Warm Roof Help

[richo106]

Hi All

 

 We are at the stage when we are sorting the roof. We have a couple of bedrooms with vaulted ceilings, flat roof section and ‘normal’ sections.

 

The plan was do a cold loft with 400mm insulation, vaulted ceilings with 120mm between rafters and 50mm over the top (below) with taped joints.

 

We are having MVHR and planning on installing it in the loft, but I’ve reading a lot on this forum and can the positives definitely in having a warm roof

 

I am going to speak to my roofer but could any suggest a typical build up of a warm roof please 

 

Any advice/information greatly appreciated 

 

[Johnnyt]

Our architect advised there had to be a greater amount of insulation above the rafter than in between with a hybrid roof.

Something I think is illustrated in the article linked.

 

https://architecturetoday.co.uk/roofing-in-detail-hybrid-roofs-best-practice/

[Iceverge]

There's mis match of terminology here unfortunately that regularly gets confused. 

 

A "warm roof" has the insulation above the rafters. 

 

 

 

This will naturally result in a "warm loft" where the attic space is inside the conditioned space of the house.

 

This can also be done with a "hybird roof" (insulation above and between rafters ) or insulation between and below the rafters. (sometimes called a "cold roof")

 

A "cold loft" is naturally where the entire cold loft is cold and all the insulation is placed above the ceiling. 

 

 

In your situation I suspect you are better off with a "Warm Loft"

You just need to decide on a

 

1.  Warm Roof

2. Hybrid Roof

3. Cold Roof

 

 

 

 

 

 

 

 

 

 

 

[richo106]

Yes I meant warm loft definitely 

 

The example you have given is for a flat roof, is this the same method used on a pitched roof with tiles?

 

Many Thanks

[Iceverge]

Slightly different as you will need battens to hold the tiles and can do without the top layer or ply/osb.  You would need to ventilate behind the tiles also. 

 

Do you have a cut roof or a trussed roof? Have you many overhanging eaves/gables etc? 

 

In fact do you have any drawings of the roof as planned? 

 

 

 

 

[richo106]

I have just spoken to my roofer, he said he can do a warm loft but would make the soffit/fascias big and bulky which we don’t want. The roofer was right in saying the architect needs to design in warm roofs to make them work etc 

 

So we can’t do a fully warm roof/loft

 

Is there another method we could use to create warm roof? Roofer suggested PIR and foamed in gaps between rafters leaving 25mm and then 50mm PIR overlay. The only issue with this is cost, I can imagine this method will cost a lot more than 400mm of loft insulation originally planned

 

I am guessing the MVHR distribution box and ducts sit under the 400mm insulation and then the MVHR unit sits on top of insulation on some floorboards/legs etc 

 

It’s just my air tightness detailing for cabling/ducting that will have to come through the upstairs ceiling 

 

 

[Redbeard]

Where is the ventilation going to be? If the PIR is very carefully and tightly installed you can run the ventilation up between the PIR and the membrane, having a dropped 'ceiling' at the ridge so that the ridge board (if any) does not block the cross-ventilation. But 2 comments: 1. BEIS Best Practice guidance now suggests 50, not 25mm gap (https://www.gov.uk/government/publications/room-in-roof-insulation-riri-best-practice). Yes, it's not really a room in the roof, but you are seeking to mimic the insulation strategy of one. And (2) You are going to all that faff of insulating the entire sloping roof simply to house the MVHR unit. Why not just stick-build an insulated 'MVHR shed', with no insulation on the 'floor' (the first floor ceiling within the footprint of the 'shed') and you can wrap it in cheap mineral wool at 300-400, rather than a relatively unexciting 75mm PIR?

 

Note that what your roofer describes is not a Warm Roof at all, though it may give you a slightly warm(er) roof space than if you had not added the PIR. It won't be that warm, though, as you have added 400mm of mineral wool on the first floor ceilings to make it cold.

[richo106]

Thank you @Redbeard

 

I think the best option for me now is the MVHR house 

 

I will make sure I pay good attention to where the ducts and cables go through the ceiling 

 

I will scour the build hub website for inspiration for a MVHR house

 

if any has done one before and got a photos or ‘drawings’ /doodles it would be great to see

Thanks again

[Redbeard]

For cable and ducts you can buy grommets from, for example, Green Bldg Store, or make your own with EPDM and air-tightness tape.

[Iceverge]

 

If you can draw it you can build it! 

 

Here a quick sketch I did. Just attach some rafter tails on top of the rafters and fit the PIR around them. 

 

Your builder is a sadist if they think it's a good idea to put PIR between rafters!! 

 

Matt shows how to do it here. 

 

 

 

[Iceverge]

For air tightness use a 600 mm strip of DPC under the wall plate on the inner leaf. Mesh it and plaster into your inner airtightness layer on the blockwork. 

 

Then wrap it around the end of your rafters and tape it to the top of the ply. Tape all the ply deck together and you'll have a really robust airtight layer and simple to execute. 

 

[richo106]

Thank you!! @Iceverge

 

I really like this idea, I will put it to my builder/roofer.

 

I know it will cost a lot more but will save a lot of time and money internally

 

Many Thanks 

[richo106]

How would the front gable fascia look though? Would that look bigger than normal?

 

Many Thanks 

[Iceverge]

@richo106

 

It can be any width you want. 

 

Have a look at the video I sent, it might be clearer than explaining it. 

[Renegade105]

Hi,

 

on an ICF build, wall plate needs to be set on top of the ICF core, was thinking of using a mixture of 1) acoustic flex sealant, 2) airtight foam, 2) mechanical fixings to clamp it all down.

 

what’s you’re recommendation here?

 

will have attic truss and slate tile with spray foam insulation

[Temp]

On 18/02/2023 at 17:39, richo106 said:

 

I think the best option for me now is the MVHR house 

 

The roof design (warm vs cold roof structure) is a separate decision to how to ventilate your house (MVHR vs Trickle vents).

 

I would recommend MVHR whatever roof design you choose. Put the fan unit as far away from bedrooms as possible. If you have an attached garage the roof space of the garage might be a good choice. The loudest part of our system is the air outlet (eg when you stand under it in the garden).

[richo106]

9 hours ago, Temp said:

 

The roof design (warm vs cold roof structure) is a separate decision to how to ventilate your house (MVHR vs Trickle vents).

 

I would recommend MVHR whatever roof design you choose. Put the fan unit as far away from bedrooms as possible. If you have an attached garage the roof space of the garage might be a good choice. The loudest part of our system is the air outlet (eg when you stand under it in the garden).

Yes we are defo having MVHR in our house, I meant a mvhr house within our loft to keep it in the air tight envelope 

 

we are having the vents out the roof so hopefully that will help with the noise 

 

many thanks