Roof insulation Air Gap

[GrantMcscott]

Hi

I am trying to find out if I need to leave ventilation between the roof covering which is 22mm OSB, breathable membrane, counter batten, battens then tile.  The eves have been filled with Thermaframe densely packed not sure if this is ok.

 

Currently I have put in PIR insulation and pushed it so it touches the OSB board does this matter or will I give it a 50mm gap.  Also even if it has a gap will it get any Air due to the eves being filled with Thermaframe

 

 

[GrantMcscott]

forgot to say the house does not have soffits so there is over facia vents

[Cpd]

Can’t answer the technical side of the question but by pushing the PIR all the way you loose the reflective values of the foil faced boards…..  not sure to what extent this effects efficiency @SteamyTea ? I know that in my build up I had a ventilation gap - slate - breather membrane - sarking board - 50mm air gap vented at the bottom and top - 170mm PIR between the rafters- 25mm PIR over the top - plasterboard  

[SteamyTea]

32 minutes ago, Cpd said:

not sure to what extent this effects efficiency @SteamyTea

Not much.

Most of the conductivity will be via convection and conductance.

[Dave Jones]

so do you have a warm roof or a cold roof ?

 

sounds like you have a cold roof. This means it needs to breath under the tiles. 50mm clear gap underneath the tiles and no blocked soffet.

 

[ADLIan]

A continuous ply/OSB sheathing is relatively vapour resistant and as such there should be ventilation below (above insulation).

[saveasteading]

This discussion so far is helping me out of some confusion.

From various sources I have seen that  PIR needs a gap to the sarking, and I assumed this was for ventilation, although mineral batts do not need it for some reason.

However, the Kingspan spec sheet shows the option to fully fill up to the sarking board, OR to leave a 13mm gap. The logic from what is said above is that the gap is only to create an air space and thus allow the aluminium foil to have an effect on insulation., and this is borne out by Kingspan's numbers. ie 140 PIR in a 150 space gives the same U value as 150 in 150.

 

Does a shiny surface (in the dark) reflect heat back into the foam? Is that affected by there being an air gap? Intuitively it makes no difference although it might work the other way and keep heat outside in the summer.

 

The plasterboard manufacturers used to claim that their inner foil surface aided insulation, but now don't mention it.

 

I am thinking that leaving the gap is a good idea for ventilating the sarking board, and I don't see why 13mm shouldn't work well enough.

 

 

[Temp]

Difference between a warm and cold roof construction (not to be confused with a warm/cold loft)....

 

* Cold roof - the insulation is between and under the rafters. This means there are structural elements on the cold side of the insulation (eg the top of the rafters).

* Warm roof - the insulation is above the rafters. This means the structural elements are on the warm side of the insulation.

 

The vast majority of houses built now and in the past are of the cold roof type. The risk with a cold roof is that water vapour created by people in the house can escape through the insulation and condense on the cold part of the rafters or other structural elements. There are two main ways of preventing this, both are aided by a vapour barrier on the warm side of the insulation..

 

* Vapour permeable: If sarking boards and the roof membrane are vapour permeable then you only need to ventilate the void between the tiles and the roof membrane.

* Vapour impermeable: if any sarking boards or roof membrane are not vapour permeable then you need a 50mm ventilated void below the impermeable layer. 

 

So the question you need to answer is : Are your sarking boards and roof membrane sufficiently vapour permeable that you can avoid needing a 50mm ventilated void below?

 

My understanding is that narrow sarking boards with 3-5m gaps are considered permeable but large sheets of OSB with no gaps might not be. Discuss with your BCO or Architect. 

 

[Cpd]

15 minutes ago, Temp said:

My understanding is that narrow sarking boards with 3-5m gaps are considered permeable but large sheets of OSB with no gaps might not be.

This is exactly what I have found, 150mm x 22mm sarking boards with a 3-5mm gap are very permeable and the roof performs really well, but as soon as you use sheet board it’s impermeable and need fairly serious ventilation below to stop moisture build up - this then leads to mould…..  don’t ask how I know ? I would personally always use 150mm x 22mm sarking board on any future roofs I build but I also put a 50mm ventilation gap below them as well ! 

[saveasteading]

My case is very specific. We now have possession of an old farm steading in the Highlands. The roof is substantially intact and I reckon was replaced about 50 years ago. There are traditional sarking boards of about 150mm, all built in inches of course and with the required gaps between. No vapour barrier. No insulation of any kind, anywhere.

The wind whips through at present, and that helps keep the dampness down where it does leak.

The roof does not need to be replaced, as rot is local and can be spliced, and worm is also surprisingly little (the wind?). It will all be sprayed.

The roof is slated, and repairs required are few.

 

Therefore our intention is for a mostly cold roof, with rafter infill using either pir or cavity batt, and I have to decide whether to leave a ventilation gap or not. The

rafters are 225mm so we have scope. It is not in any spec, and I was considering fixing Vapour barrier under the sarking, but I'm not sure there is any point.

I wonder if there is any proven logic to the ventilation gap being 50mm as that seems excessive to simply allow air flow under the boards: but from comments above, perhaps that is sensible with a 2.4 x 1.2 board, and unnecessary where draughts will require less pressure to move only 150mm.

