Subslab Depressurization

Sub-slab depressurization or sub-slab suction is probably the most reliable and effective radon mitigation technique. It involves an insertion of a pipe (usually 4 inches in diameter) through the floor slab into the soil or gravel under the foundation. This can be done either from inside the house, or inserted under the slab from the outside. As a side note, this technique is used for houses with a basement or slab-on-grade foundation. If your house has a crawl space, a different technique is used.

Just to give you a quick idea of what’s involved, you’ll need to create a hole (aka suction point) in your slab (approx 4 inches in diameter), remove as much soil and/or gravel from under the foundation as possible, put in place a pipe running out from this suction point to a suitable location, where a special exhaust fan will generate vacuum pressure that will draw radon out from underneath the slab to the outside, thus preventing the radon gas from entering your home. Right, you may say, and I know, because that’s what I said when I first read about this a few years back. Believe me, if you do this right, and there is no reason why you shouldn’t, you’ll see a remarkable decrease in your radon levels, which will often be accompanied by a fresher indoor air quality as well (this is one positive side effect of installing this system).

Now, there are different ways of going about this, but I am going to go through one example … in my opinion probably the most likely solution for the majority of home owners (remember, this is not for a house with a crawl space)

SUCTION POINT

My best advice for you is to pick the spot most convenient to run the pipe out of the basement and start there with a hole. I do think that a suction point near the footer tends to produce better results because there is often more settling there, but really that is not of the greatest importance. You should definitely determine where the pipe will have to exit the basement before you start drilling holes in your floor. Also make sure you are aware of potential drain and water pipes, and possible electrical conduits or in-floor heating.

I am going to assume that if you plan on doing this yourself, you don’t need me to tell you how to make a hole in your slab. Well anyway, one way is to drill a series of holes and then chisel and hammer out the cement chunk. The hole should be slightly larger than the 4” pipe you’ll be using.

Once you have the hole made, it’s time to dig. This is probably the worst part of the job, and it could be relatively easy or very hard, depending on what lies underneath. For best results try removing as much as possible. You are trying to create a sort of a vacuum chamber under the slab that will draw in radon and moisture from underneath the entire foundation, so this can make or break your mitigation system. No matter what you have underneath the slab, be it sand, gravel, soil, or any combination of these, try to get 20-30 gallons out, and more if possible, specially for tight soils. REMEMBER this will be the difference between a mitigation system that works and one that doesn’t.

When we did our house, I spent hours scraping away buckets of dirt and gravel till my forearms were bruised. I managed to get about 30 gallons out, and let me tell you, it was worth it.

RUNNING THE PIPES

If you have an attached garage, I would recommend going from the basement, into the garage, and then to the attic, as this is the most feasible solution. Your second choice is to go through the floor above, perhaps through a closet or some other point where the pipe is either not visible or will be easily concealed, and then into the attic where your suction fan would be. The third choice is taking the pipe outside of your house. Personally, I think an outside system should be avoided all together unless there aren’t any other options. Even if it takes a little more effort on your part, try and stick to the indoors for your piping and fan system. Colder climates especially will result in a lot of condensation that can reduce the life expectancy of your radon exhaust fan. You can purchase a condensation bypass kit if you mount your fan outside to prevent water and condensation from ruining your motor.

Back to work… Now that you have your suction point ready, you can use a coupler and secure it using a foam backer (but really you can also use whatever you see fit). Once all your pipes are in place and secure you should caulk this joint for an airtight seal. For the piping, you can use any type of good PVC piping that will fit into whatever you’re using, and make sure that you measure well before you glue your PVC to your elbows and joints. In our home we used a 4 inch sewer pipe and glueless joints/elbows (they have a double rubber seal on the inside to form a water and air proof seal, and the advantage is that you can separate the pipes if you mess up. The only downside is that they are a few bucks more, and really hard to put on, or take off for that matter).

THE FAN

Contrary to what you might be thinking, you cannot just use any ordinary fan for this job. If you are going to invest your time and energy into this, you have to do things right, and you’ll need a special fan that is capable of creating vacuum pressure under the slab without the fan burning out, that can withstand the elements, cold, and moisture, and that will do the job quietly and efficiently.

For your safety, you exhaust Fan has to be located outside of your home’s living space. This leaves you with an option to put your fan in the garage, the attic, or outside. Furthermore, the pipe leading from your fan to the outside must terminate above the plane of the roof, and must be no closer than 10 feet horizontally from windows that open. This is to minimize the possibility of radon re-entering your home.

