# Suspended Joist floor...Which Grade Plywood for sub-floor?



## shimmers (13 Sep 2014)

I have a victorian terrace with timber joist ground floors, ventilated under.
Joists are 100mm x 48mm, centres max 400mm or less, and because of a central sleeper wall, the full span of 3600 is split at apprx.1800mm, so deflection doesn't seem to be an issue with these small joists. I am going to insulate between joists with 75mm celotex and where I have some awkward small irregular gaps where I can't tight fit celotex fully, use expanding foam.

I'm assuming, unless advised otherwise, that if I lay 18mm plywood as a sub-floor on top of the joists, this will give me a choice of more or less any type of finished wood floor on top, ie, solid or engineered or even new pine floorboards. ( I haven't decided yet).

My questions are: 
With so many types of plywood, with so many different grades, I'm getting well confused...what should I ask for for this sub-floor. I started getting confused when I read 'WBP' as a technical grade doesn't actually exist anymore...but it's still used as a description at most timber yards. As the joists are sitting only about 300mm above bare earth, even though ventilated, I assumed using a WBP equivalent grade would be a good start as the air under the floor will probably be moist.
But does it need to be 'Structural' grade or will a normal non-structural WBP do?
Also, would I be right in guessing I don't need a hardwood ply, a spruce or softwood would be ok?

Also, do I need to fit any kind of vapour barrier...and if so, what's it suppose to do...and whereabouts should it be fitted?
I like the idea of some kind of membrane, as in my mind it can only help reduce draughts from under the floor, but not sure whether I need one from a technical point of view...could I even lay a sheet of dpm directly under the ply on top of the joists and wrap up the wall a bit...or would this be a spilled water trap?

(I must admit, when I read things like 'air tight breather membrane'...my little brain does a switch off aka Homer Simpson (homer) :roll: ...and I have to go have a coffee and sit down!...mmmm, air tight, but breathing...hows that work anyway?
Everything I read nowadays seems to involve membranes...breathing, none breathing, moisture this and that...but I digress...and I know I'm not the only one confused, I've seen the posts!)

So would appreciate your advice on my ply first please. Ta.


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## The Bear (13 Sep 2014)

I insulated my suspended timber floor with celotex between the joist a number of years ago. I then just laid the original victorian boards back down on the joists and sanded them. I'm very happy with the result. If you fit the celotex tight between the joists and flush with the top I'm not convinced you need a membrane. Moisture wont penetrate the foil on the celotex so the only route is through 4 inch of joist. Also if fitted well you wont have any drafts either. IMO membrane is a waste of money, others will disagree. Based on the above I don't think it really matters what type of ply you use as long as its well fixed to the joists. If you're just putting new (or old) floorboards back down I would't bother with the ply, again not necessary and raises the floor so need to think what happens at the thresholds where it meets un-raised floors.

Oh and the insulation really makes a difference. The coldest room in my house is now the warmest.

Mark


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## Chrispy (13 Sep 2014)

Most of the builders yards around here seem to stock "Far eastern ply wood" this will be "WBP" and is made from hardwood any way, or Chinese ply (this I would try and avoid) if you are going to over lay with some kind of boarding why not use flooring grade chipboard?
Vapour barriers are to stop moist air inside the building from reaching cold spots and condensing out into water, so should go on the warm side of insulation but as the celotex has its own foil you should not need it, any moisture getting into the joists can breath out the bottom and be taken away via the air vents under the floor do not block these or block the flow of air across the building.


