Your Preferred Approach Repairing Underbellies?

One of my homes is vacant and am rehabbing. In the process or taking down a lot of rotten skirting I found most of the insulation hanging down - the underbelly fabric is effectively gone - which I suppose is common for an early 80’s singlewide. This is in TX and fortunately doesn’t really get below freezing except once or twice a year at this location.

Speaking with several mobile home repairmen the consensus is to replace the pink insulation and staple chicken wire to hold it up, or alternatively nail up a thin plywood square to the joists to allow continued breathing.

Anyone else have an approach that works better for them when doing an entire underbelly or major patchwork? Doing nothing is not an option I’m considering yet, but maybe after this I will.

Not sealing it will allow it to be infested with rodents so chicken wire is not an option. There are mobile home suppliers that sell the belly material or you could substitute House Wrap.
It needs to be able to breath while sealing out creatures.

So basically a Tyvek type material…got it. Thanks @Greg

Actually that’s not the way to do it in hot climates. The best method has been found to use foil faced polyisocyanurate rigid insulation board with the seams taped and penetrations sealed with foam to make it airtight.

Gregg method is correct in colder climates. I can explain why if you want to know.

Interesting @Coach62 - do you still replace the pink stuff or just let the rigid board be the insulation and the barrier?

What about the ability to breath - I don’t think water could get through that stuff, which is a concern with condensation from the ductwork specifically.

Appreciate more info.

If it comes from the manufacturer with the woven material underbelly it is breathable and you need to replace with the same or similar material. The fact is whether it is a cold or hot climate humidity is always an issue inside homes as well as from the ground under the home. The ground should be covered with plastic and the skirting requires adequate ventilation to keep down humidity levels.
Inside the home humidity is created by heating, cooling, cooking, showers etc.
Your best choice for insulation is Roxul. Water runs right through it and it dries assuming it is in a breathable environment. If there is a water leak in the home it will go through the floor into the insulation and needs to be able to run through. Pink insulation has zero insulation value when wet or saturated with humidity and does not dry.
Also if contractors are using Tyvek on the exterior of homes in your area then breathability is an issue and needs to be taken into consideration when working the underbelly. The same principal of breathability of insulation applies to the entire envelope of the home. Tyvec allows humidity to escape from the inside while blocking water entry from the exterior…

I’m a building science junkie. I’ve taken more classes on the subject than I could count. I think it was LSU that did a study on this and concluded what I was saying. When I get home I’ll look it up.

Bottom line is that what works up north does not work in hot climates, this is just a fact. Moisture moves in the exact opposite direction down here. Not to mention our skirting is usually ventilated, not sealed as it is up north.

We heat very little and have AC most of the year. Cold air is heavy and settles on the floor, making it cool.

Under the home is what? Very hot, humid air that rises of course. It meets the floor framing, condenses and causes mold growth. The exact opposite of up north.

I even have several pics that I can post of this mold growth, I’ve seen it hundreds of times because most contractors do not understand building science.

This guy is the best.

I very highly recommend his classes, there is a schedule on his website. People, including myself fly or drive long distances to hear his lectures.

Here it is.

It’s a very good article and well written in layman terms. But here is the last paragraph of the summary, copy paste.

“. Foil-faced rigid foam and
closed cell sprayed polyurethane
foam exhibited good
performance, keeping subfloor
moisture content within
acceptable levels. In contrast,
open cell sprayed polyurethane
foam and fiberglass batt
insulation were not reliable for
preventing summertime moisture
accumulation in subfloors.”

I also noticed this, they referenced the owner, Joseph Lstiburek. Honestly, I cannot overstate how good his work is. I attended one of his crawlspace lectures a few years ago.

Lstiburek, J. 2004. Conditioned crawl space
construction, performance and codes. Building
Science Corporation Research Report 0401.
Lstiburek, J.W. 2008. New light in crawl spaces.
ASHRAE Journal 50(5):66–74.

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Thanks @Coach62 and @Greg - this is a great evaluation of options. The summers here are pretty brutal and the skirting was ventilated as you mentioned. I need to read through this in more detail but seems like the rigid board could be a good solution.

I might see if I can slide some of the rigid board between the ductwork and flooring so that any condensation from that can just hit the ground. I really don’t think there is a strong need for them to be insulated.

It will be fun to take some photos and show the results. There was an area washed out beneath the home too so have to do a relevel after adding some road base to fill it in. This was caused by drainage issue from heavy rains as far as I can tell. I need to address with some gutters and maybe french drain on one side of the home. It’s definitely a bigger job than I initially though.

More to come.

If you do it right, there won’t be any condensation, that’s the point.

2 important laws of physics. 1. heat moves to cold. 2. moisture migrates towards dry.

Up north, relatively speaking, in the winter the inside air is warm and moist, the outside air is cold and dry. We all know cold air cannot hold as much moisture as warm air. The moisture and heat both try to migrate towards the cold / dry air. So the moisture is moving from inside the house to the outside. Makes perfect sense, right?

Down south, the inside air is cold and dry (relatively speaking) and the outside air is warm and moist. So the outside heat and moisture is trying to migrate to the interior.

Read the full study, you do NOT want any air movement under the rigid insulation. If that hot, moist air hits the cold floor it condenses. You have to prevent it from hitting the floor.

Manufactured homes are designed for different climates, we all know that.

Does that make sense?

I feel enlightened by reading that article fully now and am planning to give it a try. Going to take out the pink batt’s this weekend and see how bad / if there is mold damage to address.

Was thinking about using this Rustoleum 5001 Mold Mildew Paint to address problem areas and after that vents for a week to get the foil rigid board over the joists, along with all the tiny taping and foaming for cracks as described.

I read that rigid board is easiest to cut with a little serrated knife so will take a steak knife with me. I’m in Texas, we have a bunch of those.

Actually coach, not to put too fine a point on it, Heat moves from hot to “cold” (as you said) & moisture goes to the coldest space/area. That’s why moisture condenses on the tea glass. The physics involved with moisture movement are explained in terms of pressure. Moisture laden air has more pressure in/to it. That’s why water evaporates from an open pan if left long enough. So yes, moisture in effect moves toward dry in layman’s terms. But while teaching HVAC for the last 20 years, I learned to describe it this way for them, "moisture goes to the coldest spot in the room/space. It doesn’t care if that’s in winter (outside) or in summer (inside). That’s why an AC unit dehumidifies a home. The evaporator coil (inside) is below dew point & moisture condenses on it as the warm moist air passes over it.

Hope that helps.
Recently Retired HVAC Professor

You’re getting into minutiae that is irrelevant to my point. I didn’t want to get into a lengthy lecture here on building science.

You’re statement that “So yes, moisture in effect moves toward dry in layman’s terms”. Layman’s terms was exactly what I was going for.

@Coach62 I am thinking about using a vinyl diamond lattice for skirting to improve the airflow underneath the home. Right now there is plywood paneling that was painted and had small vents installed - which seems really expensive and a lot of unnecessary work.

There has been very little airflow down there which I think has been a contributor to the problem.

Any hesitations when you hear this? I will post some pics this weekend.

Had some fun today - took down about half the underbelly and found a low spot outside the home was letting water underneath the skirting in during heavy rains - pooling over many years and now have a nice little washed out area where there is no support for the porch and beginning to fail for the home. Will have to add about 2 yards of road base or gravel to build the area back up. Debating whether to build up the area or fix the underbelly next.

Yes several posts for the porch have no support there - seems safe to crawl under :wink: