What type of insulation to minimise thermal bridging in this design?

[Great_scot_selfbuild]

I’ve received this design through as the proposed edge detail for the foundation to timber frame. I’ve only just received this, so haven’t had the answers back from the architect and so I thought I’d reach out here to you good people.

 

I’ve attached the version with my comments (and observation of a puzzling measurement discrepancy). He’s shown the insulation being continuous with the wall, apart from a DPC layer, whereas in fact the wall above the DPC is a modular timber frame (Larsen Truss twin wall). That wall is U=0.11. We’re going for 150mm PIR floor insulation (0.14U) as a cost-based decision and also to avoid adding a lot of height to the house - it’s already got to be raised off the ground, so every bit counts.

 

Specifically I’m looking to understand the most sensible option for the insulation on top of the ring beam steel, but happy to take any other comments/questions. 

 

Thanks!

 

(after-edit: the ‘90x25 batten’ has 28mm written as the height distance and then this section adds up to 208mm, but aligns with the 215mm floor depth. Have others had to pick up discrepancies like this from your designers?)

Edited June 14 by Great_scot_selfbuild
added detail

[JohnMo]

I would dump the 90 x 25 batten and add another 90 x 90, bring the floor insulation up to 200mm. Then 70mm screed.

 

Insulation mineral wool

[G and J]

I know from my own agonising how tempting it is to shave every mm to achieve a perceived vital statistic.

 

But standing back from my design is something I find hard sometimes, so I feel for you.

 

You've a brilliantly insulated wall design, so the potential heat loss through the floor to the ventilated void is unfortunate.  If you are consciously trading UFH running cost for height then that’s your right.  (But best done consciously though).

 

By a rough calculation adding the extra 50mm PIR in the floor will achieve a similar u value to your walls.  I’d be tempted.  

[Great_scot_selfbuild]

@JohnMo, @G and J Thanks - these comments are exactly why I post here. Really useful to get an objective external check/opinion. Just off to ponder...

[Nickfromwales]

If that’s a block and beam foundation then have you considered an EPS block vs the concrete blocks?

 

When you factor in logistics etc, the uplift can often be justifiable.

 

Beamshield videos show examples, but prob others out there at different price-points. 

[Great_scot_selfbuild]

2 hours ago, Nickfromwales said:

If that’s a block and beam foundation then have you considered an EPS block vs the concrete blocks?

 

When you factor in logistics etc, the uplift can often be justifiable.

 

Beamshield videos show examples, but prob others out there at different price-points. 

@Nickfromwales first I’ve heard of these. Not even mentioned as an option by architect or SE. My initial searching indicates EPS is in fact cheaper on material cost as well as labour?

 

Would I need to protect the underside from potential animals? (It’s a raised floor above ground) 

 

Have you got experience of using it?

[Nickfromwales]

1 hour ago, Great_scot_selfbuild said:

@Nickfromwales first I’ve heard of these.

 

Would I need to protect the underside from potential animals? (It’s a raised floor above ground) 

 

Have you got experience of using it?

No direct experience myself, (yet), but I'd be buggered if I would be installing concrete blocks in my own B&B foundation.

 

I have currently spec'd this for a client, where I intend to lift the blocks out of the existing B&B and replace with EPS system, and then use more of it in the wrap-around extension.

 

As for rodents, I guess a question for the manufacturers, but I haven't heard of any issues (when discussing with others on the trade shows/events).

 

1 hour ago, Great_scot_selfbuild said:

Not even mentioned as an option by architect or SE. My initial searching indicates EPS is in fact cheaper on material cost as well as labour?

Yup, I remain unsurprised when these folk fail to make any major effort to get off the tracks they've been cutting for the last however many years. Looking at one clients plans this week, and in the first (FOC) chat I had with him I listed about 6 faux-pas; a few were embarrassing oversights tbh which didn't make the chap very happy, esp when I heard how much he'd been charged!!

 

I am often discussing projects and opening peoples eye's as to 'what's out there', and it bemuses me to hear that a lot gets overlooked / under-discussed tbh. A lot of the UK's professionals around self build clientele are still walking on all 4's it seems..... A few good ones out there obvs, but they're the exception IMHO which is sad and frustrating, and often expensive! Most people are just missing a second set of eyes and ears, and ideas for alternative modern build methodology.

 

The cost-effectiveness is one thing here, but also this could leave you the additional internal floor to ceiling heights you wish to preserve. Win-win , but also you could DIY this with relative ease if the setting out is done meticulously.

[JohnMo]

If you include wood as an insulator, and with the cavity is insulated there is no thermal bridge to speak of. So not sure what the EPS blocks bring to the party.

[saveasteading]

I'm of  the opposite opinion. I would not want the soffit to be of eps or any other soft material. Concrete blocks for me.

Rodents, damage from anything being stored under there, kids playing and picking at it.

Plus I've had mobile platforms run on beam and block. It is strong even if it cracks.

 

I'd also be inclined to add 50mm of floor insulation and raise the ceiling....you have this one chance and get the cost back in 5 years.

 

But you will lose more heat from the wind under the floor. Are you going to enclose that area? 

 

[Kelvin]

On the discrepancies we hardly had a drawing that didn’t have a mistake on it. The architect even submitted the wrong drawings for planning (had to be re-submitted causing an 4 week delay) and an old version of the drawings for the building warrant (I’ll have to submit a variation when we get to final sign off)