Loft Conversation and Ventilation Listed House

[Carlymo]

I'm looking to convert our loft in our grade II listed property. My main concerns are around maintaining ventilation and finding an insulation solution.

The loft floor has already been boarded up using loft boards, and they have used 100 mm of wool insulation beneath. There is not enough head height to achieve 270mm insulation for better energy efficiency unless we were to not use the loft as a liveable space.

The pitched roofing tiles were laid over an impervious bitumen based felt sarking. The roof rafters and beams are exposed beneath. The rafters have about 100mm of depth, so again for insulation we wouldn't be able to achieve more than 50mm if we were to leave a 50 mm gap for ventilation. Since the roof is already lined with impervious sarking it is not clear to me if we can maintain breathability and therefore if it is worth using permeable insulation, cork boarding and lime plaster. To maintain ventilation is a 50mm gap enough or do we also need some vent rafters?

From my research, it seems the best solution may be to use a multi foil insulation which is thinner and can be either breathable or non breathable.

We had a building inspection done when we moved in, just over 1 year ago, and he found that the structure is sound and he didn't find dampness in the beams.

Any advice on this?

 

[freddiemercurystwin]

What part(s) of the building are in the listing? When you say the floor has already been boarded, what do you mean? The previous owners have boarded over the ceiling joists? Because I've never done or seen a loft conversion where new floor joists weren't required. You don't need 270mm of insulation in the floor except under the roof bits at the eaves behind the dwarf walls. Multifoils are misleading, to use them you need an air gap above and below and you need to use it in addition to some PIR anyway so there is little if any saving to be made in terms of space-saving.

 

[Carlymo]

Thanks for the response. It's grade 2 listed to the outside but we would likely need some listed building consent to convert the loft. Do you add new floor joists to raise the floor so insulation fits or for structure? Yes they added loft boards overtop of the joists so the loft could be used for storage. We already have sarking in the roof thought probably not as insulating as newer PIR.

 

[freddiemercurystwin]

You add floor joists to create a new structural floor, it's merely a ceiling atm. But 100mm insulation is necessary in the floor for sound (and sometimes) fire, especially if you have lathe and plaster ceilings. Sarking - do you mean the felt underlay beneath the tiles/slates or timber sarking? In anycase normally you would leave a 50mm air gap, ensure there is ventilation at the eaves and a continuous vent at the ridge, assuming your Building Control officer is happy to accept the existing roof felt is not too shabby. Here you would typically have about 50mm insulation between the rafters and about 100mm below. When a building is listed you may find you cannot achieve Building Regs compliance per se and some compromises may have to be met, it would be sensible to engage a professional to get your drawings done and plans approved whilst also liaising with your conservation officer assuming your local authority has one.

 

[ericmark]

I have lived in a listed building, and was surprised at what could, and could not be done, no problem ripping of stairs, seems they were not original, but not allowed to plane the floor to remove trip hazard where Marley tile glue has etched into the slabs leaving ribs once removed.

When I talked to my daughter who did work for the royal commission at one point, see explained it was down to how many other examples of the old building there are, so often it seems odd what is and what is not allowed.

England and Wales have different people, so only way to find out is ask. And often when you explain what you want they will give suggestions as to what can be done, without removing some special example.

 

[Carlymo]

You add floor joists to create a new structural floor, it's merely a ceiling atm. But 100mm insulation is necessary in the floor for sound (and sometimes) fire, especially if you have lathe and plaster ceilings. Sarking - do you mean the felt underlay beneath the tiles/slates or timber sarking? In anycase normally you would leave a 50mm air gap, ensure there is ventilation at the eaves and a continuous vent at the ridge, assuming your Building Control officer is happy to accept the existing roof felt is not too shabby. Here you would typically have about 50mm insulation between the rafters and about 100mm below. When a building is listed you may find you cannot achieve Building Regs compliance per se and some compromises may have to be met, it would be sensible to engage a professional to get your drawings done and plans approved whilst also liaising with your conservation officer assuming your local authority has one.

Thanks for the advise. And yes by Sarking I meant the bitumen based felt. I've reached out to our building control officer for now, but I think the felt is in good condition considering our building inspector thought so when we bought the house. I'm still confused on whether traditional breathable insulation & wall materials would work, or are worth using, considering we already have a non permeable felt lining the roof. Unless we were to replace the felt. We definitely wouldn't be able to achieve building regulations if we wanted to maintain 2.2m of head height.

 

[Mr Chibs]

Sorry, go to this party late... might have misunderstood.

Can't you do say 60mm in between the rafters and 75 below?

How much height do you have from floor to rafter level?

 

[Carlymo]

Sorry, go to this party late... might have misunderstood.

Can't you do say 60mm in between the rafters and 75 below?

How much height do you have from floor to rafter level?

I'll have to look at the heights again to see how much wiggle room we have. I think it depends on if we have to lime plaster or not as well as that adds quite a bit of thickness. I'm struggling to understand if we should use breathable materials.. ideally yes, but because we have a non-permeable felt under the roof, is there any point in using breathable materials under the roof.

 

[Mr Chibs]

Surely the plaster will only add, say 20mm or so?

Non breathable felt is fine if the ventilation etc is good, likewise you can get good and bad results from breathable membrane.