Opinions on replacing a vented suspended floor

[KneeDeep_in_it]

Hi all

I am hoping to get a little help! and clear some things up.

We have recently had an Extension built (mostly built by myself, signed off and all!) and now comes the time to tackle some of the rest of the downstairs work.

This weekend we gutted the livingroom 4M x 4M in size, back to brick and all, but now we also need to replace the suspended timber floor. Some of the joists are rotted and sheared, some have just dropped.

the cause of the rott was mostly due to the cavity wall being filled/bridged 300mm above the DPC with crap, broken down snots, blocked vents, lack of airflow and exterior ground level above DPC. amazingly the joists that were rotted were only rotten where they went into the wall!

All these have been corrected, it's now time to replace the floor, and i am having doubts... this added venting presents the obvious problem of making the floor colder and the room was never great anyway. Our kitchen has had a complete new floor build up including UFH in the screed, and now i walk into this cold room!

I am not planning on going this far again, maybe something in the middle? i want to stick to radiators in here, so i have options.. hopefully someone could cast a few

Easy option first

i could drop in 100mm Celotex between the joists as i go along fitting joists, maybe a return on the underside of the joist to keep them in place long term and also foam any slight gaps.. wrap the ends of the joists into the wall with DPC/visqueen and seal round with joint sealer. i am in favor of this due to cost and speed!
2nd option.

Fill with EPS and concrete!

the oversite is around 300-390mm deep to finished floor level. it would give me room to drop in a concrete base, lots of EPS and a screeded floor. the biggy here is the cost and also, are there any implications in doing this? its dry down there and does not seem to pool up with water from what i can see. the screed we had pumped into our kitchen was great, perfect finish and fast. however this is where the cost would bump up!

Thoughts?

 

[motorbiking]

The celotex has to be tightly fitted and there is always the risk of some thermal bridging. Are you also going to insulate the external walls? Insulation here is worth roughly 50% more than the floor.

You could use wire to create a suspended net under the floor boards and use conventional loft roll which will be around 1/3rd the cost of 50mm celotex and 200mm is roughly as good. I'd put 50mm celotex on the external walls with plasterboard foam too. Then top that off with plasterboard and you have U value of around 0.5 depending on what you already have in the walls.

 

[mattylad]

I Found this video to be very informative:

 

[motorbiking]

I personally wouldn't design in an air gap, as 1. you get one anyway when using foam adhesive and 2. if you breach it with socket wall plate fittings, downlights etc, you lose the thermal benefit of the plasterboard/air.

 

[KneeDeep_in_it]

Hi both.

Thanks for the info and the video was informative to be fair! Am still putting some prices together to compare.

Motorbiking, were not adding extra insulation to the walls, just replaced what was lost from under our window when we scraped the cavity. Part of the inner skin was removed and slab insulation added to make good.

Netting with loft roll is looking good and to be honest would be easier than trying to get boards bob on!

Also, im quoting in p5 22mm t&g boards, bad idea or not? Used them upstairs to replace some of the old floor. Only downside i see is planning the joins to make pipework access easy at a later date.

Great forum!!

 

[mattylad]

Fittings and downlights don't usually go in a downstairs floor?

 

[KneeDeep_in_it]

Think you skimmed my post a bit quick there matty, no downlights or fittings!

Just referring to ch pipes for maintenance.

 

[oldbutnotdead]

Don't worry about getting to the pipes for maintenance- if you or the plumber does the job right they won't need any. If it's placcie don't have joints in the void, if copper don't have compression couplings in the void, test it all on commissioning, insulate and leave it for 30 years plus
Worth mapping where the tubes go below the floor, just cut a hatch if you ever need one

 

[KneeDeep_in_it]

I hear you, and you would be correct i do the plumbing too. Its pex but coiled, with a 90 for copper out the floor, so access hatch at the pipe fittings would be needed. But thats no issue.

What i didnt know was this p5 stuff needed to be glued. I know theres no real need for access but the idea of glueing that down just pains me. I cant help it!

So what about 18mm marine ply? I dont mind handling the sheets and i can double up on a center joist to aid joining. Got all tooling to get nice straight cuts too. Centers are max 400, some less.

Looking at knauf insulation 44, i seen somewhere it degrades a lot less and less itchy. Ecose binding or something?? Likley to go with this!

 

[oldbutnotdead]

Have you priced proper timber floorboards (t & g or square)- last one i did there was only about £2/sq m between red pine and chipboard. The chipboard went down quick and easy but still squeaks a bit despite shedloads of 2" x 10 screws and glue.

 

[motorbiking]

Fittings and downlights don't usually go in a downstairs floor?

referring to the air gap advised in the video. The moment its breached, it no longer works.

 

[KneeDeep_in_it]

Have you priced proper timber floorboards (t & g or square)- last one i did there was only about £2/sq m between red pine and chipboard. The chipboard went down quick and easy but still squeaks a bit despite shedloads of 2" x 10 screws and glue.

Are we talking 22mm or 18mm? Iv only seen 18mm softwood stocked at most places. Would it be any good?

 

[oldbutnotdead]

Go to a timber merchant, not a DIY shed and not a general builders merchant (tho worth getting a price from MKM, they are sometimes good on timber). 18mm is ok, I prefer 20.