention of going it alone on our build - professionals with a mobile crane all the way! They have attended site already and we've agreed a construction plan for this phase.

[Jeremy Harris]

FWIW, I did some rough-and-ready calcs on probable vapour movement for the outer edge of our Larsen truss, where 200mm of it sits on a DPM that's on top of the EPS upstand around the passive slab.  The vapour permeability plus the relatively low heat capacity meant that there was an extremely small risk of that area getting near dew point, even with some pretty severe local external  temperature and humidity swings, and that was with a higher decrement delay insulation than you're using, so I think your risk is probably even lower.

[angusmorgan]

Hi @D_T , how did your build go/ is going? I am interested as we are planning a CLT construction too not too far from you.