planning for retrofit of ground floor - suspended floor - insulation - damp proof

bobsjob

First time poster long time catching parts of my hands with hammers HAH!

Ground floor in my (circa70s) semi detached house a suspended floor with about a 2 foot space below the joists. The floor is always 'cold' and drafty. Especially throughout the winter. The sitting room and hall have laminate flooring and actually are not that bad but the kitchen is tiled and you can feel the cold coming up through the tiles. We have completed some other insulating improvements in the attic in the recent past and upgraded all the window and doors at least, but this still leaves me to deal with the suspended floor. External insulation on the Gabel side is a priority also but due to the cost it could be a while.

The plan is to strip out all the tiles, flooring and floorboards throughout the ground floor, approx. 40sqyds, Insulate and make the floor airtight.

My joists are 110mm deep which I was a little surprised about; does this mean I am limited to a max of 110mm of wool insultation, or can I pack in a 170mm?

On top of the DPE layer I plan on hardwood plywood, 18mm, not t&g. Would I need to increase this 18mm any further before relaying a new laminate floor with underlay?

MuchoGracious

The Continental Op

I know this is an advert but it does explain a lot https://www.ecologicalbuildingsystems.com/post/best-practice-approach-insulating-suspended-timber-floors

On the depth of joists you can make them for the purposes of insulation as deep as you like just screw another section underneath.

If you are putting 18mm plywood over the joists I'd add good solid noggins between the joists were there are joints in the plywood boards.

Lumen

Not sure why you would use hardwood ply rather than OSB.

I used T&G OSB glued, screwed and taped on the seams and at the perimeter, with the concrete wall prepped with airtight paint.

T&G means you don't have to put extra noggins in or cut to size. 18mm is plenty stiff enough under laminate.

If I was doing it again I'd probably put a membrane under the T&G and tape/bond that up the wall. I'd also probably use solid insulation rather than wool, although that poses a different sort of fitting challenge.

Architects & engineers will tell you that it's a better job to just fill the void with an insulated slab. They're not wrong.

C. Eastwood

in most suspended ground floors, the heat loss is caused by Convection caused by draughts up under the skirting boards, and also between the tongue & grooves in the flooring boards.

The best way to avoid this heat loss is by sealing all of these draughts.

The through ventilation of the void below the floor is to prevent Dry Rot from occurring in the Joists and T&G boards.

Placing insulation between the joists under the t&g boards, may interfere with this ventilation and may cause Dry Rot to occur.

Presently the joists will have approx 17% Moisture Content (MC). The timber will be contaminated with Dry Rot spores. For these spores to germinate they need lack of fresh air and timber at approx minimum of 22% MC.

A good solution to this in your floor is to remove all on the timber and clean out the void, remove the sleeper walls (dwarf walls), clean off the oversite concrete. Vacuum all dust from the timber.

Construct a new floor with 150 mm concrete, on approx 550 mm high density insulation on a Damp Proof Membrane on the oversite concrete.

Bring the DPM up 100 mm above the concrete floor finish, and mastic it to the walls and fit the new skirting boards over same.

bobsjob

Thank you @The Continental Op, @Lumen, & @C. Eastwood for your guidance and feedback.

After stripping, everything will be 'cleaned' out, dust and debris. I will inspect the space and joists for any damp. tbh; I have been at joist level quite often and I believe they are in great condition with very little chance of ROT. Sleeper walls and existing joists therefore should be in great condition. This will be top of mind, thanks C Eastwood.

I will be wrapping a moisture barrier/membrane around the base of the joists making a 'cradle' for the insulation by coming down between the joist (over and down), illustrated below. Wool insultation as I do not want to worry about fitting insulation boards perfectly and I can pack it in pretty well so no air gaps.

Capital Valley Plastics Ltd Green 125 Micron Moisture barrier, (L)15m (W)2.5m

Based on The Continental Op's suggestion I will rip some 5mm plywood stock - open to suggestions - down to say 100mm strips/lats and I will offset this off the bottom of the joist to give me an additional 70mm (approx) capacity for wool insulation. Aiming for 170mm of insulation, more the merrier.

Knauf Eko Roll5.68x1.14m (x170mm) (18x3.9feet)

After the insultation I will cover it with a damp proof layer, based on C Eastwood's reply, bring it up behind the skirting boards, 100mm.

Capital Valley Plastics Ltd Black 250 Micron Damp proof membrane, (L)25m (W)4m

Lumen; thanks for reminding me about Noggins; what timber am I making these from; standard 4x2 planed stock?

