Taping foil backed PIR and PIR Light cut outs

[Mmark]

We have a cold roof mansard loft conversion and are fitting down lights. We now know warm roofs are better for down lights, but too late now. We cannot reduce head height further, so there is not room to have a service void below the insulation.

I have two questions. The first relates to the rectangular light cut outs and the technique of just pushing the square up (into the air flow cavity) to try and make space for the downlight while reducing the draft flow to a degree. Is this a reasonable approach? I can see why it's more practical (easier) for the builders to do it this way.

The second is on taping. The builders have done a good job in cutting and fitting. Most fit snuggly and I only see a few with 1mm or 2mm gaps. However the builders are not taping anything.They told me properly fitted foiled backed (both sides) PIR boards should be sufficient, and that there will be sufficient air flow above to avoid any issues. In short this is how they have always done it, never been an issue, always gets past building regs.

I then read a few posts online, some indicated that it's not uncommon for builders to skip taping. Some others said the tape comes away from the joists after a few years, making it pointless anyway. Which makes me believe I shouldn't worry about this too much.

I've attached pictures showing the work, they are just putting up plaster board over this, without any other further changes. Building control will be inspecting soon, but as all the ceiling plaster boards are up, not sure how they would evaluate this or not.

 

[Notch7]

The builders are wrong.

There must be a properly fitted vapour control layer fitted on the warm side of the insulation.

Foil taping to join up all the foil on the insulation is one way, although an actual control layer sheet is the recommended method.

I personally think taping or a vcl + foil backed plasterboard is a good belt and braces method.


Long term whatbthe builders propose could lead to interstitial condensation.

 

[Mmark]

The builders are wrong.

There must be a properly fitted vapour control layer fitted on the warm side of the insulation.

Foil taping to join up all the foil on the insulation is one way, although an actual control layer sheet is the recommended method.

I personally think taping or a vcl + foil backed plasterboard is a good belt and braces method.


Long term what the builders propose could lead to interstitial condensation.

Yes, that definitely seems to be what I'm reading everywhere.

Is this something that is best practice, or something that building regs would insist on? If building regs insist on it, I don't get how they do this on every build they do, without problems. They have done over 200 loft conversions in and around London. Although they use a private building regulations firm, I hope they don't just "waive" stuff through?

Considering tape costs so little and it takes so little time to do, I'm trying to understand why they wouldn't even want to take this extra precaution - but I don't get much further than "it's not necessary, we've done this for X years and Y amount of conversions, without issues".

Downside is I don't have a leg to stand on, they insist it'll be fine. So without building regs or someone qualified insisting it's taped, there is not much I can do. My main hope now is as I had a 30 year old loft conversion before, I'm just enlarging it to full mansard, so if I didn't have interstitial condensation before, I'm hoping i don't need to worry about it now. Bathrooms are well ventilated, we live in London and not directly by the river etc, so maybe less of a risk.

 

[Ian H]

What’s on your drawings? Ours was like yours then had a full layer of insulation board on the underside of the rafters, my mate did a kitchen extension and his drawings showed the insulation like yours without the full layer under.

 

[foxhole]

My loft

• foxhole
• 3 Dec 2015
• 1

100mm insulation .

Taped all mine then insulated plasterboard over top to achieve insulation value .

 

[Leofric]

The point of a vapour barrier is that it should be a continuous layer to check the passage of vapour from the warm to the cold side of the insulation. Whether insulation is required under the rafters as well as in between depends on the maximum U Value specified for the roof. Ask the builder what U value your roof achieves, but is the job being done under a Building Notice or Full Plans Submission What air gap is there above the insulation
Discuss it with BCO if they are visiting site shortly.

I then read a few posts online, some indicated that it's not uncommon for builders to skip taping.

Some builders might skip a lot of things but that doesn't mean it is right

 

[Mmark]

My loft

• foxhole
• 3 Dec 2015
• 1

100mm insulation .

Taped all mine then insulated plasterboard over top to achieve insulation value .

i’m jealous i guess that’s the advantage of doing it yourself, you know you get it right.