Insulating septic
lookloan
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New septic installed in zone 5 and has anyone insulated pipe from house to tank with foam board below grade or maybe rockwool comfort board? It is my understanding comfortboard can go below grade and it’s about $60 for 24 feet x 4 feet vs about 16 feet xps foam board for the same price. both are R6 – the reason for doing this is to help prevent freezing and better to do now during install.
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I’ve never heard of anyone insulating their septic plumbing. Septic tanks create some heat due to the decay of material inside, similar to how compost piles get hot inside. With proper pipe pitch I wouldn’t expect to have any freezing issues.
I have a septic system at my own house in the northern part of zone 5, and in the winter when there is just a little snow or frost on the ground the tank is clearly visible as a frost-free rectangle on the surface — even though it’s buried a foot or two underground.
Bill
I have doubts that fibrous insulation is *worthwhile* below grade, unless very well-drained. Mineral wool wicks moisture well enough to be used as a hydroponics grow medium (see the Grodan and Cultilene blocks). Mineral wool saturated with moisture probably doesn't insulate as well as mineral wool with lots of entrained air. Attempting to entrain air long-term in a poorly drained subsoil with waxes or binders is probably a fool's errand.
If you want to pursue this, use a high-density EPS. XPS would likely work, but is currently a ridiculous choice from a greenhouse gas perspective (until the next administration bans the blowing agent), and slightly more expensive to boot.
Or you could try a proper EPDM foam pipe insulation sleeve, like one or maybe several layers of this:
https://www.zoro.com/aeroflex-pipe-ins-epdm-4-12-in-id-6-ft-334-ac41212/i/G8641997/
But before you resort to any of this, consider this:
https://inspectapedia.com/insulation/R-Value_of_Soil.php
And consider just burying the pipe and tank deep enough to start with, well below the frost line for your area.
My architect brought up the idea of insulating since the depth can't be more than a foot or two based on ledge and elevation to home. I am in Northwest CT and the architect pointed out client systems along the warmer shoreline freezing, even though the 1st poster doubts septics can freeze. If a septic system gets regular use it may not be a problem, but freezing occurs if use is low or if there is a slow drip. So if you travel over the deep winter for over a week with no use the freezing can start. The web has numerous articles on this and suggest putting mulch and leaves 8 to 12 inches over the septic in the fall, but I'd prefer to insulate to avoid the annual labor. Other articles suggest 4 inches of foam on the sides and top of pipe from house to tank. Apparently it's a real nightmare if the pipe freezes and there is a whole process to getting it unfrozen. Apparently burying a septic too deep is not good either as it's needs oxygen to work properly
I personally have never heard of such a thing
My last house was on piers, the drain pipe was exposed to outside air for 3 feet,a and basically sat on the surface of ledge for 20 feet
Never froze
Even if you leave for weeks, the septic tank continues to create heat from decomposition and the area around the pipe is warm from use.
I think those shoreline homes were up on piers and left for months, perhaps with a leaking faucet
I will put this out there as an intermediate option:
Get a self-regulating heating cable and apply it to your 4" schedule 40 pipe. Apply a 1", R-4 jacket of continuous EPDM pipe insulation. My math says sustaining a 50F temperature difference (taking you down to -23F) across a ~12" circumference at R-4 is about 4 watts per linear foot. If the septic produces enough heat, the tape stays turned off via its internal thermostat. If you only need it for 10% of the year at that 50F difference (or 20% of the year at half of that temperature difference), you're spending around $0.40 of electricity per linear foot per year.
On the topic of burying septic:
https://terrylove.com/forums/index.php?threads/how-deep-can-septic-be-buried.65111/
https://inspectapedia.com/septic/Septic_Tank_Design_Depth.php
1" EPDM for schedule 40 is about $15/linear foot on the site I'm looking at, but as a jacket you're using far less material and labor than you would with trying to apply EPS, and the end result will be more durable.
