Green Building Forum - Replace cold roof with warm?

Replace cold roof with warm?

Osprey — Wed, 05 Jul 2023 16:05:53 +0100

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.

Replace cold roof with warm?

philedge — Wed, 05 Jul 2023 19:02:27 +0100

With an unused warm roof/loft youll be heating a large unused space and have a greater surface area to loose that heat through

Replace cold roof with warm?

revor — Wed, 05 Jul 2023 19:16:41 +0100

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.

Replace cold roof with warm?

Osprey — Thu, 06 Jul 2023 19:23:58 +0100

No pipes or tanks to insulate. Rafters are 2 x 4s.
True about larger area to heat.

Replace cold roof with warm?

tony — Fri, 07 Jul 2023 06:30:26 +0100

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

Replace cold roof with warm?

Osprey — Fri, 07 Jul 2023 11:34:01 +0100

True.
Are my thoughts re using loft as a 'dehumidifier' valid?
As I understand it, a warm roof would have a vapour barrier, and any moisture would be trapped in the house. A cold roof (as built, and as planned) would still allow a degree of breathability, and vapour removal, via the loft space.
Wall cavities will be filled (eps beads) and wall plates covered with Rockwool, so could still allow some degree of vapour transport.

Replace cold roof with warm?

jms452 — Fri, 07 Jul 2023 20:35:11 +0100

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.

Replace cold roof with warm?

GreenPaddy — Sun, 09 Jul 2023 11:44:42 +0100

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.)

Replace cold roof with warm?

WillInAberdeen — Sun, 09 Jul 2023 12:47:48 +0100

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

Replace cold roof with warm?

Osprey — Mon, 10 Jul 2023 16:10:45 +0100

Posted By: WillInAberdeenWarm 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

I think best practice is, for a warm roof, to have a VCL on the inside (warm side) of the roof insulation. So, not breathable.
You can have what I call a 'warm room in a cold roof' , where the room is surrounded by a VCL and insulation, but outside the box (including the rafters) is cold and ventilated.

Replace cold roof with warm?

Osprey — Mon, 10 Jul 2023 16:18:30 +0100

Posted By: GreenPaddyLofts 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.)

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.

Replace cold roof with warm?

WillInAberdeen — Mon, 10 Jul 2023 16:55:38 +0100

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.

Replace cold roof with warm?

djh — Mon, 10 Jul 2023 17:29:29 +0100

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.

Replace cold roof with warm?

WillInAberdeen — Mon, 10 Jul 2023 18:18:39 +0100

What roof covering do you have over the outside DJH, is it breathable? Sorry you did discuss it previously but I can't remember..!

If the outside is not breathable then the roof is not breathable and vapour control is needed, and vv.

Replace cold roof with warm?

djh — Mon, 10 Jul 2023 20:03:56 +0100

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.

Replace cold roof with warm?

WillInAberdeen — Mon, 10 Jul 2023 23:55:11 +0100

Interesting! Lots of people seem pleased with the intello membrane, which works as a VCL in winter when the vapour 'driving force' is outwards, but not in summer.

Osprey's plan is for vapour to keep on escaping from the house through the breathable ceiling/roof, so that requires a roof with no VCL in winter (tho obvs still airtight).

The Ty Mawr and Rockwool warm roof designs, support their roof coverings on battens/counterbattens, which are fastened through the breathable insulation boards and down into the rafters below. There are probably many other options, those were just a couple of examples.

Replace cold roof with warm?

Osprey — Tue, 11 Jul 2023 11:02:49 +0100

Posted By: WillInAberdeenDon'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.

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.

Replace cold roof with warm?

djh — Tue, 11 Jul 2023 17:41:49 +0100

Posted By: Ospreythe 'unvented loft with insulation at rafter level' versions have a VCL

But they aren't warm roofs?

It gets more difficult to agree a definition of a warm roof as time goes by. I mean a roof construction that has insulation outside all the structure, so all the structure is kept above the dewpoint. I suspect Will is thinking of something similar. The diagrams you point to have the tops of the rafters exposed, and thus potentially below the dewpoint, and so aren't warm roofs. The ones Will mentioned do have insulation above the rafters.

My roof as I described above is kind of an odd special case. The metal covering means there's not an easy way out for vapour (and hopefully no way in for rain) and it's a requirement for the metal roof to have a ply deck underneath it. That would ordinarily be a problem, which is why they have the fancy 'metal' breathable membranes to create a minimal gap, and why it requires a variable VCL on the inside as an insurance. We also hope that the hygroscopic nature of the insulation avoids any problems from condensation on or in the ply deck by buffering moisture.

Replace cold roof with warm?

WillInAberdeen — Wed, 12 Jul 2023 09:47:12 +0100

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!

Replace cold roof with warm?

djh — Wed, 12 Jul 2023 10:43:48 +0100

If the insulation was really not breathable, then they wouldn't need a VCL, because the insulation itself would be a VCL. The problem is that the insulation is in sheets, with joints in between and those joints tend to be imperfect and allow some air and vapour flow. So the VCL is there to prevent that flow.

Those drawings are a bit strange. They have two types of insulation, one lot between the rafters and one lot above. So they introduce all the problems of tightly fitting rigid insulation between rafters and for what purpose? Why not simply put all the insulation above the rafters? Simpler to install, probably cheaper to buy, and less chance of errors.

Replace cold roof with warm?

CWatters — Sat, 15 Jul 2023 17:23:44 +0100

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.

Replace cold roof with warm?

Peter_in_Hungary — Sun, 16 Jul 2023 09:54:57 +0100

Posted By: CWattersIt'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.

Posted By: CWattersNo impact on the overall appearance.

Putting 400mm above the rafters (or anything extra above the rafters) will either raise the height of the roof / ridge or alter the angle of the roof - either might cause the planning dept. to throw their toys about.

Replace cold roof with warm?

WillInAberdeen — Sun, 16 Jul 2023 10:32:05 +0100

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.