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## carlb40 (13 Sep 2014)

The Bear":3jdk9je1 said:


> I insulated my suspended timber floor with celotex between the joist a number of years ago. I then just laid the original victorian boards back down on the joists and sanded them. I'm very happy with the result. If you fit the celotex tight between the joists and flush with the top I'm not convinced you need a membrane. Moisture wont penetrate the foil on the celotex so the only route is through 4 inch of joist. Also if fitted well you wont have any drafts either. IMO membrane is a waste of money, others will disagree. Based on the above I don't think it really matters what type of ply you use as long as its well fixed to the joists. If you're just putting new (or old) floorboards back down I would't bother with the ply, again not necessary and raises the floor so need to think what happens at the thresholds where it meets un-raised floors.
> 
> Oh and the insulation really makes a difference. The coldest room in my house is now the warmest.
> 
> Mark


I would do the same as Mark. Adding a ply/ chipboard subfloor would as Mark suggest need a threshold detail sorting, plus trimming of doors and raising the skirting board to suit the ply plus new floor. Also i'm not sure a retrofitted vapour barrier will help much as they are normally used throughout the house in new builds etc to keep the house air tight. Used in isolation on one floor will not do much. 
However if you want it purely as an extra for draught proofing, how about the cheap weed membrane? That will do the job and be a lot cheaper than tyvec etc.


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## RobinBHM (13 Sep 2014)

The purpose of a vapour barrier is used to stop warm air, which can hold a lot of moisture, from reaching the cold side and forming condensation. A vapour barrier is therefore always fitted on the warm side. 

In your case, the celetex, or equivalent PIR insulation that you are aiming to use will be foil wrapped and this foil is a good vapour barrier. If you fit battens low down on the faces of your joists, so that the celetex finishes flush to the tops of your joists, then you could use foil tape over each joist to complete the vapour barrier. 

Provided the floor void is still vented correctly to the outside, air circulation should prevent any build up of moisture.

If you want to fit a sub floor over the joists, this wont need to be resistant against any moisture from below the floor since it will be above the vapour barrier on the warm side. If you are thinking of 12mm hardwood flooring then a sub floor of 18mm material will be necessary. I have used 18mm OSB3 boards as a base for engineered flooring. A layer of foam underlay can be laid for engineered flooring as a floating floor.

The insulation will be well worth doing. Do fill all voids with foam (if you are buying a foam gun, make sure you have a can of cleaner and leave the gun clean each time, also if you get any foam on your hands it wont come off when dry)



In theory as others have said, if you are laying structural pine flooring there may be no need for a sub floor, which may cause threshold issues


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## Andy RV (14 Sep 2014)

Heres a list of plywod grades; http://www.tradewood.co.uk/media/a8eb56d38238402db2511b517bdf0301glossary of terms.pdf

Depending on which you chose will depend on what region your supplier sources their plywood from. A low to mid quality board will be ok for your application. There is no point putting down clean faced void fee BB grade when you're going to cover it with something else. 

As others have said t&g chipboard might be a better and certainly cheaper option?


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## shimmers (14 Sep 2014)

Thank you for all the replies, very useful fellas. The conclusion seems to be that, any ply will do and no need to pay for better face qualities or WBP...or even use OSB3 or MR. Chipboard. I will foil tape the celotex across the top of joists also, as Robin suggested. Andy, thanks. I too have that plywood chart...it's EN 314 (BONDING QUALITIES) that caused me some confusion. Ply on websites is often stated as EN 314-2 WBP...but looking at the standards, there are 3 classes under EN 314... Class 1/DRY, Class 2/HUMID and Class 3/EXTERIOR....and only EN 314-Class 3 is the equivalent replacement for WBP. So why do timber suppliers state their ply is WBP at EN 314-2? Or am I missing something :roll: ?


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## shimmers (14 Sep 2014)

WBP and plywood..an interesting read. Hope the link works...scroll to section 10.7

http://books.google.co.uk/books?id=AtSp ... 03&f=false


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## andersonec (14 Sep 2014)

If it is a Victorian Terrace as you state and there is no rot in the joists then it tells you that no 'vapor' barrier is needed, it has been working fine up till now, the only thing you must ensure is that your air bricks front and back are kept clear to allow the flow of air under your floor. With the constant resurfacing (?) of footpaths it is common for these air bricks to become clogged with tarmac, the tarmaccers will say "it's not my job".
If you want a little bit of extra security then spray your joists with some form of wood preserver, Thompson's Waterseal or some such stuff.

Andy


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