Aluminum sheeting on floor joists in the crawlspace of a 1977 house

First time I’ve seen this. There was aluminum sheeting on the bottom of the floor joists in the crawlspace. There were holes poked in them.
Can anyone give information on this? Purpose, time frame when it was done, just curious, I know it had to be for insulation value, but looking for any other kind of info.
Thanks much.

Aluminum is an excellent conductor. It does not add any R-value (insulation).
This application of aluminum sheet or foil to the underside of the subfloor was not based on good building science, and there is a reason you do not see it as common practice. I would not hazard to guess what their intentions were when installing it, even if they were good intentions.

You may have seen aluminum foil or other metallic sheets or films applied in attic spaces before. In those applications the intention is to combat radiant heat, which you may have mistaken as insulation.
Radiant heat from the soil is not an issue to worry about in a crawl space.


Light reflection for an attempt at a weed grow space would be my guess.


What type of heating system and/or was it radiant ceiling heat?

I agree. Maybe it is just a draft/moisture stop for that slat sub-floor? But, I can imagine condensation issues developing between the sheet and the subfloor.

The house currently has a 2005 electric forced air furnace. Unsure if it ever had radiant ceiling heat.

Yes, but never in a crawlspace.

1 Like

Right, because it will likely condensate between the cool damp crawl and the heated floor space with no insulation between the foil and the floor. Did you see moisture issues? If not, there may not be a problem yet, just a waste of energy for whoever installed it.

My vote is the owner intended it as a radiant barrier, after reading some Internet resource on the topic.

If only they had read this instead:
Nebraska Energy Quarterly, Stay Away from Foil-Faced Bubble Wrap

I mean, at least they poked holes in it, so the process of the floor boards rotting out will be somewhat slower and harder to detect.

Recommend removal.

1 Like

Do you have a pic of the panel or were there a lot of 20 amp double pole breakers? Regardless, I see this radiant barrier only in electric homes that used radiant heating. From what I understand it was to help combat against heat loss from the floor above into the crawlspace. When I see it though it’s fastened to the bottom of joists with the air space in between.

I concur. Grow Op. Great analyses.

Having reports on several grow ops, I would have opened the main panel looking at the SEC lugs for tapping, excess cables and Hyalite grow lamps plate marks on floor joists and beams. That vent is also suspect for grow operations. 1 - 4’ x 8’ table can yield $5,000.00 every 42 days. You can put 3 to 4 tables in that space.

Old electricity bills can provide some indication but usually the PoCo SEC is tapped prior the meter equipment.

Ask if there are police records or did the previous owner declare shuch an operation.