Help required to insulate existing 'room in roof' project with PIR/Wood Fibre, in most cost effective way

[bayard1music]

Hello I'm a newbie here and in need of some pragmatic advice please. 

 

We've a Victorian detached house solid walls and double pitch roof in Kingston, Surrey. An energy analysis tells us that after the walls our roof is losing the most energy. We've a 2nd floor room conversion 'room in roof' which we use as an office .

 

It's very hot in summer and cold in winter. Entered by full staircase but without 2m head clearnance. Ther's no insulation in the roof between the slate tiles/mebrane and plasterboard. We'd like to make it more comfortable and lose less energy.

 

We've a finite budget to improve our overall efficiency of the house (lets say £25-30K) . The energy analysis says we need to spend £150K to go from F rated (current) to A. Our thoughts include insulating between the roof, replacing some strategic single glazed windows, adding lined curtains and putting a decent PV array on our roof with battery. 

 

Option 1 - Provisional quotes to remove the existing plasterboard, fill the rafter gaps with PIR/Wood Fibre and then line with 50mm insulation are coming in at £15K to £25K (includes rad moving and electrics). This would include 50mm insulation between the rafters and say 50mm on top to cover the rafters. Total insulation 100mm say. We've also requested that we cover the 3 gable walls. Advantages of this approach, gives us 100mm of insulation overall, Disadvantages, potentially blows our complete budget with existing quotes. There's 100+ square meters of plasterboard to carefully remove and rafter gaps to fill. (this alone doesn't get us a fully compliant room as don't have 2m height on stair access).

Below an alternative scheme to hopefully reduce costs and still save energy but the question is how much of an improvement might this make versus Option 1 above. 

 

Option 2 - cover the existing plaster board with say 60-75mm of wood fibre/PIR , and plaster over. (We'd still go with 100mm for the gable ends ) Screw/fix on top of the existing lining.  (Nothing to clear or dispose of less mess.)  The expectation is this would be much cheaper to carry out and leave monies say to do the PV and Curtains. My questions are simply (a) Is this approach a waste of time with no rafter insulation (b) will it provide useful energy reduction in view of the compromised approach ? 

 

 Any thoughts comment please to help steer us forward. Thanks in advance.

[ProDave]

I would love to see just what they advise will get a Victorian house up to an EPC A?

 

I think it is a safe bet your loft room has never been seen by a building inspector, and I would wager that the floor is little more than the existing ceiling joists.  So if you are hoping to achieve a building regs compliant room that you can describe as a bedroom, forget it.

 

So you have a non compliant room that is a useful but cold space.  I would not spend anything like £15K on that.  I would go for the cheap option of insulate over what is there and new plasterboard.  A lot of the improvement will be proper detailing to ensure air tightness of the insulation and boarding.

 

This is where being able to do that sort of work yourself wins. Insulating and re boarding is not that hard, do you feel up to ding it yourself?

 

Whatever you do photograph it so you have a record of the added insulation so you stand a chance of getting the next EPC surveyor to take notice of the insulation you have fitted, though that in not even guaranteed.

[Redbeard]

Agree with @ProDave that it won't, as you describe it, have been done under Bldg Regs. Agree that the 'joists' may be spindly, but until you have looked further I would not automatically say don't spend £15k insulating it. A significant percentage of the 19th Century terraces where I live (many  of which were built with attic rooms - and some which weren't) have approx 75 x 55 joists. Yes, things get a bit 'springy' if you push the stud walls further into the void, and I am not saying that a SE would be deliriously happy, but delve a bit first. In the terraced house example above there is a spine wall mid-way through the (?9m) depth of the house and stud walls which (albeit with too few nails) to some extent 'hang' the floor from the purlins reducing the front-to-back floor measurement to around 4m. If you can do the insulation work without significantly increasing the loads (and if there are no signs thus far of 'distress')then do consider it, after the aforementioned 'diligent investigation'.

 

What are the joist and rafter sections?

 

It's worth noting that although since 2010 common practice has been to put 50mm PIR between 75mm rafters leaving a 25mm vent path, the 'new' (and poorly-publicised) Gov't guidance (search room-in-roof best practice) suggests min 50mm vent gap, leaving you only 25mm of PIR. Of course we don't have dimensions but it looks like there's little room to spare. In your exploration I would include checking that cross-ventilation is not blocked at the ridge. Even with PIR it is highly unlikely you will achieve something 'Regs compliant' - that would need abt 150mm. Wood-fibre (undoubtedly 'nicer' in my view) would need nearly double that. Not much room left after that.

 

Interesting Q - although the 'conversion' may have been unauthorised it is now part of the house, and the Regs (Part L) say that if you 'add or replace a layer' (add to or take down the exg ceiling) of a thermal element you must either meet the target U value or agree with BCO a lesser std if accepted by them. I would say that you are within the remit of part L in what you do. Other views may be held.

