Views requested about ideas for minimising materials costs creation of a warm roof space insulation system

[NickV]

Hello, 

 

I would be very grateful for your views and advice about some ideas I have had to keep the material costs of insulating the roof space of my late parents house to a minimum

 

Firstly some background.

 

The house is located in a valley bottom in the Chilterns and we frequently have very low night time temperatures during cold snowy winters. The lowest temperature we have recorded is -16C. Nights of -10C are not uncommon.

 

I would like to insulate the roof of my home as it is woefully inadequate at present and a major source of heat loss from the house. The roof space has not been insulated hitherto, because the it houses the cold water tank and pipes and with the low temperatures that can occur, there is obviously a significant risk of these freezing. In the current state, heat from the living space below keeps the roof space above freezing. This also precludes putting a deep layer of rockwool or similar between and over the ceiling joists of the loft floor.

 

The house is a semi-detached, built c. 1965. It has a double pitch  roof covered in Marley Ludlow Plus tiles (actually they are Leighton tiles, but Marley Ludlow Plus are identical). Tiles are laid on battens over bituminous reinforced hessian sarking cloth. The rafters are 50x100mm, set at  c. 400mm centres. There are 20 rafters per pitch.

 

The plan...

 

I would like to install, 50mm thick PIR board between the rafters and 50mm thick PIR board backed with plaster board under the rafters giving a total PIR insulation thickness of 100mm. I would like a greater thickness of PIR board, but there are loft headroom constraints to take into consideration

 

However, besides the PIR board, it turns out that a major part of the cost would be firstly 50mm roofing batten spacers that would be attached to each side of  the rafters directly below the sarking cloth. And secondly, installation of roof vents to ventilate the space above the insulation board, between the rafters. 

 

Regarding the battens. The guidance is to install battens on each side of each rafter to set the ventilation space. Now, since there are 40 rafters (20 per pitch) and each is 4m in length, that amounts to 80 battens and a total batten length of 320m!. So what is the purpose pf the battens? Are they simply to acting as a spacers, or are they also acting as a draught excluder when the PIR board is pushed up against them? If the battens are simply being a spacer, I can just use short sections of batten, at say half meter intervals, along the top of each rafter side. Obviously doing the latter would save a lot on the cost of timber.

 

Turning to the roof vents. If guidelines are followed I would need to install one at the top and bottom of the pitch, between each pair of rafters. The cheapest I could find that are compatible with Ludlow Plus tiles are made by Ubbink at ~£18.00 ea.  With 40 needed per pitch that amounts to c £1450 to do the entire roof. So to reduce the number of ventilation tiles needed, I the idea is to bore holes through the top 50mm of the rafters to enable airflow both up and down the ventilation space and between the ventilation spaces. I was thinking of groups of 10-15mm diameter holes, at say 500mm spacing. So my question is this advisable, or would doing this dangerously weaken the rafters?

 

Many thanks for your thoughts and views.

[Bonner]

Are you intending to use the loft space as a room or just insulating it to keep the tank from freezing? If the latter, you need to rethink …

If you need to insulate between the rafters, you need a minimum of 25mm air space so 75mm PIR might work depending how much the felt sags. No battens needed, just fit the PIR flush with the bottom of the rafter, then you know there is an air gap above. Make sure you fit the PIR snuggly using expanding tape or foam. Assuming the felt is lapped, there should be adequate ventilation, no need for all those vents (IMHO!!)

[NickV]

Hi Bonner,

 

Thank you for your reply.

 

You are correct in that the roof space will not be used as a living space. only for storage.

 

It is interesting that you say that roof vents are not needed and that I could reduce the ventilation space above the PIR board to 25mm.

Do you have some links to this suggestion.

 

However, this does worry me, as this goes against all the PIR board installation guidance from manufacturers and building standards, I have seen and so might be an issue picked up on building survey, if the house comes to be sold.  Obviously, I also dont want to improve insulation at the cost of creating a condensation/damp problem. 

[Conor]

Just put 400mm of mineral wool on the loft floor. They way you are proposing is more expensive, time consuming and not as effective. Just lag the water pipes really well.

[Temp]

1 hour ago, NickV said:

You are correct in that the roof space will not be used as a living space. only for storage.

 

 

In that case it's far better to insulate the loft floor. Just don't put any under the water tank. You can drape plastic sheet over the tank then put insulation on top and sides. Let the insulation over pipes where possible and lag them well where it isn't.

 

If you want storage build a platform above the insulation. But check the floor joists are strong enough. Sometimes they are only designed to support the ceiling.

 

You should preserve ventilation at the eaves. Air should flow in one side of the house, across above the insulation and out the other side. There are special ventilation tunnels designed to stop insulation blocking air flow when you push it down into the eaves..

 

 

https://www.roofgiant.com/fascia-vents/refurbishment-eaves-panels-pack-of-50/

 

 

Edited August 5, 2022 by Temp

[NickV]

12 hours ago, Conor said:

Just put 400mm of mineral wool on the loft floor. They way you are proposing is more expensive, time consuming and not as effective. Just lag the water pipes really well.

No realistic amount of lagging would prevent pipes freezing if temperature ever fell to -10C in the roof space. Basic physics

[Conor]

8 hours ago, NickV said:

No realistic amount of lagging would prevent pipes freezing if temperature ever fell to -10C in the roof space. Basic physics

They won't tho. Temps will never drop that low due to residual heat in the loft, heat coming from below (despite insulation) and the insulating effect of the roof structure. It takes a very long, very cold period of weather to have lagged pipes in a loft freeze. We had it here back in 2010/2011 but loft pipe bursts we're almost only in unoccupied or unheated properties (we did a lot of analysis for the water company afterwards).

[TerryE]

As @Temp advises there are significant potential issues with converting a roof designed for a cold/ventilated environment into one adjacent to a warm space.  You need to be aware of the associated risks, as adding insulation directly under a tiled/slated roof can introduce a leakage trap / condensation trap that can lead to rotting your roof timbers.  This risk isn't insurmountable, but needs to be addressed in your plans.  You will need to ensure good eave and ridge ventilation with a minimum 25mm and 50mm preferred air gap between the PIR and that sarking felt so that you can get a good (cold) airflow under the felt. 

 

Is this all worth it if you are not planning to use this space for some warm function?  It will be a lot easier to do as Temp suggests.