Rules excepted, I am thinking 200 of insulation and 25mm gap will be ideal. then vapour barrier under the rafters and then plasterboard.

Whether to apply a thin strip of foam board (5mm?) to reduce the cold bridge at rafter/board is a cost/benefit consideration.

There will be some cold bridge effect through the timbers to the exposed ties, but that has to be accepted.

 

Re BCO, we have to apply for a Scottish Warrant, so we will put in a full design and await comments. I would like the design to be perfect and hence am thinking through these special details, some of which might not be standard.

Hence all comments and suggestions are welcome.

 

As background I have done only 2 Scottish Warrant applications, but scores in England. Never for a steading renovation though, and there is a lot to learn.  Architect is in the family, but with the same gaps in knowledge and experience of stone steadings.

[A_L]

17 hours ago, saveasteading said:

Does a shiny surface (in the dark) reflect heat back into the foam? Is that affected by there being an air gap? Intuitively it makes no difference although it might work the other way and keep heat outside in the summer.

 

The foil works because it has 'low emissivity', it radiates less energy to the cavity than an 'ordinary' surface (a black body radiator) The air gap is necessary to prevent loss of the heat by conduction. It will help to keep heat out in summer because they are also highly reflective of the same wavelengths.

[saveasteading]

23 minutes ago, A_L said:

 

The foil works because it has 'low emissivity', it radiates less energy to the cavity than an 'ordinary' surface (a black body radiator) The air gap is necessary to prevent loss of the heat by conduction. It will help to keep heat out in summer because they are also highly reflective of the same wavelengths.

 

I thought I 90% understood this, so can I try this?

 

Q Discuss how the foil on the faces of pir aids insulation of a house.

 

A.

 

Because somebody makes and sells stuff, and shiny looks good, it doesn't necessarily do as much as implied.

Basically it is shiny and reflects light, and other wavelengths, but only from the side that the radiation comes from? 

So in a heated house the inner shiny face of pir reflects heat back into the house, effectively back into the plasterboard that is touching it. But as aluminium is conductive it also transfers heat into the foam board and out to the world. 

 

In summer the radiated energy that comes through the roof covering is reflected back towards the sarking and ventilated away,  only if there is an air-gap?

The size of the gap will be irrelevant as long as there is air flow?

 

Therefore it works to keep out some summer heat, if there is an air gap and ventilation.

For winter internal heating, the foil is only there to keep the foam in a sandwich?

If fully enclosed and touching other materials on both faces, then the foil has no benefit?

[Temp]

4 hours ago, saveasteading said:

Therefore our intention is for a mostly cold roof, with rafter infill using either pir or cavity batt, and I have to decide whether to leave a ventilation gap or not. The

rafters are 225mm so we have scope. It is not in any spec, and I was considering fixing Vapour barrier under the sarking, but I'm not sure there is any point.

 

I expect you meant under the insulation. The vapour barrier should be on the warm side of insulation.

 

4 hours ago, saveasteading said:

I wonder if there is any proven logic to the ventilation gap being 50mm as that seems excessive to simply allow air flow under the boards: but from comments above, perhaps that is sensible with a 2.4 x 1.2 board, and unnecessary where draughts will require less pressure to move only 150mm.

Rules excepted, I am thinking 200 of insulation and 25mm gap will be ideal. then vapour barrier under the rafters and then plasterboard.

 

As I recall the 50mm gap is to allow roofing membrane to drape 25mm between rafters and still leave a 25mm ventilated gap (eg when there are no sarking boards to support the membrane). So my guess is 25mm would be ok if the BCO allows it.

 

The main problem with ventilation is usually at the eaves and ridge. Builders tend to forget and just mortar on ridge tiles. Later when there is a problem you are faced with fitting tile vents one per rafter bay as the rafters stop lateral air flow.

 

Typical suggested ventilation schemes for different roofs are something like this. Yours would be the one top right. Note that the dimensions are per meter. eg at the eaves (marked B) you need the equivalent of a 25mm wide slot the whole length of the eaves. If using round vents you need enough to provide 25,000 square mm of free area per meter.  

 

 

If you were to fit this type of common round soffit vent ..

 

https://mytradeproducts.com/products/manthorpe-circular-soffit-5-colours-g700?variant=37765056233625&currency=GBP&utm_medium=product_sync&utm_source=google&utm_content=sag_organic&utm_campaign=sag_organic&gclid=CjwKCAjwq7aGBhADEiwA6uGZp6QEf45bAAjO8jSqSrNeEZ7XuVKpoEqsHIx7wO44fffOpy19cZL1BRoCwkgQAvD_BwE

 

..it has only 2150mm² free area per vent. Meaning you need ELEVEN per meter !!  You probably cant fit that many in a single line as the rafters get in the way. You need two staggered rows. That's a lot of vent holes to drill.

 

Not so bad at the ridge where you need 5mm gap or 5000 mm² per meter each side.

