Deep dive – everything you possibly want to know about concrete sandwich panels

So at a more detailed level – what are concrete sandwich panels??  Basically they are panels consisting of concrete as the “bread” of the sandwich with insulation as the “meat” of the sandwich.  In the case of the panels offered by International Precast they consist of 3” thick high psi reinforced concrete wythes of concrete with either 2” or 4” of Polyisocyanurate insulation as the interior.


A surprising and very important differentiator of the International Precast concrete sandwich panels is that International Precast claimed that they could deliver panels where the interior and exterior wythes are not connected by any thermal bridging material – they would be completely isolated by the interior insulation.  We quizzed them at length on this claim but they were able to demonstrate to our satisfaction that they could indeed deliver on this claim.  This opened up the possibility of completely isolating the interior and exterior of the house from one another with insulating material and eliminating all thermal bridges.  There would be no through concrete in the panels!!  This started the thought of the house as a cooler – more on this later.

The concrete wythes would be connected via non-conductive cast-in wythe to wythe fasteners.  These are described by their suppliers as being made from “high performance heat and alkaline resistant engineered polymers” or “high-strength, low-conductivity, not-corrosive and chemically resistant” polymers.   These are spaced at defined intervals in the panels per well tested engineering specifications.  They are poked through the insulation at the center of the sandwich panels and have heads that end up being embedded in the concrete with a very small diameter high strength fibrous material connecting the heads together.  These connectors work in tension to prevent any wythe movement under load – basically keeping the exterior concrete surfaces from moving relative to one another under loads and enabling bending loads to be shared between the 2 wythes.  To be 100% technically correct these panels are referred to as “structurally composite concrete sandwich panels” because both the interior and exterior wythes act compositely to carry the external gravity and lateral loads.


The concrete itself is high psi concrete with reinforcing wire and rebar embedded in each of the wythes to enable the concrete wythes to carry structural loads.  Both the interior and exterior wythes are reinforced so both can carry structural loads – this would turn out to be a key factor in enabling thermal isolation for other parts of the house such as the roof.

Why concrete sandwich panels for our exterior walls? Part 3 – Step 2 toward concrete sandwich panels – the interior walls

Having bought into the concept and design aesthetic of concrete exterior walls we started brainstorming about interior walls surfaces.  Out initial thoughts were to use wood to warm up the interior of the house and avoid the dreaded sheetrock as a wall material.  We bounced around a lot of ideas on how to execute interior wood walls while keeping with the evolving design aesthetics.  We came to the conclusion that it would be better to use wood as warming / accent material via furniture, cabinets, doors, etc. where it made logical sense to use it.  This then led to the crazy, wild assed idea of using concrete on the interior walls – an idea that grew on us after initially us being skeptical about it.  So why not??

In parallel with our discussions on interior wall materials we had continued our internet search for precast concrete wall manufacturers.  And during this we stumbled across a company that claimed that they made concrete sandwich panels – panels that had both a concrete exterior surface (known in the trade as a “wythe”) and a concrete interior surface or “wythe”.  And it turned out that this company, International Precast, was fairly local to us – offering the ability to meet with them in person, see their facility, etc.  The fact that they were local also helped from a LEED perspective as it minimized the environmental and cost impacts of shipping the finished panels.

So our next steps were an initial telephone call and then a visit to International PrecastIP_SampleThe net out of these discussions and site tour was that it appeared that they could do exactly what they claimed they could do – deliver concrete sandwich panels that could form the outer walls for both buildings for our project.  So our crazy wild assed idea to use concrete on our interior walls was actually possible.  Not only was it possible – it seemed that properly executed concrete sandwich panels would deliver both our interior and exterior finished walls in a single product.

Why concrete sandwich panels for our exterior walls? Part 2 – Step 1 toward concrete sandwich panels – the exterior walls

In considering the wide variety of exterior materials that we could use I half jokingly made the statement that I wanted a house built of concrete, steel and glass (design aesthetic #1).  Note that all of these are lousy insulating materials.  So why these materials??  From my personal perspective these were part of the commercial / industrial / modernist design aesthetic (we can call this design aesthetic #2) that I like.  So we asked ourselves why not a concrete exterior to the house??  Properly mixed and cured concrete certainly is a very durable, low maintenance cost material that does not require any exterior finishing as is proven in the millions of commercial buildings that are built out of concrete.  And if it is used without trying to make it look like something that it isn’t then it meets a design aesthetic of “truth in materials” (design aesthetic #3) – it is what it is so don’t try to hide this.  And if we use it this way we get the bonus of not having any additional cost for exterior finishing.

concrete exterior

So how could we execute concrete exterior walls??  Certainly we could do poured in place walls – but these required lots of work in on site forms, dealing with all of the quality issues caused by weather, variations in concrete batches, etc.  Our architect / builder had done poured in place walls on a previous project so they were very aware of the issues with using them.  In particular the waste material from building custom wood forms was an issue.