 

Plenty more thoughts to come if required. Some sort of structral opinion once stripped out would be desirable. (IANASE*) * I am not a structural engineer

[bayard1music]

Hello ProDave,

 

Happy to share the key points. Broadly : Insulate the walls, insulate roof, replace all of our single glazed windows with double glazing , replace our existing  double glazed units, insulate between floors. install a heat pump and uprate rads to accommodate.  Probably £150K of work their estimate. But matches a colleagues expenditure.

 

I fear you might be right re the flooring . We're not builders and the floor itself has only recently come to our understanding. In truth the house was bought knowing the room wasn't fully compliant but we didn't fully appreciate what that might mean in detailed tersm.

 

I  thank you for your comment and advice about not going for massive expenditure , it would need a complete rebuild then including the floor to be effective.

 

Have you any recommendations as to the most suitable material to use to fix on top of our existing structure ?  (I'm 66 now and not a builder , so not sure I'd say this a diy for me but thanks for the tip.)

Edited September 18, 2023 by bayard1music

[saveasteading]

1 hour ago, bayard1music said:

complete rebuild then including the floor to be effective

There are ways of stiffening rather than replacing joists. Doubling up, and/or fixing plywood above or below. Needs an SE, an interested one.

[billt]

2 hours ago, ProDave said:

I would love to see just what they advise will get a Victorian house up to an EPC A?

 

Adding PV, which the OP mentions and 150k should pay for a lot of PV. You can get virtually anything up to an A with PV.

[bayard1music]

Thanks Redbeard for your comments. 

 

My belief is that we have 100mm space to fill but as you say we need to leave some space (say 50mm) means we can only put 50mm behind the existing plasterboard line . I know we've only a 100mm or so distance between the floorboads and the ceiling below. Guess confirming we haven't got the requisite strong joists. 

 

As you suggest the room is part of the house I'm not sure how we might convince any local regs people  that we only need another 50mm of insulation as a total of 100mm PIR won't comply. More will impact on the height space available , and tend to defeat the reasoning for keeping this as an office.

 

I fear this is project is starting to loose all traction as there are too many conflicts / issues. All solvable no doubt with large amounts of cash that we may not have.

Perhaps we're simply better off double glazing (prioritising on the poor windows)  and just living with the weak roof insulation? At least the windows are notionally simple and don't interfere with other aspects such as building regs.

Thanks again for all the comments.

[Redbeard]

"Perhaps we're simply better off double glazing (prioritising on the poor windows)  and just living with the weak roof insulation? "

 

Noooooooooo! Please don't think that. You are in the relatively rare position (in that most of us inherit part-insulated buildings) of potentially going from 'nothing at all' to 'something' in terms of insulation, and that is the steepest part of the curve. It's all diminishing returns after that. If you think you can add 100 mm (50 between, 50 under, resulting in a net loss of 50mm 'head-space') then, with PIR, you should get a U value of around 0.25 - 0.3W/m2K (includes a very crude adjustment for the 'timber fraction' - the amount of the insuation depth which is not actually insulation, but timber instead). The target is 0.16W/m2K, but the un-insulated 'base case' which you have is 2.00!

 

Do it, do it, do it, please!

 

Windows can be subjected to 'quick fixes', particularly with regard to air leakage. If you do not have to open them why not tape them up for the winter? And don't discount the 'cling film' 'secondary glazing' - remarkably good. Both short-term measures (and make sure you retain enough 'intentional ventilation') but can yield good results while you concentrate your major interventions on the roof.

[ProDave]

As above.  you are never going to make this comply with current building regs so don't even bother trying.  Add whatever insulation you can.

[bayard1music]

Hello Prodave again,

 

My only fear now in just adding insulation is the earlier comment from Redbeard stating that we should operate with building regs since 2010. Should we choose to sell the house subsequently might we be penalised by a prospective buyer say ? Sorry so many questions but thought this would be straightforward at a level. 

 

In addition not sure why there's discussion on the joists? Are there concerns based on adding weight to the roof structure .  Or is this comment purely with attempting to meet building reg standards. Thanks agian. 

[ProDave]

Any prospective buyer would be advised by their surveyor that is is not classed as a bedroom as it fails to meet building regs and it should therefore just be treated as loft storage.  So don't bank on it being classed as a bedroom and increasing the value of the house.

 

As the stairs don't have enough headroom there is little chance of you ever getting it to comply.  It is what it is and I see no harm in making the best of it that you can.

[Russell griffiths]

Have you considered moving that room back down stairs and turning that back into a cold loft space like it was before.