[Cpd]

When I did my roof I vented the soffits at each rafter bay and then a 50mm gap all The way up below sarking and then vented the ridge with tile vents along the top x 4 The result is a VERY well vented rood, you can feel the wind blowing up each bay really well. In the picture you can see the area that is fully vented at the top. The bits of wood running length ways are so there is something solid to butt the insulation up to and adds another fixing point. I put 50mm of insulation up first (foamed and taped)  as in the picture - We used foam glue to stick up the first 50mm and held the board in place with battens until the glue had gone off and then removed the battens - that’s what’s going on in the second pic and then added another 170mm over the 50mm and attached this with foam glue and screws. It’s probably not how the “book” says to do it but it’s been up 8 years without a hitch. The last picture just shows the vents on the roof. 

 

 

 

Edited June 19, 2021 by Cpd

[saveasteading]

Excellent thanks for the info , and what a pretty roof.

Yes I meant fixing vapour barrier under the rafters and insulation, on the warm side.

And NOT fixing any under the sarking, just relying on the draught to remove any dampness.

 

 

As the construction is of slates direct to sarking boards with the traditional gaps every 150mm I would have thought that local draughts would do the job, rather than linking the whole under-sarking void to vents. (Yes you need an awful lot of circular soffit vents. I once talked a BCO out of vents by taking him into my attic to feel the existing draught. That is my current house (change of use) , with tiles onto sarking boards above a cold attic, and the wind whistles through it.) 

I might just show that existing detail, with comment,  and see what is said. There will be no soffit so I think a pragmatic solution will be acceptable.

I agree that 25mm vented space is sensible as it allows for local deviation.

 

I have read every word of a very useful blog on a steading rebuild in Aberdeenshire, and gained some info on what is the accepted way for a steading. It is clearly not a mainstream method of construction and there may be  many solutions, but it is great to hear of proven ones, and the reasoning.

 

With all that in mind I think I will do a detailed sketch and let you all see it, and await praise or disdain...better from this hub than the BCO.

The final version will then be drawn up in CAD for clarity and the ease of the BCO.

 

If we get it right between us then it can go on this hub as an approved solution.

[Wil]

On 19/06/2021 at 11:47, saveasteading said:

My case is very specific. We now have possession of an old farm steading in the Highlands. The roof is substantially intact and I reckon was replaced about 50 years ago. There are traditional sarking boards of about 150mm, all built in inches of course and with the required gaps between. No vapour barrier. No insulation of any kind, anywhere.

The wind whips through at present, and that helps keep the dampness down where it does leak.

The roof does not need to be replaced, as rot is local and can be spliced, and worm is also surprisingly little (the wind?). It will all be sprayed.

The roof is slated, and repairs required are few.

 

Therefore our intention is for a mostly cold roof, with rafter infill using either pir or cavity batt, and I have to decide whether to leave a ventilation gap or not. The

rafters are 225mm so we have scope. It is not in any spec, and I was considering fixing Vapour barrier under the sarking, but I'm not sure there is any point.

I wonder if there is any proven logic to the ventilation gap being 50mm as that seems excessive to simply allow air flow under the boards: but from comments above, perhaps that is sensible with a 2.4 x 1.2 board, and unnecessary where draughts will require less pressure to move only 150mm.

Rules excepted, I am thinking 200 of insulation and 25mm gap will be ideal. then vapour barrier under the rafters and then plasterboard.

Whether to apply a thin strip of foam board (5mm?) to reduce the cold bridge at rafter/board is a cost/benefit consideration.

There will be some cold bridge effect through the timbers to the exposed ties, but that has to be accepted.

 

Re BCO, we have to apply for a Scottish Warrant, so we will put in a full design and await comments. I would like the design to be perfect and hence am thinking through these special details, some of which might not be standard.

Hence all comments and suggestions are welcome.

 

As background I have done only 2 Scottish Warrant applications, but scores in England. Never for a steading renovation though, and there is a lot to learn.  Architect is in the family, but with the same gaps in knowledge and experience of stone steadings.

 

In the example you describe here, what stops the water getting between he tiles and onto your insulation if there's no breatheable membrane directly under the tiles. I ask this as I'm still trying to work out how I insulate my roof from the inside with a non-breatheable felt directly under the tiles. If you can effectively ignore/ cut the felt out (or in your case, not have any in the first place), what stops the driven rain coming the other way?

[Cpd]

4 hours ago, Wil said:

In the example you describe here, what stops the water getting between he tiles and onto your insulation if there's no breatheable membrane directly under the tiles. I ask this as I'm still trying to work out how I insulate my roof from the inside with a non-breatheable felt directly under the tiles. If you can effectively ignore/ cut the felt out (or in your case, not have any in the first place), what stops the driven rain coming the other way?

in my experience if the roof leeks with driving rain then you need a fully vented cold roof, not infilled with insulation. My house roof is made up of old small slates with no membrane and sarking boards, when it’s windy and wet, rain is driven under the slates and it runs down the inside of the sarking boards and roof joists but gets no further as the wind then dries it out before it gets into the ceiling where it’s insulated with rock wool. (The roof is about 120 years old) If there was insulation between the rafters then the water would get stuck between the timber and insulation as it would not dry out and this would cause the timber to rot.