I need to replace the roof on my 1920s house (clay tiles, hips no gables) and was planning to replace it with an updated cold roof. Breathable membrane, dry verges etc to get a nice ventilated space. My thinking was that this could act as a sort of 'evaporator' for the main house: as we improve insulation, if there IS any additional moisture, it may find its way to the loft and be dispersed (no vapour barrier to the loft). I have concentrated on making sure there is a continuous insulation layer (cavity-loft) in my plans.

However, one roofer asked if I had thought of changing to a warm roof, where no ventilation is needed.I have no plans to put a living space in the loft.

I want to do the roof replacement right: which way should I go?
Thanks.

Posted By: philedgemain

Depends on how well you insulate the roof. Such a construction has advantages. No need to insulate pipes ducts tanks etc. No need to worry about covering electrical cables with insulation. I guess a 1920's house will not have very deep rafters so you may be looking at insulating in between rafters and over-boarding.

Larger volume, larger area to insulate , loads more opportunities for air leakage

Posted By: OspreyAre my thoughts re using loft as a 'dehumidifier' valid?

bluntly I think not. You don't want warm water vapour to go from your house into your cold loft, It will condense and cause issues over time.

I'd go cold loft (if you didn't need to heat the loft space) and do what you can do stop moisture getting up there - probably mainly sealing air leakage from the house to the loft.

Warm roofs can be breathable and allow moisture to escape - just depends whether the insulation and membranes are permeable or not.

Just an example: https://www.lime.org.uk/warm-roof-insulation-system.html
https://www.rockwool.com/uk/products-and-applications/product-overview/pitched-roof-solutions/rockfall-en-gb/#Overview

<blockquote><cite>Posted By: GreenPaddy</cite>Lofts were used as dehum's for a long time, so yes it will work. The question is, is it a good way to dehum the house. If the loft is well ventilated, then better to have vapour exiting the house, to the loft, rather than being trapped inside the house, raising AbsHum, if there's not a planned alternative for vapour removal (passive or active ventilation.)</blockquote>

Thanks. With a 1920s house, I don't think I am ever going to get it perfect, so I would rather err on the side of 'too much' vapour removal. From what I read, MVHR is probably not suitable. Wetrooms have extractors, as will the kitchen.

In our roof we do have a VCL on the inside. It is an Intello membrane with a variable Sd depending on ambient conditions. The idea is that it blocks vapour going into the roof but allows it to escape when conditions are suitable. So breathable in some conditions, not breathable in others. The exterior has a breathable membrane. The Intello is also our airtight layer for the roof.

Alas things aren't that simple. The outer covering is standing seam aluminium. Under that is the breathable membrane, which is of a 'metal' type. i.e. the membrane is topped with nylon 'wire wool'-like stuff so there is a smallish air gap to allow gas and liquid movement. Supporting the membrane is a ply deck, with the timber roof structure under that. And under the structure is (drum roll :) a layer of muslin, a layer of panelvent, the Intello membrane, a layer of paper-faced plasterboard and a plaster skim. The whole structure between the top and bottom surfaces is pumped full of Warmcel (i.e. shredded recycled newspapers). The condensation analysis for the warranty was done by Ecological Building Systems. So it's an unventilated roof, neither cold nor warm.

Pure warm roofs are fairly rare, I think, because there's usually a need for structural support for the roof covering.

My point was that a VCL isn't always necessary, but that doesn't stop you fitting one if you want. There's no reason not to, apart from cost.

<blockquote><cite>Posted By: WillInAberdeen</cite>Don't think it's good practice to have a VCL inside a warm breathable pitched roof, as the idea is to avoid vapour/condensation risks altogether, so vapour control is not required.

You might be thinking of an airtightness layer (?), but that can be breathable - Ty Mawr are using lime plaster in that link above.

Or you might be thinking of flat roofs which have a vapour/water tight membrane on the outside that stops vapour escaping? For a pitched roof, this can all be breathable, as it is covered by tiles.

Think about the warm breathable roof as thermally working the same way as your ceiling insulation, just mounted a few feet higher up! There's nothing to stop the moisture leaving, and plentiful ventilation on the cold side to carry it away.</blockquote>

I may be wrong (which is vey likely), but https://www.buildingregs4plans.co.uk/roof_detail_drawings.php shows various roof types, and the 'unvented loft with insulation at rafter level' versions have a VCL where I suggested, unless you have specifically included a ventilation path above the insulation.

As DJH said,
The "warm" roofs are further down that page, "Warm Pitched Roof Insulation Between and Over Rafters (U-value 0.11)", with an insulation layer over the rafters to keep them warm and dry.

But... those are not "Breathable" warm roofs because they are using non-breathable insulation, foil-faced PIR.

As their insulation layer is not breathable, moisture cannot escape, so they need a VCL to stop moisture getting in in the first place.

If all the roof coverings and insulation layers were breathable materials, then moisture could freely escape and so they wouldn't need a VCL. Then it would be a "Warm and Breathable" roof.

Here's another example https://www.mikewye.co.uk/STEICO-pitched-externally-fitted/


There are lots of options and it's a personal preference, whatever you feel comfortable with. Just a possibility to consider!

Normally I'd say go for a warm roof (insulation above the rafters) but....

If you have no plans to convert the loft the best place to put the insulation is on the floor of the loft. The surface area of roof is about 1.4 times greater than its footprint so a warm roof would need that much more insulation.

It's also much easier to just pile up say 400mm of insulation on the floor of the loft than put the equivalent above the rafters.

No impact on the overall appearance.

A warm roof doesn't raise the roof that much. Very roughly, 100-150mm insulation goes above the rafters, and about the same amount inbetween the rafters, obvs thinner layers if the insulation is higher tech.

When replacing a roof, there is a general right under permitted development for it to be +150mm above the plane of the previous roof, to allow for different build ups like this without involving the planners. That much thickness on the roof will not visibly change the appearance from ground level.