Concerned about BCO’s advice (floor insulation install)

ToastyToes · 12 Dec 2018

I have replaced all ground floor joists due to damage and the next stage is to insulate them now. The building control officer came round to inspect mid job and advised to install 80mm celotex between our 95mm joists and leave the extra 15mm gap at the top of the insation. (Detached Victorian (1890) with no wall insulation or cavity)

He didn’t mention anything about vapour barriers or taping between where the joists and insulation meet to keep it sealed. He even advised to not fill the gaps between the wall and outer joists. I’ve seen many comments about filling this gap with insulation or sand/cement to prevent drafts.

Is his advice good or should I be taping the gaps?

EDIT: the subfloor is very well vented around the whole house with an original DPC and no signs of rising damp. Some corners of the subfloor are wet after heavy rain fall but remedy work outside with drainage should help this.

wwwebber · 12 Dec 2018

So your floor joists are only 95mm ?

ToastyToes · 12 Dec 2018

wwwebber said:
So your floor joists are only 95mm ?

95x75 with 1m clear spans due to honeycomb walls at 300 centres. Very strong and sturdy.

KenGMac · 14 Dec 2018

So the BCO has asked you to install the insulation, but leave an air gap between the insulation and the underside of the flooring? given there is a well vented underfloor area, I am confused?

Surely if there is an air gap as above the benefit of the Insulation will be lost when the air whistles along between the insulation and the underside of the flooring??

Ken

Notch7 · 14 Dec 2018

I dont understand the logic....

I would be thinking flush to the top, foil tape and foam around edges.

KenGMac · 14 Dec 2018

I am with Notch7.

Any gap between the insulation and the underside of the floor wi totally negate the benifit of the insulation, because in a well vented under floor the draught will drag all the heat from the room above???

Ken

ToastyToes · 15 Dec 2018

KenGMac said:
So the BCO has asked you to install the insulation, but leave an air gap between the insulation and the underside of the flooring? given there is a well vented underfloor area, I am confused?

Surely if there is an air gap as above the benefit of the Insulation will be lost when the air whistles along between the insulation and the underside of the flooring??

Ken

Notch7 said:
I dont understand the logic....

I would be thinking flush to the top, foil tape and foam around edges.

KenGMac said:
I am with Notch7.

Any gap between the insulation and the underside of the floor wi totally negate the benifit of the insulation, because in a well vented under floor the draught will drag all the heat from the room above???

Ken

I thought his advice was odd...
I wanted to use 100mm Celotex but BCO said 80mm was enough. Given my joists are 95mm, I would have to make up the 5mm on the underside with batons which shouldn't be an issue.

Could filling the entire joist for insulation cause any issues with preventing the wood from getting ventilation? Or is having only the underside of the joist enough?

ToastyToes · 16 Dec 2018

Any thoughts on using expanding foam around the edge of the joists to the wall?

Leofric · 21 Dec 2018

If you can look on The Planning Portal - Part L conservation of fuel and power - Approved Document L1B -Accredited construction details - under masonry
cavity wall insulation you will see detail MC1-GF-03 timber suspended ground floor which should answer some of your queries. Discuss it with the BCO.

ToastyToes · 21 Dec 2018

Leofric said:
If you can look on The Planning Portal - Part L conservation of fuel and power - Approved Document L1B -Accredited construction details - under masonry
cavity wall insulation you will see detail MC1-GF-03 timber suspended ground floor which should answer some of your queries. Discuss it with the BCO.

Lovely. Page 3 if anyone else is wondering.

https://webarchive.nationalarchives...ry_external_wall_insulation_illustrations.pdf

tony1851 · 22 Dec 2018

If you can get hold of a copy, this book:

Stirling, C. (2002) Thermal insulation: avoiding risks. BRE Publication. (p63)

also advises that PIR board insulation should be level with the tops of the joists, to avoid heat loss through air circulation
directly under the floor boards.

Your bco seems not to be up to speed on this by advising something sub-optimal.

Notch7 · 22 Dec 2018

tony1851 said:
also advises that PIR board insulation should be level with the tops of the joists, to avoid heat loss through air circulation
directly under the floor boards

That makes sense to me.

And, as Ive just done a garden cabin with PIR, flush to the top, Im rather glad its not wrong

diynot

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Concerned about BCO’s advice (floor insulation install)

Leofric

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