Insulating an existing skeiling

[Kenbob007]

Hi!  First time poster here...

 

I have noticed of late that condensation and mould is forming on the inside face of the plastered skeilings in each of our upstairs room.  Went into the loft the other day and found that there no insulation at all behind the skeilings!  I have researched this loads over the last few days but thought it would be best to run my plan and some questions past those who know better   In one of the rooms, the skeiling is peppered with cracks as well as the mould/condensation so I plan to start there.  Here is the sequence I'm looking at...

 

1. Pull away existing loft insulation and clean up dirt inside the loft around the skeiling.

2. Remove existing plasterboard.

3. Fit roofing battens to side of rafters, set 50mm back (100mm rafters).

4. Cut and friction fit 50mm PIR insulation between rafters (leaving 50mm air gap to roof membrane), angle the bottom edge to sit flat on top of wall plate (low expansion expanding foam to fill any gaps with wall plate) and oversail top edge past plasterboard on ceiling (possibly easier said than done), foil tape to joints between PIR and rafters.

5. Cut and screws 25mm PIR insulation to rafters running across the skeiling, underneath the rafters and 50mm PIR, angle top and bottom edges to fit tight to existing wall and ceiling (low expansion expanding foam to fill any gaps), foil tape to joints in PIR.

6. Cut and staple vapour control layer to 25mm PIR, foil tape to all edges to seal it against the PIR

7. Cut and screw standard plasterboard to rafters through VCL and PIR, angle top and bottom edges to fit tight to existing wall and ceiling using low expansion expanding foam to fill any gaps

8. Get a professional plasterer to do a better job than me of plastering the skeiling!!

 

Is this a sound plan? Have I missed anything out? And can anyone help me with these questions?

 

Q1: is this 25mm PIR totally necessary?

Q2: is the VCL necessary and can anyone recommend what type to go for?

Q3: should I use double sided butyl tape on the face of the 25mm PIR in line with the rafters to avoid any holes in the VCL?

 

Looking forward to hearing people's view on this...cheers  

[ProDave]

Hi and welcome.

 

Your plan there is on the right lines.  As you have already identified, the key to success is good detailing.  So well fitting panels of insulation fitted and all imperfections filled with foam and taped.

 

As to is an additional 25mm enough, many would argue not enough.  How much celiing height can you afford to lose?  50mm would be better.

 

But whatever you do will be vastly better than nothing.

 

Re detailing the plasterboard when finished.  Look at dry lining it aka tape and fill.  You only fill the edges and joints between plasterboard and screw heads.  I am the first to admit I have tried and failed miserably at plastering, but I can tape and fill a wall quite reasonably and can give my opinion on what products are good.

 

Out of interest how old is the house to have been built like that?

[Redbeard]

Don't know which part of the UK you are in (assuming you are in UK), but the 75mm you propose would not comply with Building Regs. 125 or 150 is commonly required, depending on the 'timber fraction' (how much of the 'insulation' is actually rafter or joist). The Regs (Part L) refer to Thermal Elements and, when doing work from the inside, refer to the room you are in, not to the whole house, so when they say that if you 'treat' more than 50% of a thermal element you either have to comply with Part L or show why you cannot (and pay for a Building Notice, it is no good to say 'well, it's only one of 3 rooms, so it's not even 35%'. If you are insulating more than 50% of the roof in that room, you have 'triggered' Part L. My comments refer to England. I think Wales is probably the same or similar, and I understand Scotland is significantly different.

[Kenbob007]

1 hour ago, ProDave said:

Hi and welcome.

 

Your plan there is on the right lines.  As you have already identified, the key to success is good detailing.  So well fitting panels of insulation fitted and all imperfections filled with foam and taped.

 

As to is an additional 25mm enough, many would argue not enough.  How much celiing height can you afford to lose?  50mm would be better.

 

But whatever you do will be vastly better than nothing.

 

Re detailing the plasterboard when finished.  Look at dry lining it aka tape and fill.  You only fill the edges and joints between plasterboard and screw heads.  I am the first to admit I have tried and failed miserably at plastering, but I can tape and fill a wall quite reasonably and can give my opinion on what products are good.

 

Out of interest how old is the house to have been built like that?

Thanks for your reply. Glad to hear I'm on the right lines. Any thoughts on the VCL and butyl tape? 

 

25mm with the 12.5mm p/board on top as much as I can lose really. 50mm would come very close to the top of the the window reveal and look odd imo. 

 

The reason I'm leaving the plastering to a pro is that room in particular has a horrible textured paint ceiling (not artex) so I'm going to get the pro to skim the entire ceiling flat as well as the skeiling....but will probably tape and fill in advance to keep cost down. 

 

The house was thrown together quickly after WWII. In having an full width extension to the rear a few years, the builder found all sorts of interesting construction techniques hidden inside the walls 🤣

[Kenbob007]

34 minutes ago, Redbeard said:

Don't know which part of the UK you are in (assuming you are in UK), but the 75mm you propose would not comply with Building Regs. 125 or 150 is commonly required, depending on the 'timber fraction' (how much of the 'insulation' is actually rafter or joist). The Regs (Part L) refer to Thermal Elements and, when doing work from the inside, refer to the room you are in, not to the whole house, so when they say that if you 'treat' more than 50% of a thermal element you either have to comply with Part L or show why you cannot (and pay for a Building Notice, it is no good to say 'well, it's only one of 3 rooms, so it's not even 35%'. If you are insulating more than 50% of the roof in that room, you have 'triggered' Part L. My comments refer to England. I think Wales is probably the same or similar, and I understand Scotland is significantly different.

