Under Floor Insulation Project - Completed

[thefurz]

Hi all,

I recently installed PIR insulation under the floorboards in my lounge. Hopefully some of the details here might help someone, given there aren't many tradesmen who offer this service (given the time involved, I can see why!).

The house is 1930's with a suspended floor and it's about 18 m2.

I ended up buying Celotex GA 4000 PIR boards with a 70mm thickness.

Here are some of the learnings/thoughts:

- Take up all the floor. It was taking up to 10 minutes per floor board to lift them and remove the old nails. Because it was time consuming, I decided to leave the odd board or two in situ and just batten under them. This was a mistake. The time spent trying to knock in battens under these boards, ended up being way in excess of the time it would have taken to just lift the floor boards.

- Lifting and removing all the boards, also makes the area easier to work in and safer (often I was treading on boards which were unstable, resting on others etc.

- Make sure you clearly label or number the boards, so you know where they need to go back to

- I used wooden battens to hold the PIR boards in place, which worked well (you just spend a long time cutting them to shorter lengths).

- The PIR sections don't slide very well on the battens when they are a good, tight cut. When I tried to slide the sections under the floorboards I didn't lift, it was very difficult. Another reason to lift all the boards!

- The sub-floor area was a mass of old pipes, dwarf walls, electrical wires etc, which made moving around, installing battens and cutting the PIR very tricky.

- The gap between the joists aren't consistent and the joists themselves often aren't horizontal. Every section of PIR had to be cut specifically for that space. It's very time consuming.

- We also found that one of the joists was just 7 or 8 cm from the wall and the skirting board made access from the top very hard. I then cut the PIR board to make a thin strip and pushed it up in to the gap but holding that section in place was very difficult. Putting a batten in there was nearly impossible, so I ended up screwing through the joist in a number of places, to give the board something to sit on.

- If you have a nail gun this will make the whole process much faster but they are expensive bits of kit.

- I ended up buying a multi-tool to cut some of the floor boards, where the board itself went under the hearth tiles or extended in to the hallway.

- I made sure to keep the sub-floor vents clear.

- I used expandable foam to fill some of the gaps around the side and silver insulation tape over the joints.

- I would say it took about 20-25 man hours to do the insulation work and replace the floorboards. It took another full day to lift the old boards and remove the old nails.

- Costs wise....I've spent about £540 (excluding the multi-tool and new saw). Insulation and foil - £370, lagging for pipes - £70, foam - £12, battens - £60 and nails/masks/glue - £20.

It was hard physical work and very time consuming!

I don't claim to be an expert but I'm happy to give thoughts on my experience if anyone is interested.

 

[dannymassive]

Looks like a good job done to me. No doubt the room will be much warmer

Are you continuing through the rest of the house?

 

[JohnD]

Well done!

Round the edges you can push in mineral wool quilt (as used in lofts). It squashes to fit small or irregular gaps and is very good at blocking draughts (which mostly come up round the edges of the room and under the skirting)

I like to use screws, especially over pipes and cables and round the edges of the room.

 

[thefurz]

Looks like a good job done to me. No doubt the room will be much warmer

Are you continuing through the rest of the house?

Thanks. I've also got the new fire being fitted in a few weeks' time, which will mean the chimney flue is blocked, which should also help. Hopefully the combination of the insulation, new fire, new radiators and new carpet will make the room warm.

My next job is to insulate the attic room (I've just put a post on that forum) and then next year I will do the hallway.

The rooms on the ground floor at the rear will be changed when we have an extension next year, so I'll have those done when that work is being done.

It was hard work and very time consuming but I'm hoping it will make a big difference.

 

[thefurz]

Well done!

Round the edges you can push in mineral wool quilt (as used in lofts). It squashes to fit small or irregular gaps and is very good at blocking draughts (which mostly come up round the edges of the room and under the skirting)

I like to use screws, especially over pipes and cables and round the edges of the room.

Yes....mineral wool in the smallish gaps between the joist and the wall would have been much easier to fit in.

Keeping the PIR (or expandable foam) in that space was a challenge because I couldn't knock a batten in along the bottom of the joist, facing the wall. There just wasn't room.

Perhaps attaching plywood on the underside of the joist and then mineral wool on the top would have been the easiest way to do it.

There were plenty of pipes in there......often sitting between the joists, which meant I had to cut the PIR with a channel to give room for the pipe. That was painful

 

[Notch7]

the joys of cutting / fitting insulation

theres a reason its a job often given to the labourer -its laborious

 

[foxhole]

Could have saved money and time by omitting the baton , a row of nails or screws does the same job and doesn’t need to be very accurate as you can push the insulation down thru the heads till perfectly flush.

 

[geraldthehamster]

Could have saved money and time by omitting the baton , a row of nails or screws does the same job and doesn’t need to be very accurate as you can push the insulation down thru the heads till perfectly flush.

That's what I did with mine. Cut all the insulation fractionally under size, rested on screws driven into the joists horizontally, then squirted a bead of expanding foam to fill the gap and hold it together

 

[camerart]

Hi,
I was talking about this, in the timber side of my kitchen, and the person said he went ot the bean bag factory in Birmingham, got masses of polystyrene beads, and pumped them under his floor, and it worked
C

 

[rangy]

Are you happy with the results warmth wise and how did you arrive at 70mm PIR boards( rather than 50 or 100)

 

[aveatry]

I was under the impression that you are supposed to leave a small gap between the top of the insulation and the top of the joists to keep an airflow under the floorboards - sort of defeating the insulation but the gap warms up ?

 

[thefurz]

Are you happy with the results warmth wise and how did you arrive at 70mm PIR boards( rather than 50 or 100)

Yes, the room now holds the heat when it is warmed up.

Blocking the chimney has obviously also helped with retaining the heat but the combination of the two has finally made the room habitable!

The joists were 100m deep, so if you used 100m PIR you'd have to use another method to hold the boards in place, rather than rely on the batons along the bottom of the joist.

I tried to use as thick PIR as I could fit.

 

[thefurz]

I was under the impression that you are supposed to leave a small gap between the top of the insulation and the top of the joists to keep an airflow under the floorboards - sort of defeating the insulation but the gap warms up ?

I didn't come across that when I did my research and spoke to Celotex.

The key air flow issue seems to be to keep the flow under the PIR, so the sub-floor area is dry and well ventilated.

 

[robinbanks]

went ot the bean bag factory in Birmingham, got masses of polystyrene beads,

Always wonder how people who do that transport the beads..

 

[camerart]

Always wonder how people who do that transport the beads..

Hi R,
I think they came in large polystyrene bags.
C