Sunday, August 11, 2013

Foundation Thermal Bridging

One of the most difficult spots on any project is where the building meets the ground. On a Passive House where we strive to eliminate as many thermal bridges as possible this moment is especially fraught. The typical house detail (which usually include building a floor and then simply sitting the walls on top) is a massive thermal bridge and lead to huge energy waste as well as cold and uncomfortable areas inside. 

Wisconsin Cabin Double-Stud Foundation Detail: The ext. stud layer overlaps the floor , insulating the edge and reducing the thermal bridging of the floor framing members. 

Thermal Bridging?
In the building-science world we refer to a thermal bridge as some element (usually wood or steel) which is continuous from the conditioned interior of a building all the way to the outdoors. These 'Bridges' (from one side to the other of a wall/roof/floor) move heat from the warm side to the cold side (look at that: high school physics in action!) - so in the summer the gross heat outside is invading your nice AC'd interior and in the winter the warmth from your toasty wood-fire is being drawn out into the cold outside. 

No matter the season - you are wasting money and energy and increasing possibilities for discomfort when you have many bad thermal bridges in your building. In a typical home, there are LOTS of bridges; every stud, every rafter, every floor-joist, lots of electrical conduit and wires, plumbing pipes, HVAC ducts, Bolts and other fasteners, window frames, door thresholds and a thousand other spots all contribute to wasting energy in a building. 

So what can be done to reduce thermal bridges? Lots. Its not rocket science - it just takes attention and care during the design and detailing phase and an understanding of the principals of heat flow. 

Typical Foundation Details:
There are a LOT of factors that go into a foundation detail. First though are the code required sizes and configurations. Here in NY we can use several types of foundations - for this project though we are looking at two real options - a full stem wall and a slab on grade. 

The Stem wall needs to go down at least to the frost-line as defined by the local code inspector. Where we're building - that's 48", and then a spread footing at the bottom of the wall. Since there is no basement, the foundation slab is then poured on gravel which will allow for proper drainage as well as a continuous Moisture barrier. There are lots of good descriptions of these systems but the best are (as usual) the folks at BSC

The way the wall, foundation and floor all come together are crucial to the energy performance and this is often the weak link in even the best insulated walls.

A Really Good Foundation Detail:
On a Passive House we are very concerned with thermal bridges, and a lot of our decisions about our wall framing on these projects are driven by these sill-detail concerns (and the eaves too). We like the double stud a LOT because it allows us tons of freedom to create a full thermal separation between the interior slab (which will be at the interior air-temp) and the exterior Structural Foundation wall which is outside the thermal envelope and therefor will stay the temp of the outside air / ground.

Most PH projects feature a lot of Foam insulation underneath the foundation slab. Check out especially the Hammer and Hand folks here for a great example some great foundation work. So we initially drew up our detail using this technique - works pretty darn well - except that the local building inspector doesn't really like it and wants us to hire and engineer to assess the slab, though that's certainly not a big deal.

Below you can see the CAD detail on the left and a thermal bridge simulation on the right. The colors represent the temperature of the assembly from Interior (left - white) to exterior (right - black).

To asses our 2-D thermal bridges we use a simulation tool called THERM, and a huge thanks go out to David White from Right Environments for posting his amazing THERM tutorials for PHPP use - invaluable for anyone getting started in thermal bridge analysis.

A Really Good Foundation Detail: lots of Insulation under a concrete slab

You can see on this detail that we achieved a NEGATIVE thermal bridge of -0.0374 W/m-degK. Whats a negative thermal bridge? Well, because of the way we do our energy modeling in the Passive House spreadsheet, we can actually get a credit if our thermal bridges are designed well enough. These credits offset some of the heating / cooling demand in the house - so the bigger the negative number the better. 

An Even Better Foundation Detail:
Now -this is certainly unconventional - and I know lots of builders that might have a worry or two about this assembly. Admittedly, the fixing / fastening here is not quite worked out yet so if anyone has a suggestion please send it my way! But what we're proposing here is to move the insulation above the Typical concrete slab, then to install the wood floor on-top of that foam. This is pretty much a 'retrofit' detail that you might see on a basement or garage renovation like this

You can see here that when we use THERM to assess this detail, we get a negative thermal bridge of -0.0547 W/m-degK.

So quite bit better that the insulation underneath the slab. How much better? Well, with the first example (-0.0374 W/m-degK) our house would have an annual heating demand of 14.234 kWh/m2-a - now if we shift to this over-slab detail, we drop our heating demand to 13.868  kWh/m2-a.

That's a 0.4 kWh/m2-a (3%) drop in our yearly heating demand just by moving the location of the insulation in relation to the slab. I  know that sounds small - but to hit the PH standard every little bit is crucial) So not bad considering we didn't increase any material quantities and we might have even made our life easier (cheaper) since now the local building inspector doesn't need me to hire an engineer to keep him happy about the slab being poured on-top of the foam. 

Of course, there are moisture and constructability issues here  - but if we can seal under the slab well to prevent moisture from getting into the slab in the first place-  I think this detail could work quite well. We're testing quite a few different versions though in order to find just the right balance of constructabilty, thermal performance, moisture resistance + durability and especially cost.

An Even Better Detail - put the insulation ABOVE the slab