A study in the risk of excessive internal moisture in SIP panel homes

Warm healthy homes
Warm, dry, airtight, well ventilated, resilient, healthy homes are my passion.  

So, it’s no surprise that I’m a big fan of SIPS.

Structurally Insulated panels are inherently more insulating, airtight and quicker to build with.  

There are few downsides to building with true Sips in my view, depending on the products used.  

Recently an article appeared on Linkedin about what happens if SIPs are not made vapour tight from the inside.  https://www.energyvanguard.com/blog/how-to-avoid-the-flaw-in-structural-insulated-panel-construction/ As my fast brain took flight, I immediately reacted with “Huston, we have a problem!” Recently, this was raised again following a post on sealing naked Sips on a build in Queenstown, I’m involved in. https://www.linkedin.com/feed/update/urn:li:activity:7086783687341608960/

But do we really?  

Firstly, what are SIPs?

What are SIPs?
Structurally Insulated Panels (SIPs) in New Zealand are made of a three-layer sandwich of an outer structural skin; Strand board, oriented strand board (OSB), plywood, or sometimes magnesium board (MgO/MgS) or steel, and an inner insulation core of expanded polystyrene (EPS) or polyurethane foam (PUR). If you are not familiar with SIPs, think ice cream sandwich.
‍

Fig 1. A NZSIP build in Cromwell, showing the componentry that goes into a SIP panel build.  

What’s the problem?
‍The problem outlined in the article, is unless all the internal joints and penetrations are properly sealed, there is a risk that humid, moist air could make its way between the joints and condense on the cooler outer skin, causing decay.  

There is evidence of this happening overseas, but not in New Zealand as far as I’m aware. Or not yet, anyway.

Firstly, most suppliers of SIP panels require all joints to be sealed with tape such as provided by Pro Clima or Siga.
‍

Fig 2. A passage on sealing joints from the NZSIP technical manual.

All panel providers recognise that as their panels are inherently airtight, mechanical ventilation is required. Just relying on opening windows for ventilation isn’t good enough. High quality, continuous, balanced mechanical ventilation reduces humidity levels, thus reducing the risk of moisture entering a gap or crack in a panel.  

The potential problem is the increased prevalence of leaving the panel skin unlined as a money saving measure or for personal preference.  

This means that taping is not an option as it would be unsightly, unless battened, so another way may be needed to reduce the risk of moisture entering panel joints.

NZSIP recommends that “If the panels are left without a lining, it is recommended that the panel joins are taped and battened for airtightness.”

But is there actually a potential problem?

What’s the potential pathway?
‍Firstly, it’s important to understand the vapour permeability of the constituent materials.  All of the common skin materials; Strand board, OSB, and MgO are vapour permeable to a greater or lesser extent.

Expanded polystyrene (EPS) is also vapour permeable, but as Polyurethane (PUR) is a closed cell foam, it is vapour tight.  However, the glue that is used to bond the EPS to the OSB likely renders the skin vapour tight, at the inner surface.  

Therefore, it’s unlikely that vapour would enter through the panels themselves, so the remaining pathways are at penetrations, joints and intersections of panels.

‍The following are some standard SIP joining details I use every day in my designs. The orange arrows show the potential vapour pathways.
‍

All male to female connections are spray foamed before final installation.  If the panels are to be lined, then all joints can be covered with tape. However, this is not always possible when SIPs are left exposed, or “naked.”

So, as you can see from the details, there are some junctions potentially at risk in a naked SIPs building.  

Is there an issue?
‍There is a potential issue, for sure.  
However, if the foam core is Polyurethane, then the type of failure mechanism shown in the first article is impossible, because polyurethane will not allow the vapour through in the first place.

So, with polyurethane SIPs, the issue is limited to moisture laden vapour getting between joints in the panels.  

Most of the joints are pretty well protected, in that the weak point is through quite large sections of treated timber, (Boron treated H1.2), directly towards the outside, or through a tortuous path via a tight, rebated camlock system or a thermal spline.  

Polystyrene on the other hand is vapour permeable, so the described failure mechanism is possible with this material. The jointing systems are similar, sans the camlock, so the panel joint pathways are also similar, but there is also a greater risk with penetrations too.

What’s the solution?
‍Firstly, it’s incredibly important to state Prevention is Better than Cure.

All SIPs built housing should have properly commissioned balanced ventilation systems to keep the relative humidity levels down with the 40-60% ideal range. (mechanical ventilation also provides decent internal air quality. https://www.healthyhome.kiwi/the-clever-heat-exchanger)

Secondly, is important to understand that there is a potential issue here if moisture is not managed correctly.  If ventilation is installed correctly and working properly, the risk is likely low.  

Thirdly to mitigate any future issues it incredibly important to ensure very good spray foam coverage as the first line of defence. That is not always easy to do, with cans of foaming goo.

And finally, when detailing joints in naked SIP homes, consider how you are going to provide a second line of defence. Possible suggestions could be battens or silicone sealants.  Sealants need to be investigated to ensure they will last for the life of the building, otherwise, should they fail, shrink or lose flexibility, they won’t do the job intended.


‍