I am a veteran at sealing HVAC duct systems but am new to air sealing. Approximately half of my work is in the manufactured home community. I recently started a project in Louisiana. High humidity and high heat area. I had a common issue in about half of the homes. When bringing the home to 50pa, then sealing everything our crew could find, the linoleum floor would bubble.... entire room bubbles. In most cases, the air losses through this situation would virtually kill the savings achieved on the other sealing. Obviously we are pulling air from somewhere below the home. Any suggestions or ideas for improving this?

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Hi John,

The primary air barrier of all mobile home is the subfloor and that is what should be air sealed, just like in a site but home. The road barrier material is designed to hold up the insulation and protect everything above it when being transported down the road and is considered a secondary air barrier which when sealed /patched will also help to stop airflow.

Road barriers materials are not all the same, it depends on the manufactuer and the climate zone. Depending on the material some will block more air than others but your best air sealing results are gained from sealing the holes in the subfloor.

In your air conditioning climate the road barrier might be designed as a vapor barrier and should also be sealed/patched to prevent warm moist air from condensing on the cool ducting. 


I agree, and then notice that puts the duct system outside of the house.  So the duct systems frequently have to be rebuilt, sealed and then insulated.  And if the return system is a series of holes along the edges using the underbelly as a plenum, then a proper return system needs to be made - can be a single return if pressures are sttended to. 

Thanks Ed,

Yes the duct system is outdoors. It came from the factory that way.

After working on Mobile homes for 25 yrs in the WAP program I've found very often the bulk of the air leakage reduction often comes from the duct sealing/rebuilding.duct%20end%202.jpg

If you intend to reinsulate the underfloor (90% of time this is done in the WAP program) you better rebuilt/seal your ducts or you'll have a duct system full of fiberglass.

Who ever thought up a belly return YUK! 

Notwithstanding the typical low budget for these homes, it seems like encapsulating the crawl might actually cost less in some cases.

Unless the home is on a permanent block or concrete foundation, attempting this is a lost cause.  Air sealing and insulating skirting is futile.

Depending on the local climate, soil conditions, you then have the same challenges associated with any crawl-space homes including ground moisture and local zoning enforcement that will require crawl space vents.

You didn't mention anything about the age of these homes.  Early mobile/modular homes (1960's) sometimes had walls that were made with 2" thick walls,  HVAC duct coupling between sides of a double wide - depended on the units be in touch with each other -- no positive sealing.  No  sealing the seam at the ceiling, outside walls and floor - just wood trim over the connection. 

If this is an OLD modular home - there really isn't a lot that can be done.  Tightening up the envelop means that you'd also trap the moisture movement - and - you might find that one problem causes another.

So first question is what's the age of the home... if its modular how are the two halves joined.   Can you confirm the air ducts do not have large gaps between the sections that allow movement?  (A camera and scope can help with this...)

Actually, it was only in 1976 that any standards at all were applied to mobile homes:  U.S. Department of Housing and Urban Development (HUD). The Federal Manufactured Home Construction and Safety Standards (commonly known as the HUD Code) went into effect June 15, 1976.

Before that, anything could be found.

Yup,  I'm not sure you want to even try tightening up a modular home built before 1975.  I think it's pretty risky.  These were not homes designed to last fifty+ years.  I've seen the fiberglass in those old walls - get wet with moisture and then pull down to the floor level - because of the moisture that condense on the insulation -- with an outside metal wall.

Modular homes (double wide where they are delivered in 2 pieces them attached on site) typically in the center where they attach the 2 pieces together there is have an open airway from the belly to the attic. Through stack effect cold air from the belly air flows up into the attic.   I find that the contractors who install modular homes  never air seal the marriage wall in the belly or in the attic.  Many times, I have listened to a homeowner complain that when they are sitting in their armchair (located on an interior wall) that they always feel cold.  Air sealing these gaps (belly & attic)  with spray foam is inexpensive and effective.

I observed a similar situation made worse by having the thermostat mounted on this "interior" partition wall. The cold wall fooled the thermostat into thinking the room was actually colder than it was.  On top of that, the heating system was greatly imbalanced resulting in severe depressurization which drew even more cold air through the partition wall encouraging the thermostat to call for even more heat. It was a positive feedback loop resulting in almost constant furnace run time.

We have worked extensively for a couple of modular (not manufactured! - these are trailers = double wide) builders, one completing Energy Star Certs.  In my experience in the mid-Atlantic, they all now do a pretty good job of gasketing the mate walls (walls that mate - side/top/bottom) with a big spongy gasket.  It is still worth being on site as they are set to add a little single-part foam where needed.  On older ones, seal the joints from the attic and the crawlspace, and if instruments show a problem, you can peel back the siding where needed to expose a joint and seal it there too.

We have found that a 2-module ranch built before about 2000 can be in the 3-4 ACH50 range if it has baseboard electric heat and no A/C.  If you add an A/C unit or heat pump in the attic (it always seems to be the attic) that number can climb to 8-12 ACH50.  One builder has an optional crown molding package where they do not tape the ceiling corners that will get the crown.  The drywall is painted and the stained and varnished crown is put up, and this can add 1+ ACH50 to the leakage rate.


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