Correct way of installing PIR insulation boards in a suspended underfloor

[nailscrew]

Hi All,

we hired a crew to install floors and put insulation into underfloor on the ground floor in our house. We ordered 120mm PIR insulation boards , even if underfloor joists are 100mm, contractors didn't warn us that it will be pretty inconvenient to install 120mm boards, we just wanted extra warmth So now they are doing a lot of extra cutting into those PIR boards so that they fit on top of beams joists are resting on.  One thing I noticed is that these boards are not secured at all. They are kept in place with friction + some expandable foam in gaps applied with the gun. Contractors say that it is secure enough, but I'd like to hear other opinions.

My worry is that as timber contracts and expands with temperature and moisture fluctuations,  PIR boards will sag and eventually drop on the ground, leaving portions of the floor completely uninsulated.  I asked to put a tape across boards to prevent PIR boards from moving down over time, well as long as it remains sticky. My request was received with the look that I don't  know what I am talking about, which is true, hence my message here

Do I worry over nothing? How PIR boards should be secured in this situation? With a PIR boards thinner than joists I'd ask to instal battens so that panels can rest on them, but what to do when panel goes deeper than joists? That is if friction is not enough of course.

Many thanks in advance.

[Cpd]

Photos please…. 
I would always fit the boards and then fully foam and then cut back excess foam and then silver foil tape all junctions

post pics showing the work so people can see if it’s been done well or not. 

[Thedreamer]

I foamed our insulation boards but also I screw little bits of wood underneath the joists, these would span the joist, to support the boards on each side and resulted in each board being supported by four bits in total.

 

I used a mixture of PIR and glasswool to fill the joists, our boards sat flush with the bottom of the joists, with the bottom exposed to allow the timber to breathe.  

 

There are probably other ways of doing it.

[TonyT]

Expanding foam via a foam gun to give a controllable bead.

silver tape over all the joins and joists.

 

[Iceverge]

You probably don’t want to hear this but i’d rip it out.

 

Take the unused PIR back to the shop. 

Chuck any used offcuts into the loft.

 

Buy some membranes and mineral wool and do it properly.

 

[nailscrew]

11 hours ago, Thedreamer said:

I used a mixture of PIR and glasswool to fill the joists, our boards sat flush with the bottom of the joists, with the bottom exposed to allow the timber to breathe. 

When board depth matches joists, so that they can sit flush with the top and bottom this method would give me peace of mind. Unfortunately our boards go deeper than joists, so it is very hard to make support for them :(
 

10 hours ago, TonyT said:

Expanding foam via a foam gun to give a controllable bead.

silver tape over all the joins and joists.

 

Nothing at the bottom? Is friction + foam + tape enough to keep them in place for years? Wont tape lose stickyness over time allowing boards to "migrate" down?

 

9 hours ago, Iceverge said:

You probably don’t want to hear this but i’d rip it out.

 

Buy some membranes and mineral wool and do it properly.

 

What's wrong with properly installed PIR boards?

[nailscrew]

13 hours ago, Cpd said:

Photos please…. 

Added photos showing finished underfloor with PIR boards installed.

[dpmiller]

is that an offcut supporting it ^? Is the whole area done like that?

[nailscrew]

47 minutes ago, dpmiller said:

is that an offcut supporting it ^? Is the whole area done like that?

No, nothng supports these boards other than friction and some foam on the edges, hence my worry. That offcut just happen to be there, I believe it was removed  later. Last photo is to show depth of the panel relative to the joists.

[ToughButterCup]

48 minutes ago, nailscrew said:

... Last photo is to show depth of the panel relative to the joists.

 

I think you are right to be concerned.

17 hours ago, nailscrew said:

...
...My request was received with the look that I don't  know what I am talking about,

...

 

... and, I assume,  no evidence-based explanation of why the boards should - over time - stay there.

 

Until you get a definitive answer, there's going to be that leeeettlll niggle - have the boards dropped a bit or not?  You are right, timber moves - and so nobody can ever be certain sure what's going to happen. And so in my book, I'd assume it will happen.

 

I suggest you consider  securing them in some way that - never mind the glances or grimaces - reassures you. Stuff everyone else.

 

The cost? Less than sorting it out later.

Edited November 16, 2021 by ToughButterCup

[Cpd]

it will be ok, the boards are level with the top off the floor joists so little chance of cold air bypassing the insulation, just make sure to squirt plenty of foam into all gaps, just jam the metal nozzle into a small crack 1-2mm and give it a squirt as this will go a long way to helping secure the boards. Silver foil tape over the joists and insulation for extra protection against air movement and additional help in securing the boards for the long term. 

[Dave Jones]

can see why they didnt batten out the underside of those joists to support it as the insulation wouldnt fit.

[SteamyTea]

If there is some expanding foam between the joists and the insulation boards  that will act as an adhesive. Just a matter if how much.

As you have around 20mm difference, you could fit some counter battens on 20mm off stands from the joists.

Just depends how easy it is to get under the floor.

And builders can be twats, they forget who is paying them.

[Mr Punter]

Walk around on the boards.  at 120mm they should feel solid.  Any that move, get them refitted.  The Pink Grip foam adhesive is very sticky if the foam you have has not worked 100%.  Use with a gun and force the nozzle in so it is about halfway down the joist and squirt as you withdraw (oh err).