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change a 3 season room to 4 season

I have a room to insulate. The exterior ceiling and walls were insulated prior to interior finishing, however, the floor was never insulated.

The room is a second floor porch (on posts). The floor joist are exposed from underneath. The subfloor was laid on the joist, and I can only assume no vapor barrier was placed between the subfloor and finished floor.

Before I insulate, I'd like to create a vapor barrier. I can use "Faced" insulation and install it paper side up, but the integrity of the barrier will be suspect :cool:.

My question is; Is there a nice way to create that vapor barrier? Is there a better alternative?


Re: change a 3 season room to 4 season

What is the finished floor material ?

You could apply a plastic sheet vapor barrier from the underside..... this will be tricky but it can be done.

You would need to go over each joist ... across the the underside of the floor to the next joist .... and so on. The perimeter framing would also be covered from the underside with all seams taped with house wrap tape ( not duct tape) to ensure a sealed envelope. then apply the insulation and cover the underside of the joists with sheets of foam insulation which would be covered with wood.


Just use 2 lb closed cell spray foam insulation and cover with wood and finsh.... no need for vapor barrier.

Hope this helps.:)

Re: change a 3 season room to 4 season

I thought of the poly wrap also, but then I thought otherwise.

2 lb closed cell spray foam insulation?

I have one 12' x 20' surface to cover. The property is well beyond the third tier suburb; I’m thinking it may be cost prohibitive, however I don’t know until I make the calls. Do you know, is it possible to rent equipment?

Flooring Material: I'm told there is only one layer of 3/4" t&G decking, plus carpet and Pad. Can a guy pull the carpet and pour a heavy coat of poly or something along those lines?

Re: change a 3 season room to 4 season

There can be factors such as where you are located and what local codes that may influence what can and should be done.

If you go with the spray foam insulation it will be more expensive than conventional products but if installed correctly will eliminate the need for a separate vapor barrier applied and will provide a proper air seal ....which is a key point to any insulating.

As far as renting the equipment and obtaining the materials for DIY spray foaming ... I'm not aware of any service available ... not that it doesn't exist.

Considering the sub flooring is T&G and depending on the floor height difference you may use sheets of plywood or OSB on top of the existing sub floor .... for example in some areas 5/8 and thicker OSB has an acceptable perm value to be considered as a reasonable vapor barrier for the floor in conjunction with an acceptable insulation and air seal of the underside.

If you decide to use more cost effective and conventional method of insulating .... cover the top of the sub floor with sheets of plywood or OSB and fill the joist cavities with bat insulation and cover the bottom of the joists with 3/4 or 1 inch sheets of rigid foam which provides the thermo break to the joists. Ensure that all seams of the sheets of foam are sealed ... it's important that no outside air infiltrates the cavities in the underside of the floor.

Just a couple of suggestions.:)

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