Is hybrid roof evil ?

[bmj1]

Hi all,

 

I've got a flat roof problem to solve. Roof is already built and sealed watertight.

 

Was spec'd as 125mm above / warm roof.

 

Built as 100mm above / 25mm between rafters.

 

Architect wants us to rip out and add above.

 

Building control says it's fine if the insulation between is pressed up against the bottom of the plywood, to form 1 layer.

 

All insulation is as 0.022.

 

Options

1) Rip off roof and rebuild

2) Remove the insulation between the rafters

3) Something else ?

 

Advice gratefully received !

 

Many thanks in advance.

[kandgmitchell]

Guess you're not going to get the full benefit of 125mm insulation as the bottom 25mm is bridged by the rafters. Did a quick check with a U value calculator and it's 0.178 with it all on the joists and 0.183 with it as you have it. Both round up/down to 0.18W/m2K. Put a decent vapour barrier behind the ceiling and it's unlikely to come to much harm as the deck material has 100mm insulation over it which used to be the spec anyway. This should keep the underside of the waterproof layer above dew point.

[Jilly]

My understanding is you need a 50mm ventilation gap above,  the insulation if you have that build up. Presumably this why the architect is insisting? I have heard horror stories of condensation peeing out thru light fitting. Search back through previous posts 

[ProDave]

We have a hybrid roof, 100mm above the roof and 200mm between the rafters.  No problems so far and it is a very warm very effective roof.

 

Why would you not want to full fill between the rafters to get as much as you can?

[ADLIan]

Do not use a hybrid flat roof unless insulation between joists is minimal (which becomes almost a waste of time & effort). Old rule of thumb, if you had to go hybrid, was at least 2/3 of thermal resistance above the deck/joists. Not sure with current insulation standards if this still applies.  Also requires an highly efficient AVCL at ceiling level which is very difficult/impossible to achieve. BS6229 on flat roofs and BS 5250 on condensation and moisture in buildings both advise against use of hybrid flat roofs. Problems can take many years to become obvious as all well hidden within roof structure.

[Gus Potter]

51 minutes ago, ADLIan said:

Problems can take many years to become obvious as all well hidden within roof structure.

@ADLIan sums it up.

 

4 hours ago, bmj1 said:

Built as 100mm above / 25mm between rafters.

You may just get away with this but not ideal.

 

Your Architect is rightly cautious and probably nervious about approving this.

 

To solve the problem you may want to pay your Architect a bit more and ask them if they will arrange to get this analysed for the dew point. If it comes back clear cut that the dew point is above the VCL on the top side of the boarding then you are on the right track. but you need to make sure the VCL is the right type and has been installed correctly for your own benefit.

 

Next you need to check that you are still achieving the U values for the roof as the 25mm below is subject to the repeating bridge of the joists. The repeating bridges will have reduced the thermal performance C.f having the uninterrupted insulation on the top side.

 

4 hours ago, bmj1 said:

Building control says it's fine if the insulation between is pressed up against the bottom of the plywood, to form 1 layer.

Lucky you.. that suggests you have a bit of spare capacity in the insulation or BC have missed the repeating bridge effect and the shift in the dew point.

 

For you own piece of mind get the dew point checked. If the roof starts getting wet that will cause big problems later on, a bit extra heat loss?.. well use common sense and beef up the insulation elsewhere for you own interests. When we get problems like this we take a compensatory approach.. add insulation in easy places to counteract the builder cock ups to some extent.

 

If it all goes horribly wrong.. probably not.. then before you rip the roof off consider adding more insulation and a new membrane on top of the existing. Often you find that if an extension you need to get the roof flashing in under the upper floor windows. Or you could take the ceiling down, take the 25mm insulation out and compensate else where insulation wise.

 

There are often plenty ways round this so don't worry too much about this for now.

 

Talk to your Architect, make sure you understand the potential problems, make an informed decision.. then you will feel much happier. On this ratio of insulation above and below the VCL you may find your are ok.

 

Mind you what on earth was your builder thinking about when they deviated from the Architect's spec?

 

 

 

 

[Dave Jones]

9 hours ago, Gus Potter said:

 

 

Mind you what on earth was your builder thinking about when they deviated from the Architect's spec?

 

 

 

 

 

That did make me chuckle.

[Temp]

In winter its obviously cold outside and warm inside. The 25mm insulation effectively "moves" part of the rafter from the warm side towards the cold side reducing its temperature. This increases the risk that water vapour may condense on the rafters.

 

Ideally you would get a condensation risk analysis done. However my understanding is that normally upto 1/3rd of the insulation between and 2/3rds above the rafters is ok.  You have 25/125 or 1/5th between and 4/5ths above so I agree with the BCO, it should be fine.

 

To reduce risk perhaps seal the gaps between the 25mm insulation and rafters to reduce the amount of water vapour,pour that can get through to the cold side and add a vapour barrier before plasterboarding.

 

Avoid down lights that need big holes cut in the vapour barrier.

[bmj1]

Thank you all for the advice !

 

Does having MVHR help at all with the condensation risk flagged ?