Insulating Cabin Floor

[georgevacabin]

Hello All,

I hope that this entry finds everyone well.  The question of how to insulate the cabin floor - and keep critters out - has been posted in several places on the forum.  I thought it might be helpful to centralize the topic.  I apologize if this is a repeat.

So here is my question (finally right?  ):

Would it work to insulate over the floor joist?  I'm thinking a layer of the vent screening (critter protection), 1 inch external moisture resistant sheathing insulation, and then your TG flooring. 

Any thoughts?  Thanks!

[glenn kangiser]

I think it would work - I think you would get more R value and no critter problem if you used fiberglass over 1/4 inch hardware cloth though.

[georgevacabin]

Thanks Glenn!

I get the hardware cloth.  I don't get the fiberglass.    Do you mean fiberglass insulation?  If so, what type?  Sorry if this is a silly question. 

[glenn kangiser]

Just regular batts or rolls - the heaviest you would like - I don't know what is suggested as far as proper in your area - vapor barrier toward the heated side.  We have discussed that the 1/4 well stapled to the bottom of the joists is the largest size that will prevent mice from getting through.

Generally most here like to overinsulate and save the energy.  Without the hardware cloth the under floor insulation gets to be a disaster with critters of all kinds - - raccoons - dogs - rats - mice - even with single wire supporting it every foot at every joist as I saw on a house with enclosed crawl space that was open at the access hole sometimes.

[John Raabe]

Yes, this is the best way to insulate a floor - between the joists not on top of the subfloor. You would have to float foam insulation (expensive) and then put down another subfloor... Then there are all kinds of problems with how to anchor the interior walls, plumbing flanges have to be longer and on and on...

Also, you don't want to break the connection between the subfloor and the joists by putting foam board insulation down before the subfloor.

[MountainDon]

Fiberglass is probably the way to go for a DIY'er. Spray in foam would be nice anywhere, but that's usually a job for a professional installer and his truck of equipment.

We used 3/8" CDX plywood to enclose the bottom after the insulation was stuffed in between the joists. We used those spring wire insulation supports to keep the insulation up against the subfloor. That left an inch air space approximately before the plywood.

Prior to installing the 3/8 CDX we ran a strip of 1/4" hardware cloth along the inside edge of each main support beam. The plywood was nailed over that leaving a two inch air space by each of the two beams. The floor joists overhang the beam by about 12 inches. Those overhangs were also sheathed with the 3/8 CDX.

[georgevacabin]

Thanks for the feedback everyone.  No wiring or plumbing will run under the cabin.  I was just trying to think of a way to insulate that didn't require crawling, stapling, nailing, etc. underneath the joists.  Not a lot of room to work.   

[MountainDon]

Is it built already?

[georgevacabin]

Hey There Don,

The platform is not yet complete.  Heading down on Monday to hopefully finish it.  I'm doing the ole one step forward, five steps back.   

[glenn kangiser]

Any possibility of doing it from the top?  Before the sub floor goes in?  Staple the hardware cloth in to the sides of the joists or underneath from the next joist over then drop the insulation in on top it - staple it then put on the sub floor? hmm

[MountainDon]

  the danger there is keeping it dry until the structure is dried in.

[georgevacabin]

Thanks Guys.  Yep Glenn.  Looks like that is the way to go.  I'll keep you posted. 

I looked at Youngins posting/pics again.  http://countryplans.com/smf/index.php?topic=2634.60 He used tar paper prior to laying the insulation.  Anyone know how that worked out?

[MountainDon]

I don't know why he put tar paper in the bottom against the wire mesh, and he has dropped off the fce of the earth, so to speak.

To my mind the tar paper is in the wrong place. If it's a tight enough fit with the insulation stuffed on top it would likely act as a vapor barrier. VB should be on the other side of the insulation normally.

Maybe the tar paper was to keep moisture from the ground, rain spatter and so on from getting into the insulation?   

You likely are already aware, but just to drive the point home or bring it to the attention of new readers... the main concern about insulating the floor from the top and then building up from there is the worry of wet weather. It's difficult to water proff a subfloor for certain. You don't want water to get into the insulation. Not that fiberglass would be ruined; it can not absorb water. The real concern is mold growth.

[georgevacabin]

Thanks again MD,

I think I remember Youngin saying that he just wanted another layer of critter protection.

Thanks for the excellent feedback as always.  I just want to get it as right as possible the first time around.