Installing PIR inbetween floor joists

[padstar]

I am going to start insulating the floor in my extension and the rest of the ground floor.

I will be using 100mm PIR and pan to staple a 10mm thick 100mm deep foam insulation strip to the joist and then cut the PIR 10mm less than the gap between joists so that it will fit in nicely when the perimeter foam is compressed to give hopefully a tight fit.

I appreciate this is overkill and would not be done if i were paying a builder but surely it will give a better result. The same idea as gapotape i see a lot of people on youtube use but a fraction of the cost. Is there any issue in using this method?

 

[martygturner]

The only issue is that wooden floors move and whatever you do with rigid PIR will have gaps...You would need to fix the boards down with some batten or skew nailing. The ventilation wind under the floor can be a right pain, cold spots appear in the gaps, moister finds its way in and no matter what the boards will move in the joist channel. Sealing the gap with foam wlll only last a bit as the movement breaks down the foam..

Personally I would ditch the PIR and use a membrane ( roofing felt) lapped up and over the joists to create a hammock for fibrous insulation to rest in. Its easier to cut and you only need a staple gun to fix it, pipe work and noggins can be cut round and sealed with tape.

Cap the insolation off with another sheet over the top of the joists and you will have a gap free, wind free, moisture free insulated floor that will allow movement.

 

[padstar]

Thanks marty. I have a lot of PIR left over/available FOC and my BCO seems to prefer the PIR route even though he excepts both methods are acceptable. Obviously this would be my preference as i have all the materials or certainly most available without buying in more.

 

[martygturner]

In that case I would batten top and bottom and still use some membrane lapped over the joists. Getting boards to seal permanently is a right pain in joists.