The next thought was precast walls that could be cast in more controlled conditions that would then be set in place on site.  We looked at these fairly closely and even visited a factory where they were manufacturing walls that had a concrete exterior and were fitted with various materials to serve as studs in conventional walls.  We were convinced that we were on the right track on finding a material and method of making walls that met our exterior requirements – the panels would be “cool” as a way to meet our evolving design aesthetic and accomplish the our exterior durability / minimal maintenance goals  .

Further analysis showed that these panels that we had found still had some issues that we needed to consider.  First, they would still require insulation and interior finishing.  In that sense they solved our exterior goals but not our interior goals.  And they did not achieve the goal of thermally isolating the exterior and interior surfaces – we would still have to accomplish this.  They were clearly a big step away from conventional 2×4 construction – but were they a big enough step away??  Another concern was with this particular company’s ability to execute the panels to the detail and precision that we expected our project to require.

Coming up next – Why concrete sandwich panels for our exterior walls? Part 3 – Step 2 toward concrete sandwich panels – the interior walls

Why concrete sandwich panels for our exterior walls? Part 1

This is the first post from the engineering geek guy for our ShopHouse project.  I will be explaining some of the more technical reasons for the design and material decisions that we are making as a couple for this project.

So why build using concrete sandwich panels for our exterior walls??  There are a  variety of reasons but this decision started as a desire to find a better way to build a house and shop.  And a better way to achieve our goals of having the lowest possible operating cost and the lowest possible maintenance cost.

wood framingI look at conventional 2×4 construction as “lowest common denominator” construction – while it is cost effective I find it to be incredibly wasteful and sloppy.  It also has a fundamental problem that we needed to avoid to achieve our “cooler” concept of thermally isolating the inside of the house from the outside as much as possible.  Every 2×4 creates thermal paths between inside and outside surfaces, thus creating thousands of holes in the thermally isolated “bucket” that we are trying to achieve.

There are certainly a number of ways to reduce the thermal  paths or bridges between the inside and outside surfaces but these add complication and cost to the building process.  One way is to build 2 separate walls so that there is insulation between the inside and outside surfaces.  Again this can give a good thermal result but essentially you are building 2 separate houses, one inside of the other.

Another factor with conventional 2×4 construction is that exterior wall surface materials must be added to project the exterior surface from the elements.  The exterior sheathing, typically plywood or OSB, must be covered to prevent degradation from rain, sun, etc.  There are obviously a very wide variety of materials to use but most are susceptible to damage from the elements and require some level of maintenance.  Brick can be used as an exterior material but you are again essentially building multiple walls in order to achieve the desired end result.

In almost all cases conventional 2×4 construction assumes sheet rock for the interior wall surface.  I find this to be another “lowest common denominator” construction material.  It is very forgiving of construction errors, easy to find people to install and finish, etc but I personally find it to be a material that I would like to avoid if at all possible.  It is messy to install and finish, susceptible to damage from everyday household activities (and especially damage from dogs!), and difficult to hang items from the walls because of its low structural strength.

The net out of wanting to find a better way to build a house is that I challenged the architect/builders that we considered to design a house with no 2×4’s and no sheetrock.  Needless to say the reactions were interesting.  I think that most didn’t believe that I was serious – but at least one took it to heart and embraced the philosophy that this challenge required.

Coming up next – Why concrete sandwich panels for our exterior walls? Part 2 – Step 1 toward concrete sandwich panels – the exterior walls

We’re building a cooler.

A cooler what, you ask?  No, a cooler.

The reason a cooler works so well at maintaining the temperature of the things inside it, is that it fully insulates the contents from the ambient temperature.  For the most part (except for the opening lid) there is no thermal path that connects and conducts outside temp to inside temp.  Therefore, your beers stay icy cold!

coolerWhile we might not want to be icy cold, certainly the thermal envelope of any building is the single most important part of the building process, and minimizing the thermal paths, from outside to inside is paramount (as is sealing it up, but more on that later).

Conventional construction – whether 2×4’s or 2×6’s or even 2×8’s have wooden thermal paths every 16 to 24 inches or so.  Therefore, you can stuff some high R value insulation in those spaces, but your still fighting those paths.  Folks have gotten around that but essentially building a house within a house and offsetting the studs, but that seemed, to us, to be a little complicated.  So – what will we use for a wall system?  What can minimize these thermal paths, and is durable and cost effective?  SIP panels?  These are a really good product to minimize the thermal paths, but still very conventional and you still have to put something on the inside of them for walls, and then something else for your exterior finish.

We’d heard about some concrete panels with insulation – essentially the Superior Walls method, but that still wasn’t the answer.  Finally – we found the concrete sandwich panel.  3 inches of concrete, 4 inches of insulation, and then 3 more inches of concrete.  What this did, was allowed the panels to provide our exterior finish (remember – we like the minimalist aesthetic), our insulation, and our interior wall – which we can paint, adorn, or leave in all its grey glory.

Interestingly, this “all in one” material started us down a path for a couple of themes, including one material providing finished inside / insulation / finished outside and also working to minimize the subcontractor on the project to try to contain costs.

So – we now have the walls for our cooler – and we’ll talk about how the insulation could be continuous around the house…..