MuchoGracious

The Continental Op

Personally I think you have the best plan there OP. I don't like the idea of using plywood simply because it seems to be the first thing that woodworm go for? I'd use something nominally 2 x 3 rip it to the same width as the joists then predrill and prestart all the screws so its easier to fit them - might need bigger size starting timber if you can't get one that is near enough the width of you current joists. But that all assumes you can get a drill/driver in from underneath. This is just me being me but I'd take the opportunity to soak all the timber in a water based larsons preservative. But do a test first that the preservative won't stink the place out most of the water based ones are fine. I use the spirit based ones a lot and they do stink.

The Continental Op

Just a thought on the noggins one of the Kreg pocket hole tools might be useful but there are loads of cheaper version like the Trend one or even plenty of cheap chinese knock offs that work perfectly well. Just check they are adjustable for thicker timber as some only put the pockets holes near the edge so only any good for thinner timber up to about an inch thick. Don't worry about the special screws you don't need them especially not for noggins.

bobsjob

Thank you @The Continental Op, @Lumen, & @C. Eastwood for your guidance and feedback.

After stripping, everything will be 'cleaned' out, dust and debris. I will inspect the space and joists for any damp. tbh; I have been at joist level quite often and I believe they are in great condition with very little chance of ROT. Sleeper walls and existing joists therefore should be in great condition. This will be top of mind, thanks C Eastwood.

I will be wrapping a moisture barrier/membrane around the base of the joists making a 'cradle' for the insulation by coming down between the joist (over and down), illustrated below. Wool insultation as I do not want to worry about fitting insulation boards perfectly and I can pack it in pretty well so no air gaps.

Based on The Continental Op's suggestion I will rip some 5mm plywood stock - open to suggestions - down to say 100mm strips/lats and I will offset this off the bottom of the joist to give me an additional 70mm (approx) capacity for wool insulation. Aiming for 170mm of insulation, more the merrier.

After the insultation I will cover it will a damp proof layer, based on C Eastwood's reply, bring it up behind the skirting boards, 100mm.

Lumen; thanks for reminding me about Noggins; what timber am I making these from; standard 4x2 planed stock?

MuchoGracious

bobsjob

Hello, I am back with an update and a new challenge I am facing. 95% of the flooring is now completed; old boards out, fully membraned, insulated and airtightness layer in place and it’s looking good.

I have arrived back in hall now to where I started initially at the front door. I didn’t know what to do with this then and honestly I am still a bit frazzled.

There used to be a porch at some point in the past and it was taken out and the main door was pushed out. The porch part was covered with 20mm MDF and this was/is finished right under the front door. There is a double step outside but there is a gap between it and the door and at that level the finish is PVC into a piece of 6x1 which is attached to the new joists. This is not water tight NeverMIND airtight the 6x1 piece is showing a bit of rot in the extreme corners.

Here are couple of pictures of the situation; where do I start folks?

37B973A0-1A4C-4983-90D2-0F346649174A.jpeg

2CF40330-EBB3-4ADE-A02A-6D84831EEAC6.jpeg

10-10-20

If that step's being retained then I'd fill the gap with a stiff mix of concrete (so it doesn't shrink), but prime the surfaces with watery cement first. Build the concrete up higher than that step and slant it outwards to aid run-off. After it cures seal it with mastic along the wall to be sure. That should fix any leak in there, but you'll need to check that it's not coming in elsewhere too.

As for that under-floor, the gap between those joists looks wider than ~30cm? Is it? The supports aren't great. How big is this porch area; and you're not planning on tearing that up and replacing it? You could just go for an insulated concrete slab and be done with it....?

Nothing to add but do have a request. Any chance you can post a few pictures of your work to date as have a similar project coming up thanks

bobsjob

I will update with some photos shortly

bobsjob

Hi All,

Some photos; starting with the initial destruction of the kitchen area :) Remove floor boards and do an inspection and general tidy up. Now is the time to get anything else done at this level while it’s all open.

Complete inspection no issues found, got some new electrical circuits in while floor up. It’s also good idea to properly sand down the joists before wrapping them in the membrane.

wrapping over and across, using nice little battens to hold the membrane right across the gaps.

Kitchen need supporting noggins and as much insulation as I could squeeze in without making the membrane sag down. It actually stiffens up quite tight after this stage.

This is how the hall finished up:

Very tricky getting the floor boards and studs from the walls out from under the stairs but I went slow and eventually I got them out.

