The footings had just been poured, the plumbing was roughed in and now it was time to fill the void. For fill, I used more of the same gravel that we had used for the pad. The form work that was left up around the exterior perimeter of the footings prevented us from driving a skid-steer over top of them. Instead we used the excavator that we had on hand to reach over and drop the fill in one scoop at a time. We then shoveled, raked, leveled and packed until the tamper bounced off the surface. The finished height of the gravel was three inches below the top of the footings to account for the thickness of the sub-slab insulation.
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| In this photo, my nephew Parker and I are finishing the gravel between my new footings. Parker did a lot of raking while I ran the packer. It was hard work but we had it all done in less than a day owing to the help I received. Even my mom pitched in - I warned her that the gravel was very heavy and hard to move but she replied "Don't worry Lucas, I'm a gardener." |
Once the gravel work was done, I installed all the sub-slab insulation (reclaimed 2'x4'x3" Dow "Roofmate" XPS). The rigid foam boards installed easily enough but the heat was intense. Don't let the grey sky in that earlier photo fool you. We had many long stretches of sun and above average temperatures last summer. The colour of the insulation boards was a sort of dusty light blue that seemed to reflect a lot of the sun's energy - making it almost impossible to work over by early afternoon.
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| Insulation installed. Notice the glare. |
The next day I rolled out 10 mil poly over the rigid foam boards to serve as an air barrier. The slab concrete was going to need to make direct contact with the rough surface of the top of the footings so I trimmed the poly back from the footings. I used sheathing tape to secure the poly around the edges, seal the seams between sheets and to the plumbing. By noon, the heat was very intense but I had to get the job done because the concrete guys were coming out the next day to lay out and tie together the steel reinforcing for the slab. Despite the heat I managed to get the poly installed - then the fun began...
The heat melted the adhesive on the tape and a wind came up to intermittently lift the poly up and make a mess. Then, as I was busy with repairs, I noticed that in one spot (where the poly had stayed in place despite the wind) several of my rigid foam boards seemed to be warping from the heat! They bowed outwards towards the sky, necessitating the removal of the poly for their replacement. I can only guess that the transparent poly I was using was creating a sort of "greenhouse effect" between the top surface of the rigid foam and the poly... Anyway, while I was busy replacing these warped boards, the wind came up again and undid all my previous repair work.
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| One of the "Roofmate" boards that I pulled from service. Notice the curvature - I put that broomstick across the top for effect. This board had been nice and straight when first installed but warped after several hours of sun exposure under the clear poly that I was using. |
And so it was that in a moment of frustration, I called my building inspector to discuss whether or not I needed the poly there at all. Given the difficulties I had had that day I was concerned about a number of things:
- With the weather forecast for the coming week being at least as hot as it was that day, would my rigid foam be at risk of being damaged like the boards that I had replaced?
- Since the sheathing tape I was using was melting and would not "stay stuck" was I going to be making a career out of repairing the poly every time the wind blew?
- Would the poly stay in place while the concrete guys worked on top of it or would I be running around after them fixing it all the time?
During a phone call and site visit, the inspector and I threw onto the balance the following additional considerations along with the concerns that I had raised:
- Would there be a moisture management problem if I omitted the poly?
- Would there be air (soil gas) leakage problems if I omitted the poly?
Regarding the migration ("wicking"/capillary action) of ground moisture into the concrete slab:
We decided that omitting the poly presented a low risk of moisture problems considering how well drained the gravel pad is and the fact that the concrete was wrapped on all sides by 3 inches of rigid foam which is a pretty good capillary break (1" "Roofmate" = 1 perm/inch so, 3" = 0.3333 perms).
Regarding air (soil gas) infiltration:
We decided that the risk was small enough to not require the poly given the following: I don't live in a "high risk" area for radon gas - not to say that its presence isn't possible. The slab was poured over the footings (like a lid) so there is no "seam" around the perimeter where the slab would normally meet the concrete wall (see "planning for the foundation"). The slab is in direct contact with the rigid foam underneath, so any leakage that might occur at a crack in the slab will be restricted to where the crack bisects a seam between insulation boards. Lastly, I have committed to sealing the few plumbing penetrations with neoprene seals as I will be doing with other envelope penetrations.
I think it's fair to say that not including the poly may have been a mistake... I "lost my cool" in the heat and maybe made the wrong call - but frankly, I'm not too worried about it. And besides, my plans don't call for any kind of sub-slab "VB" anyway. I'm more concerned that my foundation makes it through the winter without heaving (despite precautions). We'll see how things go with blower-door testing.
Soldier Jane - Beck
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