An investigation into the options for insulating a slab foundation: from the top or from the bottom - which performs better and is easier to build?

1. Newsletter Article 1
Foundation Insulation
We have been working up some details for a Passive House retrofit in
Oakland and it has provided a wonderful opportunity to investigate a
few specific details. I will be reviewing each of these in our upcoming
newsletters, starting here with the foundation detail.
Some of you may have seen (or own) a copy of the book ‘Details for
Passive Houses – A catalogue of ecologically rated constructions’
published by Springer Wien, New York. I picked this up at our favorite
local bookstore, Builders’ Booksource. We’ve been wading through
this for a while here at Quantum, trying to find ways to translate these
construction details into our construction projects. I’ve been
particularly intrigued by a detail on Pg 188 (AW1 03 EFo 01.) This
detail illustrates an option for providing insulation above a
conventional spread-footing concrete slab foundation: a variation on a
building detail which both contractors and structural engineers are
much more familiar with around these parts vs. a common Passive
House foundation detail, which specifies insulation below the slab.
Our discussions surrounding this above slab insulation detail have
ranged from skeptical, to enthusiastic, and as we build more Passive
House projects, I’m sure our opinions will change again. To illustrate
this point, Josh, one of our Project Managers, called in from our Palo
Alto project after pouring the slab foundation to say he’s now willing to
consider insulating above the slab! He’s been working on a Passive
House project where the architect specified rigid insulation below a
floating slab foundation. He had just had to deal with keeping a thick
layer of EPS (expanded polystyrene) level, clean and undamaged,
before pouring a finish slab over it – not an easy job when re-bar and
various utilities needed to be laid on top of the foam. The idea of
insulating above the slab was starting to look more attractive.
Meanwhile, back in the office, we ran a couple of THERM simulations
for our Oakland retrofit project, to see just how insulation above or
below the slab performs. (For the record, these were modeled for a
northern California climate, with the outdoor temperature set at 32◦ F.
We also assumed a 6” compacted gravel bed, with 2” of sand and a
poly-barrier but didn’t include that info in the detail.) Here are the
results of those simulations:

2. Fig. 1a – Isotherm image of foundation with 4” EPS above the slab.
For the first option, we modeled four inches of EPS above a four inch
floating slab. No allowance was made for a finish floor in this case, but
I would assume a plywood sub-floor and some form of finish
treatment. From the lovely rainbow color infra-red image below
(Fig.1b,) you can see how the top of the foam keeps an even 65-69◦ F
surface temperature across the interior surface, but the slab below
maintains the same temperature as the ground below, somewhere
between 50 and 53◦ F.
Fig. 1b – Infra-red image of slab foundation with 4” EPS above the slab.
We were really surprised to see the outcome of the alternate detail in
this study. Here I placed the four inches of EPS below the finish slab -
a fairly typical detail for many Passive House projects. In the rainbow
thermal image simulation (Fig2.b), you can see how effective the four
inches of insulation is in keeping the whole interior floating slab warm.

3. Most the full 4 inches of concrete maintains a consistent temperature
between 65 and 68 ◦ F – an incredible result for a material that we
typically associate with being cold!
Fig. 2a – Isotherm image of slab foundation with 4” EPS below the slab.
The implications for this detail are fairly significant. The THERM
illustration makes a strong argument for the redundancy of radiant
floor heating when the slab is insulated from below. I would
speculate that this already warm slab would be almost impossible to
heat sufficiently with hydronic (or electric) radiant tubing in order to
provide sensible heat i.e. heat that can be felt by the human body.
This large thermal mass would also help maintain a very stable interior
temperature, helping to moderate large temperature swings inside the
building – typical for a classic thermal mass, but one that does not
require direct heating by the sun.

4. Fig. 2b – Infra-red image of slab foundation with 4” EPS below the slab.
This leads me to conclude that insulating below the slab is a better
detail for this particular situation. We have the added benefit of not
needing to install an additional finish floor surface, since the concrete
slab serves as both structural foundation and floor finish.
I do want to add a word of caution about this construction detail.
Concrete requires a long curing time and releases a large amount of
moisture into the building interior. When you insulate beneath a slab,
you create a barrier that prevents moisture dissipation. Since EPS is
open-cell, that moisture could theoretically slowly wick thru the foam
and dry out the ground below. Unfortunately, if you install a
polyethylene barrier either above or below the EPS, you will prevent
moisture from seeping into the ground below and will force the slab to
dry out to the interior only. I have read about a few projects where
the slab took more than a year for the surface temperature to equalize
with that of the rest of the building’s interior ambient temperature,
and most likely released a load of moisture into the interior. These
caveats make me want to learn more and proceed with caution.
Perhaps the in-field trouble of pouring a slab over insulation will be
compensated for by superior thermal performance numbers? Let’s
keep our minds and our options open!
Bronwyn Barry
Certified Passive House Consultant