Insulating a raised wooden sub-floor?

Started by Bluemoon, April 11, 2011, 01:33:04 PM

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Bluemoon

Hi Folks, 14 X 24 plans are on the way and we are hoping to start construction of our "bunkie" in late May.  It will be built on a raised wooden floor (concrete piles, PT posts, built up PT beams and 2 X 10 PT joists) with 12 to 20 of crawl space underneath.   We are located in Northwest Ontario so I would like it to have an insulated subfloor to allow for three-season use.

I would like to get your opinions on using a layer of 1 1/2" rigid foam insulation between a bottom layer of 3/4 PT Plywood and a top layer of 5/8 T&G Plywood as the floor system.  This would essentially create a sandwich (plywood/insulation/plywood).

Thoughts?


rwanders

Gonna require some long nails or screws (4"?) to tie everything together and down----if you are willing to do that, I assume it would work----fiberglass batts would be quite a bit cheaper.
Rwanders lived in Southcentral Alaska since 1967
Now lives in St Augustine, Florida


mpls_ham

A couple of the people on this site have tried this and have been unhappy with the results.  I wish I could point you to the threads but I could not find them.
Northern Black Hills - South Dakota

rocking23nf

I put insulation and covered it with chicken wire.  It was crappy work, but it works.

firefox

When you use the fiberglass batts you have all those joists
acting as thermal bridges. Remember, Wood is not that good
at thermal insulation. If it were me I would be using 6" foam between
the two sheets of plywood. You can get screws designed for this.
Bruce
Bruce & Robbie
MVPA 23824


MountainDon

It can be made to work. There has to be a good solid connection between the walls (which the roof is attached to) and the joists (which are securely attached to the foundation, we hope.)  One help would be to plan to bridge the foam with OSB or plywood sheathing from the rim joists across the foam to the wall studs; a good place to use 4x9 or 4x10 sheets of OSB to span the distance.
Just because something has been done and has not failed, doesn't mean it is good design.

Don_P

I'd look into that further before bearing the walls on the sandwich. I was envisioning this being inside the walls. A SIP would have a 2x rim around the perimeter.

JRR

"Thinking out loud" ... I would consider attaching 5/4" wood decking, normally spaced, atop the joists (this would allow all support wood to breathe freely downward) .... then two layers of foam, staggered seams, no plywood ... then perhaps a layer or two of felt (optional) ... followed by a heavy bed of wire reinforced grout bed and tile ... or just a thick (2") covering of reinforced sand-concrete, to be later painted or carpet covered.  You could stop the floor construction just as soon as the decking is in place .... to be completed when the rest of the building is dried-in.  This would keep the elements from being a concern of destroying your floor during construction.

This scheme puts most of the weight to be in the very top layer ... holding the rest underneath in place, fewer tie-downs required.  

Squirl

I will second or third (whatever we are up to) the idea of insulation in between the joists.  If you are concerned with the thermal value of the wood, size up the joists and spread them to 24" O/C.  A 2x12 SPF No. 2 joist with L/480 deflection (above code) can span 14'-7".  That is more than you need.  It will probably be cheaper than the foam with more insulation.  In addition you don't have to worry about the weight compression issues mentioned by Don_P.  In my research I noticed that for energy star and energy code compliance, floors have R value requirements closer to walls.  The reason is heat rises, not falls.


Squirl

I'm also a little confused about the use of PT wood for the girders, joists, and floor.  Is this in a flood zone?

rocking23nf

Its a code requirement withing a certain distance to the ground in Alberta.


Bluemoon

Pressure treated timber is also a code requirement in the Ontario Building Code (OBC 9.23.2.3) when there is less than 6 inches clearance from the ground.  Also for me it provides peice of mind at minor extra costs.

mnboatman

Quote from: Bluemoon on April 11, 2011, 01:33:04 PM
 We are located in Northwest Ontario so I would like it to have an insulated subfloor to allow for three-season use.

I would like to get your opinions on using a layer of 1 1/2" rigid foam insulation between a bottom layer of 3/4 PT Plywood and a top layer of 5/8 T&G Plywood as the floor system.  This would essentially create a sandwich (plywood/insulation/plywood).

Thoughts?


I put 1.5" foam over my PT plywood subfloor in my Bunk House in Northern Minnesota. I have OSB on top of that. It gives me 7.5 R value and is firm and not bouncy. I used 96" x 22.5" strips of foam with a 2x3" running at right angles to the joists. The floor stays warm enough for three (or four) season use.
http://countryplans.com/smf/index.php?topic=7324.msg94245#msg94245