Foundation Work


The foundation work has been underway this week and will be done by the end of the week. Block work started on Monday, insulation work was yesterday, and, weather permitting, they should be wrapping up the remainder of the work by the end of this work. Guess what this means?? Yep! The house will be delivered and set next week. We are quickly going to go from nothing to a two story structure in a matter of days. Ah…the beauty of prefab.



As you can see, we opted for a crawl space on this house. This was partially driven by economics and also by the prefab process. When working with prefab homes, you have to go with either a crawl space or a basement, as a slab-on-grade simply won’t work. This is because of the way prefab homes are manufactured. In order to build walls (both interior and exterior), you need a floor structure on which to build them in the factory. This necessitates building the floor structure in the factory. As a result, a slab-on-grade simply would not work.

We will wait to do any concrete pours (the garage, garage apron, front porch, sidewalk) until after the house gets set. By the end of next week, the site is going to look dramatically different. Keep your eyes open and look for the crane mid week next week!




The Slab

Yesterday the slab was poured for the first floor.  As you may or may not have known, the home is designed as slab-on-grade construction.  However, the slab has been slightly lifted out of the ground to avoid the slab appearing to be right at grade from the front entrance.  Further, you will notice in the photos that the slab at the front entry is lifted 8″ higher than the rest of the first floor to give that space a bit of definition and also allow for a nice transition from interior to exterior.

The final finish of the slab is to remain it’s natural gray tone, simply sealed with the soy-base sealer Acri-Soy.

In terms of the slab itself, we did some research into proper insulation for both the slab itself and also the foundation walls.  We made some changes along the way and have learned some things during the process as well.  We feel the end solution will perform much better than our initial details had shown.  We wanted to make sure the slab was properly insulated but still stayed within a reasonable budget.  The initial thought, before additional research, was to use 2″ rigid insulation up the interior face of the foundation walls and then to extend it parallel to the slab 2′ in around the entire perimeter.

There were two places in particular, however, that we modified this initial insulation strategy.  We decided, after some research and recommendations, that it would be beneficial and more efficient to provide some sub-slab insulation under the entire slab…as opposed to simply this 2′ band around the perimeter.  As a result, we continued the rigid insulation under the entire slab which will provide a greater insulation value and allow the floors to stay warmer in the winter.

The second modification was to address the issue of the thermal bridge between the foundation block and the slab itself.  Originally, and what is typical in many slab-on-grade details, is to simply allow for the slab and the foundation to essentially butt up directly to one another…with some sort of expansion joint.  This, however, creates a thermal bridge and can cause a temperature differential between the edge of the slab and the underside of the slab.  As a result of this bridge, we placed tapered 2″ rigid insulation between these two elements, eliminating this thermal bridge.  The reasoning for the tapering was to allow the slab pour to extend completely to the interior wall edge.  Below is an illustration found during our research which demonstrates this detail (ignore the radiant heating aspect though):

We are very pleased with the results of the pour and can begin to envision the living spaces a little more.  In other news, we received the money from the city today for the manufacturing of the SIPs (Structural Insulated Panels).  So, we should be moving forward with the walls as final shop drawings come in the near future.  Can’t wait till we see some walls going up on site!

Fill it up

Last week the foundation was filled with a compacted granular fill and the perimeter foundation insulation was installed.  The water lines are installed and pouring the slab will come next once the weather cooperates.

Solid Foundation, Part II

The week has ended with the construction of the block foundation walls.  Now we can finally begin to visualize the footprint of the house.  It’s great to see it slowly rising out of the ground.

Foundation, view from north

We did encounter some design changes as a result of the grading of the site.  As you can see, the block rise 5 courses out of the ground…more than we originally anticipated.  This is not a major difference, the biggest result is simply the slab will be elevated higher than originally anticipated.  As a result, the interior of the foundation walls will simply be filled with a larger volume of compacted pea gravel as a solid base for the pouring of the first floor slab.

Foundation, with skyline in background

Foundation, 8" raised slab for entry