SIP wall thickness: diminishing returns?

[joshwk]

Interesting, thanks both. I actually made a separate post on the MVHR sub-forum about this topic as I was trying to determine the pros/cons to both. In the end, we opted to proceed with a rigid branch system using a who have experience working with our appointed SIPs supplier. We were led to believe the benefits of a rigid system was that it's more "hygienic" and PasivHaus themselves opt for rigid ducting in their testing (which may have just been sales-BS).
 

Currently, we've opted for supply only. The MVHR supplier is quoting ~£4K to carry out the install and although I appreciate it's a premium expense, I'm tempted to have them undertake this so there's a single point of contact for any issues in the future (and also avoid the headache of installing this myself). Alternatively, I could ask with my builder if they'd be prepared to take it on (and get a quote), but wonder if people have had positive/negative experience with this?

[markocosic]

On 10/11/2024 at 12:19, joshwk said:

I've noticed the quote from my SIPs supplier hasn't allowed for supply/fix of VCL to walls and roof. Is this something I need?

 

They're saying that the entire inside of the house will be covered in foil faced insulation, and the joints of this will be taped, and that this forms your airtightness layer. (a vapour control layer is not so critical; but it also forms this)

 

Airtightness is bloody difficult. Throw in windows, doors, internal partitions that need to hang off walls, cable and pipework penetrations, battening to hang stuff off walls etc and there's loads of detail to get right. I think they're not doing this; they're just offering to pop up the insulation and tape the seams. (the easy part of the job)

 

Airtightness is essential with SIPs, as any air that does leave he building carries moisture with it that will then condense on the external OSB layer; turning it into wet Weetabix. Most common at the ridge of the roof.