Internal insulation

Rosia

Hi all, I am trying to get a quote from a QS on building work on a detached 1950s bungalow I'm purchasing. Our budget does not stretch to external insulation so I want to install insulated plasterboard board on the inside of all the external walls and on the ceiling.

I was able to find a recommendation for the thickness of the board on the ceiling (300mm) but for the walls the only guidance is that it meets the target u-value 0.27 W/m2K. The construction of the house is cavity walls with pumped bead insulation. Could anyone provide some guidance on what kind of board would be needed so I can get a more accurate quote? Thank you!

Mellor

https://www.boards.ie/discussion/2058350816/internal-insulation

300mm boards on the ceiling. I think you need to check that.
Also, it's not really possible to recommend an internal board thickness without know what the current cavity insulation. It's a 50s cavity wall, and the cavity has been pumped, that was obvious relatively recent.

10-10-20

https://www.boards.ie/discussion/2058350816/internal-insulation

300mm is the depth of glass or rockwool insulation between the joists to meet the min requirement. That doesn't translate to the equivalent of 300mm of PIR boards on the same ceiling.

Rows Grower

Get as thick a board as you can afford money wise and space wise, it's an investment that you won't regret.

We went 100mm urethane backed plasterboard. House is transformed. Ran to about €100 a board but far cheaper than any external system. Don't forget to price in all the airtightness tape which is essential to avoid condensation behind the boards.

10-10-20

https://www.boards.ie/discussion/comment/122110838#Comment_122110838

Which tape is this?

This is the tape we used, hits the sweet spot between price and function. Sticks well to plasterboard.

https://www.ecologicalbuildingsystems.com/product/uni-tape

You want to pay particular attention to sealing around windows.

TimHorton

https://www.boards.ie/discussion/comment/122110902#Comment_122110902

Please Remind us of the new internal build-up on the wall you went with.

https://www.boards.ie/discussion/comment/122111177#Comment_122111177

I used foam glue and mushrooms to attach to the wall boards like these

https://www.insulationshop.co/112.5mm_xtratherm_xttl_thermal_liner_dot_and_dab_insulated_plasterboard.html

The stuff we got had foil on the inner surface of the PIR and behind the plasterboard.

We taped all the seams, added spray foam to any gaps and into the shaft of the mushroom fixings.

All reveals were finished with 25mm insulated plasterboard. We airtight taped behind the plasterboard reveals to the windows and then again between the plasterboard fronts and the windows.

We also taped the edges of the plasterboard to the internal walls and taped the the tops to the ceilings.

I think we achieved a good level of airtightness. The temp drops by 2C over 24hours with each internal to external delta of 10C. We installed MVHR for moisture control and cannot see any signs of condensation anywhere in the house.

Rosia

https://www.boards.ie/discussion/comment/122110721#Comment_122110721

You're right. The vendor has confirmed that there is 8 inches of fibreglass insulation in the attic and the QS said that 62mm on the ceiling would work.

Rosia

Thanks, that's very helpful.

Is insulated plasterboard on the ceiling actually OK? I'd assumed there'd be fire safety issues?

In my garage conversion i cross battened for ventilation, then 100mm PIR mechanically fixed to the battens,, then joists through which the cabling for lighting is run, then plasterboard. Part of the reason i did it this was to have extra support for the PIR in the case of fire- i also got a Phonewatch monitored smoke alarm in there for reassurance.

With 8inch of rock wool in the loft already you only need another 4inch of rock wool to max out on spec. Can you afford a 4inch ceiling drop ?

Dropped ceilings with a good taped vapour barrier will be a significant improvement which will significantly improve your overall airtightness.

However in an intense fire rock wool burns.

Mellor

https://www.boards.ie/discussion/comment/122115210#Comment_122115210

No issues. Insulation plasterboard or separately on battens it is the same.

The resistance to surface fire spread is good. But neither build up is fire resistant .

Rosia

Yes, the ceilings are 3 metres high so we can definitely afford to lose a little bit and still keep the height.