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Insulating the floor when there is no reasonable access below

Hi, I own a floating home build sometime in the 20's. It was probably originally built with stolen lumber as a budget alternative to a landed house. Needless to say insulation is lacking. We've added what insulation we can to most of the walls, sealed all known air leaks (there were lots of places where you could see water through the floor, wall, etc).

Now I'm working on the living room floor. The floor seems to be 5/4 inch t&g. I'm planning on adding electric radiant heat mats, and engineered bamboo over that. To maximize the heat, I'd like to insulate the floor. The issue is that I do not have much access to under the floor so I need an over the sub-floor, under the heating mats solution. I'm considering foil products (like Prodex), but am not convinced that they will help enough. Any suggestions?

For those doubting the under floor access: The house is built on a raft of old growth logs (~2-4 ft in diameter), on top of them are stringers that function as the foundation (~14"x14" creosoted), the floor joists run across the stringers. So to access the floor I would need to scoot across the 14" gap on top of the logs (think rusty spikes, algae, moss, etc. plus 1-4' gaps open to the water). The only way I can think to do this would be to go in through the top and I am unwilling to rip up the floor,


Thanks, Tony (no longer onHiatus), Seattle, WA

Re: Insulating the floor when there is no reasonable access below

Are you willing to rip up some of the floor? What if you took out some boards so that you had enough access to put in some kind of backer between the floor joists and then added some low expansion spray foam insulation? This is kind of what TOH did on the East Boston house when they insulated the living room walls except that was a vertical (wall) application and it already had something to contain the foam on the back.

The backer would have to be secured well enough so that it holds tight until the foam sets and doesn’t get blown out by the foam.

When all is said and done it might be easier to just rip out the floor to get easier access to the joist space.

Another option is to lose some height in the room and build a new insulated floor on top of your existing floor. Would that work for you?

Re: Insulating the floor when there is no reasonable access below

Hmm, I hadn't thought of that. I wouldn't mind taking up a few boards, but I'm not sure I could take enough up to attach a backing. I think the room is a little over 30' long - I might be able to get in a few feet from the sides. It seems like there should be a long handled stapeler type tool that I could use to tack up some tar paper - something on the end of a 10' pole?

as far as building up the floor - I actually have the ceiling space, but it would make an odd transition out to the deck. I'm already going to be adding about 3/4" in flooring / heating / etc. If I could do it in another inch it might be doable - but 3-4" would definitely be a problem.

Does anyone have any experience with Prodex? They claim R14.5, but I'm guessing that's an "effective" rating - I don't see how an 1/8" product can have that kind of thermal effect.

Thanks for the suggestions, I will definitely think about the foam - if I can only think of a way to hold a backing in place.

Any other thoughts / ideas? It does not need to be a perfect solution - any insulation will be an improvement!

Thanks, Tony

Re: Insulating the floor when there is no reasonable access below

I would have to see some pictures of this one to give an accurate opinion. If you could please email me a few @ [email protected] Thanks.

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