Sheet goods might be appropriate for the tank itself, if you're going to bury it under a bare minimum of fill and you're concerned about leaving the house unoccupied for long periods of time.
James,
I have a cottage with septic close to Zone 6 conditions. Never had issues with the septic freezing even when it is only used for a couple of weeks in the winter time.
There is short run of the drain near the house that was pretty much at grade, that is insulated with one of those jackets used for steam pipes. Foam would work just as well.
It is really hard for drain lines to freeze as houses use a lot of water. Even if you are flushing 5C well water down the drain, it takes a lot of heat loss for it to freeze. The heat loss through a plastic pipe is just not enough for it to happen before it gets to the tank.
If you are worried about freezing, just get 4x8 sheets of 2" XPS, break them in half , into 2 2x8 sheets and install over the pipe. I do this frequently when connecting sewer laterals to sewer man holes and the laterals are a little shallow or run underneath paved areas where there is less frost protection. It's cheap insurance.
Where I live, the frost depth is 3-4 feet, which is below pretty much every septic drain. I've never heard of anyone insulating the drain, and I've never heard of one freezing. My old house had a drain that was about 2 feet below grade, was there for at least 50 years without issue, including entire winters without use. My current house has a drain less than a foot below grade, covered mostly by pea gravel which probably is close to R0. The lid of the tank is covered only by a dusting of sand. No problems so far through two winters.
During the "polar vortex" a few years ago there were widespread stories of septic freezing.
The reason you don't read much about septic systems freezing is that if it happens often it's a high priority to fix it.
It is common to insulate above water pipes when they can't be buried below the frost line. The rule of thumb I've heard is a foot of dirt equals an inch of foam. You should go a foot out horizontally for every inch of foam.
Let's say you have a new septic and you have a moment in time to insulate the pipe, why not take out cheap insurance and do as some have suggested. If you look at the responses, there are some intelligent detail, math behind them answers vs 'Oh I had this or that and it never happened to me"
I want to thank the detaiedl replies. I am wondering if a small leaking appliance such as a dripping fraucet is behind most freeze ups? I'm sure there are those that would say just fix the leak, but there may be a time like changing a property to a VRBO that the tenant doesn't say something - just giving an example and readers would have to think of their own. From what I read, if you have a freeze up, it is a pretty nasty event and expensive and time consuming to fix. I think looking back, someone would kick themselves for not taking an inexpensive insulation step.
How deep will the pipe be and what is the depth of your local frost line?
Do you have a septic contractor selected yet?
Often you can find used foam sheets form commercial roofs at very low cost on Craig’s list.
If your soils is that shallow septic will be a challenge finding a good local contractor and listening to him is a good bet.
Walta
James,
That's always been the dilemma: You want the pipe to the septic tank as shallow as you can get it so that the outlet leading to the field stays high, and the runs are at their correct depth. Less so now that most systems are pumped, and the tank can be any depth you want.
Use 1 1/2" thick 24" wide foam directly above the pipe. Consider getting borate treated stock if ants are a problem there. It does loose its effectiveness over time, but helps keep them at bay for a while.
+1 for using reclaimed foam here if you choose to insulate the pipe. You could even use the extra cheap reclaimed stuff that has damage — it’s going to be buried so who cares how it looks? Save some money and use cheap reclaimed XPS here.
I don’t buy the “septic systems need oxygen to work properly”. If it’s buried, it’s not really getting much if any oxygen from the atmosphere. The pipe is likely far too long to really provide any air exchange too. I’m pretty sure the decomposition going on in a septic tank is all from anaerobic bacteria anyway which don’t need oxygen to survive.
Bill
Bill,
Yeah - most common septic systems are anaerobic. The only part that is dependant on depth to function effectively is the field. Ideally the tops of the infiltrator tubes should be around 12" below grade, allowing around 1/3rd of the effluent to diffuse upwards and evaporate.
FYI - My septic consisting of a low boy tank and reserve field are installed by escavator based on my site engineer. My septic it not a DIY project.