I am working on an internal / marketing document where I would like to address or dispell common preconceptions and misconceptions with regards to SIPs.

I'd like GBF input please.

Please let me know your hates, concerns and /or other potential negative comments with regards to SIPs.
Be as bitchy as you like as I hope to address them / educate potential doubters' (if possible).

EDIT: please include any actual limitations, in your view with regards using SIPs.

For example a common preconception maybe that SIPs require a huge crane onsite, thus causing potential access issues and increased build costs?

I do not like the basic philosophy of combining structure with insulation, future problems will result for extensions, alterations, adding insulation, potential problems with pests/vermin,

Insufficient insulation in the first place,

I think there must be some old threads on SIPs too as I have commented on them before.

Presumably the actual limitations are of primary interest here, not the misconceptions...

My reading of the above posts by Steamy, tony and Gavin is that these are actual limitations or problems.

As I said, GBF-ers are likely to be primarily interested in actual limitations. (And yes, I do understand the meaning of what I've just written.)

how about:

designs amendments can't be made onsite
lack of flexibility with extensions/new windows etc
long lead times
complexity for thermal bridging calcs
can be damaged by weather if exposed during construction
require 'specialist crews'
fire risks during construction
difficulty for mortgages/insurance

Have a look at the UKSIPs association technical bulletins on SIPs. They might help you with a few questions/answers (perhaps not!).

A lot of the problems/misconceptions listed above are entirely dependent on the system. Depending on the design, modifications or changes *can* be easy (they can also be really hard and destructive). Thermal bridging calcs are no different to any other system, although truly knowing the quantity of timber in the structure can be hard.

One big thing to remember is that ALL SIPs are proprietary products, i.e. there is NO generic SIP system.

Also, for me, one of the big mis-conceptions is about air tightness - SIPs are not inherently air tight. It depends on the system and the detailing. Also the mis-conception that SIPs don't need VCLs and that they can have metal roofing systems placed directly over them.

Ok, although I know we have discussed this before...

VCL - Dew point calcs on SIPs can be a bit hit and miss when it comes to predicting condensation risks. In my experience of running calcs they can be made to pass or fail depending on the specific resistances of the foam and OSB that you use. Typical moisture vapour resistance values quoted for OSB are a bit on the low side, and actual tested values tend to be a bit higher. If you use higher resistance values for OSB it makes things a bit worse.

It also depends what insulation materials are used in the SIP and their moisture vapour resistances as well.

As I say, generally speaking calcs show that you do need a VCL. Also it is worth noting that most 3rd party certification for SIPs requires VCLs. VCLs also helps greatly with air tightness as some manufacturers don't seal the panel joints (although they should).

As for metal roofing - SIPs are cold roofs and so need a ventilation void between the SIP and any vapour impermeable roof covering. If using metal roofing, there should be some battens, ventilation and then the metal roofing on a suitable carrier. Without the ventilation void, condensation can occur leading to decay of the outer SIP skin.

I know we can debate the VCL point but I am firmly in the camp of VCL is required until I see something that proves to me that it isn't.

Posted By: borpinI do worry that any VCL risks trapping moisture behind it and actually making the situation worse.

I feel the same, is there a recommended method of fitting exterior and interior finishes to SIPs (another thread maybe).

Borpin - the foil faced insulation board will themselves act as a vapour control layer only letting moisture vapour through the gaps. If the gaps are sealed, you will have a fairly effective VCL anyway (certainly more than 5x the resistance of the OSB and EPS core (if using EPS)).

Viking - Sorry, I am going to have to politely disagree with you, although I know that angle you are taking about diffusion verses bulk movement. I have seen condensation issues arise (with my own eyes) with incorrectly constructed roofs (although not SIP construction specifically). If we are constructing buildings designed to last, why take such an obvious risk?

Posted By: TimberBorpin - the foil faced insulation board will themselves act as a vapour control layer only letting moisture vapour through the gaps. If the gaps are sealed, you will have a fairly effective VCL anyway (certainly more than 5x the resistance of the OSB and EPS core (if using EPS)).

Yes I do get that but it does actually concern me!!! Still I have the recommendation from the manufacturer (Kingspan) that this is an acceptable buildup!

It will be sealed with Soudal Fix & Bonds (great stuff!) although there will be gaps where the MVHR ducting is.