Brian and Colleagues,

Your intuition about a high flow fan is most likely correct. However, this may be a case for a non-ASD approach such as ventilation and/or basement pressurization.

While, to the best of my knowledge, there is no published peer-review research on mitigating "All Weather' wood foundations, they are are constructed with a great deal of aggregate under and surrounding the foundation. There also ca be significant variations in construction of wood foundations, for example:
- the footings may be a treated plate on compacted aggregate or concrete; and
- the floor may be treated sheathing with furring strips on aggregate or a conventional concrete slab.

Almost always there are significant, difficult to seal air leaks.

Personally, I would be extremely cautious in quoting any post-mitigation radon concentrations.

Keep me posted,

Bill

We have done 2 to date and both luckily had a moisture barrier under them, so we verified it’s integrity by reading communication under poly and building pressure simultaneously- in both cases we were again lucky and ended up moving +/- 80cfm at 1” WC - btw- 6” hole in floor, is much easier to get your hands into-to seal your suction pipe to poly- we just blue skinned it in place and slipped a flange onto floor for stability.

We have done a number of these buildings. As the floor vapour barrier was not in good shape we had to install a new vapour barrier on top of existing floor (pt plywood) , we removed base board and tied into wall vapour barrier and installed a new plywood floor over top of. Our communication testing allowed us to use a small fan and these buildings communicated right up the walls. Significant radon reductions from 1200 -1800 Bq/m3 down to less than 20 Bq/m3.

Have done several. All have had concrete slab, not wood floor. I use Fantech FR150 or Radonaway equivalent. Never a problem. Always plenty of crushed stone in a wood foundation design. Radon mitigator’s dream.

This home does have a concrete floor as well as a sump pit and perimeter drain. The first thing I will do when I go is get a sniff test at sump pit.

Use the sump pit for your vent location and you’ll be fine. If you can’t use the pit, core your hole over the top of the perimeter drain at your chosen “vent out” location. Suck out enough stone with your shop vac to expose the drain pipe. Cut the top of the pipe off and install your vent and complete your install. Seal the sump.

As always with anything new/weird....DO DIAGNOSTICS before quoting mitigation. The only similar thing i have seen was a house with below-grade wooden exterior foundation walls. It had a traditional concrete slab over crushed stone.

We have very few wood foundations in PA but lots of superior walls. Both manufacturers and installers worry about water pressure and water entry because they typically have no footers. If the lot is sloped it will surely have drains to grade. If it has window wells they will be a direct leakage. Leakage can be a bigger problem than lack of permeability. Design your system to minimize the impact of leaks you can't seal rather than just use the sump. good luck

We have installed at least a dozen or more ASD systems for these kinds of homes over the last couple of years. In every case, we chose to put our main suction point near the center of the basement. Some have been with raised wooden floors and others with concrete floor slabs. In all the cases we did, there was good crushed stone under the wooden floor or concrete floor slab. The weak point with these types of basements is the joint between the PT foundation walls and edge of floor. There is a potential for a lot of air leakage at this point. By drawing from the center of the basement and controlling the flow from the fan to get only a minimum of 1 Pa across slab pressure at the edge of the slab. we minimize the loss of conditioned air in the home and at the same time, minimize the risk of backdrafting combustion appliances. We use dampers on the suction side of the fan to accomplish this. In most cases by using this technique, we are able to use either the smallest fan or medium sized fan. The simple fact that we are moving some air under the floor causes a decrease in the concentration of radon levels under the floor which in itself is sometimes sufficient to solve the radon problem. Whenever we find a raised wooden floor in these kinds of homes, we also look at the possibility of cutting an access hole through the wooden floor in a closet or mechanical room to see if they have a vapor barrier over the soil under the floor. If so, we have also in certain cases cut a hole through the vapor barrier for our piping and created a sub-membrane depressurization approach with that particular home. In all cases, we have had a 100% success rate at reducing the radon levels below Health Canada`s actionnable levels in the homes we have mitigated. It is also a good practice to leave behind a carbon monoxide detector ($25) with the homeowner as a safety precaution.

This particular home wound up working out well. After some diagnostic work, along with having access to the building plans, I determined to install the suction point near the center of the basement. Luckily in this home that is also where the mechanical room was. I did not use the sump pit as the suction point as I believe the tile exiting form there was a daylight drain. Post mitigation average radon level was 1.1 pCi/L. Thank you all for your input!


This home brings me to another question that I should probably open a new thread for but I will start here. I was really concerned, with this type of construction, about what looked like a lot of moisture staining/damage on the basement walls. I struggled morally with if I should bring this up with the new owner, or if I should assume that the issue was pointed out in the home inspection and just do what I was there for. Thoughts?

Brian,
You hit the nail on the head.
Most of the similar construction we have discussed here had a bad mold problem behind the FRP curtain insulation that I witnesssed.
One home literally had it's PT wall board almost down to wet cardboard consistency from foundation water.
In all cases, I made the problems known to the homeowner. That's just me. I suppose this would make a good ethics topic on another thread.

Mention it and let the Home Inspector be responsible for his own competency, or lack thereof.