I had to go with a lot of noggins here and some new sisters as the original joists in that spot where in bad condition. Not bad as in rotten but not secure/solid as I would have hoped. Original plumbing had cut deep through the existing joists.

Next the Airtightness layer:

Put the airtightness layer down making sure it overlaps up the walls.

Tape up any overlapping edges of the airtightness layer.I choose not to finish up to the front door in this effort as it’s a bit tricky so I had to come back to that once the floor is down throughout.

I decided to go with 18mm OSB T&G for the new floor and this went down pretty well and is cost effective.

It took me a lot longer to complete then initially estimated. I took a week off work at the start of the month and just barely got the hall and that first third of the kitchen done. I have been at it nights and weekends since and while the new floor is now fully down there is still a lot of finishing bits to complete like the front door area.

Hope this helps you guys!

loyatemu

Are you going to put a wooden/laminate floor over the top then? Did you do all this with the family still in-situ or did they move out for the week? Total cost?

bobsjob

@loyatemu hey there.

Yes plan is for new laminate throughout (picking a high ware rate with water resistance).

First week during the first part of the kitchen and hall family had to move out to my parents, only for a few days during the worst of it. No safe access to kitchen or upstairs during that first week so it was a bit hellish. I had to pick up the pace as I knew then I may have bitten off a little more than expected.

Once the hall and first part of kitchen completed everyone home. Nights and weekends since with family home. One section at a time making sure everything ‘covered’ again before morning. Other areas of the kitchen and sitting room got easier as I progressed.

Not fully calculated the actual cost yet based on receipts but I expected core materials to come in around 2k including electrical first fix.

Cheers!

GreeBo

Fair play, and we've all been that soldier who under estimates how long a "simple" job will take.

Cue photos of GreeBo with the front of his car up on jacks so that the headlights would light up the first floor roof so he can get the felt and tiles back on after a "quick roof repair" one Saturday evening in November.... :)

bobsjob

Ha I can just imagine. This feels like it just keeps on giving! I just finished fitting new insulated plaster board around the back of the kitchen cabinets under the counter top totally unrelated to the floor. What was there was in bits and a major source of draft so had to deal with it. A fair bit of work undone on the plumbing and sockets now all needs to be done/finished again :( was hoping to have some cabinets back in today but that won’t happen.

SuperBowserWorld

Fair play. Brave. You'll learn a lot from this.

bobsjob

Thankfully I’m able for it, always been ‘handy’ but this is a biggy alright. Love to know how much I’m saving by doing it myself. I’m doing it to the best standard I can muster but those insulated boards got lashed in today as they are out of sight ;) still more filling to do on the joints and a bit of bonding in spots. There is a 5 inch gap at the base of all the walls and the floor (walls are dot n dab). Going to use as much of the insulated boards as I can to fill the spa e and use the jointing compound to finish might try my hand at a bit of skimming at some stage.

GreeBo

For the first time ever I just taped and skimmed the joints last year and to be honest it was a revelation. You would have no idea once painted other than the fact that you don't get any cracking of the skim when you try to fix something to the wall!

10-10-20

Yeah, I picked up skimming over covid, it's fine as long as you have the time and patience and have a good setup. The key thing that I found was that you fire the plaster onto the wall as quick as you can and then you have to "sit on" the plaster for the first few hours and make any corrections early on before it all fully sets, and also run the float over it every 20 mins to give a nice finish, again making corrections as you go. Make sure you practice on a hidden wall first and don't bite off more than you can chew!! Also know how to gauge the stiffness of the mix - you need it to sit on the hawk and not be sloppy. Loads of Youtube info on it.

Get a good quality float - not the standard Draper affair in Woodies and also a wide jointing knife.

GreeBo

That hardest part (similar to concreting) is that you have to just get it on the wall and then leave it until its workable. I found that I would be trying to make it too perfect too early, especially the first coat.

10-10-20

https://www.boards.ie/discussion/comment/119986448#Comment_119986448

True. I found that having a flow where you trowel it every 15 or 20 mins as you apply helped. So apply the plaster in the top-left working rightwards for 15 or 20 mins - then spend about 3 mins troweling the very first part. Then apply 15 mins - re-trowel all of it... then 15 mins and re-trowel... I felt that I got as good a job as I would have paid for and I was able to get the switches and sockets to my liking.

bazzjazz

https://www.boards.ie/discussion/comment/119984423#Comment_119984423

In terms of how much you are saving, I have been quoted about €11,000 to do the ground floor of a house, approx 65 sq. m (Oct 2023). Whatever about the materials it's certainly labour intensive.