RP145 fan with couplings and  Dynameter

RP145 Fan by Radonaway and a manometer on a 4 inch 100mm sewer pipe

The size and price of your fan will vary and depends on the square footage of your foundation. The type of soil  also plays a role in the fan decision, as you might need a more powerful fan for tighter soils.

As an example if your foundation is anywhere up to 1200 Square feet and your sub-soil is a gravel mixture, an RP145 by RadonAway should be quite sufficient. Most retailers will have a chart or table of different fans, ranging from 3” to 8” duct systems, the latter being used for large projects, such as a school.

Finally no matter where you end up placing your fan, you will need to bring it power. For this, you might need to call an electrician, or not, depending on whether you have a plug-in nearby. Luckily, my father is an electrician, and so we have our fan wired in directly to a switch for ease of maintenance. As you might have guessed we have the RP145 fan installed in the attic above our garage, and it is virtually silent.

As a side note, each fan comes with a suggested maximum operating pressure. This basically informs you how well your entire mitigation system is performing, how much pressure is being created by the fan, and whether you have sufficient airflow under the slab. To keep an eye on your system and to see that it is functioning nominally, I strongly suggest getting something like this Installation Kit for RadonAway System . It includes a manometer which  basically tells you what pressure your system is holding. It’s inexpensive, easy to install, and well worth it. These kits also include couplings, which are intended to tie in your fan to the PVC piping(shown in the picture with the RP145 Fan) and I’d recommend using these, as they allow not only for a perfect tight fit with your piping, but also easy future fan replacement or maintenance.

FINAL NOTE

Our radon levels in the basement were always hovering at around 25 and up to 31 pci/L, and around 20 pci/L on the main floor, during the cold winter months. Summers were ranging from 8 to 12 pci/L. After we installed the subslab mitigation system our levels are averaging 1.4 during the coldest months, and 0.4 to 0.8 pci/L in the summer. The picture to the right is from our own Safety Siren Pro 3 Radon Detector. Click on it to see a larger version. It was taken on January 31, 2010. We have been using it for over 2 years now, and had several of our neighbours borrow it for some time as well. If you find that you have  a radon problem, you are probably better off getting one of these instead of the charcoal or alpha track test kits that need to be sent in to a lab.  These are not cheap, they go for US $130.00, but they are very accurate and convenient to use.

If you are interested in prices or would like to purchase one, here are a couple of suppliers in the States. They also provide a lot of additional information about their products.

Radon Zone – These guys are offering  free shipping, unfortunately they only ship to the States (sorry Canadian customers).  They supply other, short and long term, radon test kits, as well as the Safety Siren Pro 3.

National Safety Products – These guys dont offer free shipping but they do ship to Canada, and they do offer discounts if you plan on purchasing more than one item. They also supply other radon products.

I wish you all the best and hope you get those radon levels down just like we did.

8 comments to Subslab Depressurization

  • admin

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  • I am very thankful to this topic because it really gives great information

  • Jason

    I have one of these tubes in my house but it dosen’t have a fan it is just a PVC tube that runs from the foundation out through the the attack. I’m having a problem with condensation building up and dripping down from the floorboard of my basement durning cold temperatures. Can someone recommend a remedy to this?

  • admin

    Have you looked into getting a condensation bypass kit? That should do the trick for you –

    http://testproducts.com/guard-ra760402-condensation-bypass-schedule-p-1143.html

  • dude

    thanks for the info. the replies you get saying your website is really good, asking for host info and are written in terrible english etc are spam. delete those mofos

  • Kim

    Hi I live in a heavy clay area and am wondering if sub slab makeup air is ever required. If so I am concerned that the blower will exhaust the make up air and not the radon. Maybe I should have asked first if the menometer will tell me wether makeup air is needed or not needed?

    Thanks

  • kevin

    ca a subslab system be used in a condo with 2 units above it?

  • George

    Have high radon (10-15 as per pro series II detector).

    Dug pit only to find there was NO pea gravel under slab. Installed system which did nothing.

    Created 20 foot trench through existing slab and removed earth. Added drainage tube and gravel to trench. Poured replacement concrete. This crosses my entire basement covering 50% of the rooms.

    Radon levels haven’t moved much – what else should I check / try?

    System is using 25W fan and 4 inch PVC.

    Drawing about 1 inch of water column vacuum…

    Ideas greatly appreciated!

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