Can you use rigid ducting for subfloor ventilation?

[TheHandySqirrel]

Hi all,

we have had subfloor ventilation installed by a contractor, but its quality is not very good so I want to redo it.

As flexible ducting always has a greater resistance to air flow I was thinking, "what if I used rigid ducting, i.e. 150mm PVC pipes?"

Is that just a mad shower idea or has one of you experience with this? Yes, the ducts might be more expensive to buy, but my logic is that air flow then exits quicker and I get more changes per hour (plus the installation is damaged less when accessing the subfloor).

Any advice?

Thank you very much!

[MitchellMc]

Hi @TheHandySqirrel,

You're absolutely right that a properly installed straight pipe configuration will outperform a corrugated pipe setup. In terms of flow, straight pipes over extended distances offer less resistance compared to their corrugated counterparts.

If your flexible ducting is not kinked and installed correctly, then the airflow should be quite comparable to rigid piping. Ideally, the flex duct should be pulled tight and supported adequately to ensure straight runs. Corners should be swept and not at a tight angle. In some applications, it can perform better than straight when multiple right-angle bends are required for the straight pipe installation.

Before you swap it out for straight, I'd recommend having a ducting specialist come in and inspect the installation to see if there are any obvious reasons for the low flow.

Regarding using PVC pipes, you'd need to purchase some designed to be used as ducting, but specialised PVC is commonly used for duct work.

Please let me know if you have any questions.

Mitchell

[TheHandySqirrel]

Hi @MitchellMc,

thank you for your helpful answer. After a little more research I will (re)use the flexible ducting.

How would you fasten the ends of it, given that when you stretch it there is horizontal pressure on the ends?

I've attached my current design below. The access door to the subfloor is at the top, the flat ovals in the walls is where a human can crawl through to access to each of the "rooms" of the subfloor. Existing air bricks are indicated. The ground slopes upwards from the top of the sketch to the bottom and from the right to the left (i.e. the highest point is in the garage).

Can the duct layout be improved? Where would you place the fans? I have three available.

And one more question: A lot of drawings and photos show the input ends of the ducts dangling from the joists towards the ground. Since moist air rises up, my thinking is to have these ends also near the joists, or am I mistaken?

Thank you!

[MitchellMc]

Hi @TheHandySqirrel,

Can I ask what issue you're addressing? Did you not see any improvement in moisture level after the installation? Where are the exhausts currently positioned?

I'm not a duct installer, and it would likely be worth discussing the layout with one. However, air will likely be drawn predominantly from the closest outlets with the least resistance. The furthest rooms will need some form of additional force to draw from them. As an educated guess, I've added to your diagram where exhaust fans need to be placed. I still feel that the bottom exhausts will prioritise the closer rooms with shorter ducts, and the further rooms with longer ducts won't receive as much suction.

It's more about aerating the space rather than sucking up moist air. You are creating a negative pressure that draws in fresh air from your wall vents. The more fresh air that is drawn in, the more moist air is displaced and sucked out. So, where you put the entrance to your ducts might be somewhat irrelevant, but your thinking of having them at the joists is sound.

Mitchell

[TheHandySqirrel]

Hi @MitchellMc,

the issue is that the "professional" installation of the existing setup is not working. The installers kinked the flexible ducting, created sharp bends and didn't stretch the ducting as you suggested. I did moisture measurements with a sensor over a period of several weeks and the moisture barely drops when the fans go on.

I did contact a local company but the moment they realised the I'll be DIY they refused advice, even after I offered to pay them for their time.

Maybe, to address the imbalance in suction within duct branches, one should make them equal length?

Thank you! 

[MitchellMc]

Hi @TheHandySqirrel,

When you say, "the moisture barely drops when the fans go on," are you running the fans constantly, and did moisture levels drop at all over the several-week period? It's unlikely you'd see moisture levels drop when the fans are turned on, it would be a situation where the area should dry out over several weeks. However, if it's raining, moisture levels might stay the same or even increase if groundwater penetrates beneath the house.

Is the moisture sensor one that works in the air? If so, check the humidity of the outside air on a warm day. Then, check the air coming out of the duct from beneath the house. If the air that is drawn out has higher humidity, then your system is working.

I'd certainly sort out those kinks in the ducting, and you could consider making the branching the same lengths.

Mitchell

[TheHandySqirrel]

Hi @MitchellMc,

sorry for the delay in replying. I had placed a moisture sensor on the soil below our bedroom and measured 24/7, but the fans were only running at the time programmed by the installation company.

I have since done some work: 

• Removed the entire ducting installation (bar the power cable)
• Reinstalled the ducts with solid end caps and stretched out as much as possible
• Hung the fans from the floor joists

I'm currently working on the external duct grills where the air is expelled to the outside. But from that first reinstallation alone I see vast improvements:

• Much more air is exiting the subfloor
• The noise transferred to inside of the house has dropped dramatically

I've also changed the timer: The higher subfloor is now vented 12 hours a day (7am to 7pm), where the subfloor is closer to the joists and the moisture problem is worse, I have the fans running 24/7.

Once I've finished the outside work I can share this project with initial problems & what I learned along the way

Thanks!

[MitchellMc]

That sounds very promising @TheHandySqirrel.  

Many thanks for the update.

Mitchell