Hi! Thanks for your reply. Yes, I am in England. 

 

Being the skeiling I don't have that sort of space to play with. The rafters are only 100mm deep so only 50mm PIR can fit between them (50mm air gap above PIR). That would mean a minimum of 75mm PIR would need to go below the rafters bringing the line of the ceiling below the window reveal. The rest of the flat ceiling is covered with around 250mm of loft insulation....does it that help me in terms of regs? 

[Redbeard]

The insulation status of the rest may not help you, but 'practicalities' probably will. Different BCOs have different views, but the 'ideal'being entirely impracticable (or in some cases 'looking stupid' - although that's subjective) should get you some sort of acceptance and a waiver. Alternatively you may not even be asked how much insulation you are installing. I always insist on telling them - and usually showing, too, via a 'step-back' in the work, even if the rest is near-complete - even if they don't want me to!

 

There are a few 'get-out clauses' with part L for refurb. One is that your proposals (for internal insulation of solid walls, for example) might cause moisture issues in a building originally built to allow the beneficial 2-way passage of water vapour, another involves pay-back time (but try to make that stick - it is hard to make it work), another relates to unacceptable loss of floor area. 'Practicability' and 'not looking stupid' are, unsurprisingly, not actually written that way in the Regs, but (if you actually get to see and talk to a BCO) you should be able to reach an agreement.

 

You also asked:

 

Q2: is the VCL necessary and can anyone recommend what type to go for?

 

It is advisable. In reality your average apex void probably has a bog standard ceiling below it with no VCL, so water vapour can enter the void. Hopefully, though, there is a Howling Gale in the void, which blows away the moisture-laden air before it can condense out. I think in your case, you have the opportunity, so take it, and 'fit a VCL'. This could simply be the foil on the PIR, taped at all joints and perimeters. Or you could use a plastic sheet. When I do, I tend not to use the thin (often green) VCL, but heavy-grade DPM, as it's more difficult to make accidental holes in.

 

Q3: should I use double sided butyl tape on the face of the 25mm PIR in line with the rafters to avoid any holes in the VCL?

 

Well.... The optimist's view is that a hole (in a VCL) with a screw in it is not a hole at all. The pessimist's  view is that it might still be a bit of one! Your idea is not unreasonable. I used to use 25 x 50 battens over (under!) the over-the-rafters layer. If I was feeling obsessive I would pre-drill the holes, gun in silicone mastic, then drive the screw through the silicone, so each screw gets its own air-tight and vapour-tight (??) grommet. Not fully proven, but pragmatic! Clean up the silicone carefully or your joint-filler will fall off!

[Kenbob007]

1 hour ago, Redbeard said:

The insulation status of the rest may not help you, but 'practicalities' probably will. Different BCOs have different views, but the 'ideal'being entirely impracticable (or in some cases 'looking stupid' - although that's subjective) should get you some sort of acceptance and a waiver. Alternatively you may not even be asked how much insulation you are installing. I always insist on telling them - and usually showing, too, via a 'step-back' in the work, even if the rest is near-complete - even if they don't want me to!

 

There are a few 'get-out clauses' with part L for refurb. One is that your proposals (for internal insulation of solid walls, for example) might cause moisture issues in a building originally built to allow the beneficial 2-way passage of water vapour, another involves pay-back time (but try to make that stick - it is hard to make it work), another relates to unacceptable loss of floor area. 'Practicability' and 'not looking stupid' are, unsurprisingly, not actually written that way in the Regs, but (if you actually get to see and talk to a BCO) you should be able to reach an agreement.

 

You also asked:

 

Q2: is the VCL necessary and can anyone recommend what type to go for?

 

It is advisable. In reality your average apex void probably has a bog standard ceiling below it with no VCL, so water vapour can enter the void. Hopefully, though, there is a Howling Gale in the void, which blows away the moisture-laden air before it can condense out. I think in your case, you have the opportunity, so take it, and 'fit a VCL'. This could simply be the foil on the PIR, taped at all joints and perimeters. Or you could use a plastic sheet. When I do, I tend not to use the thin (often green) VCL, but heavy-grade DPM, as it's more difficult to make accidental holes in.

 

Q3: should I use double sided butyl tape on the face of the 25mm PIR in line with the rafters to avoid any holes in the VCL?

 

Well.... The optimist's view is that a hole (in a VCL) with a screw in it is not a hole at all. The pessimist's  view is that it might still be a bit of one! Your idea is not unreasonable. I used to use 25 x 50 battens over (under!) the over-the-rafters layer. If I was feeling obsessive I would pre-drill the holes, gun in silicone mastic, then drive the screw through the silicone, so each screw gets its own air-tight and vapour-tight (??) grommet. Not fully proven, but pragmatic! Clean up the silicone carefully or your joint-filler will fall off!

Top work Redbeard. Appreciate you taking the time to reply in such detail. It really helps me, thanks 👍

Edited November 14, 2023 by Kenbob007
Typo

[Kenbob007]

2 hours ago, Redbeard said:

heavy-grade DPM,

Hi @Redbeard....what if I change the DPM for aluminium foil Membrane? I really don't need to buy much for what I need and I can get the ally in a much smaller roll.... 

[Redbeard]

Not a problem if the alu foil is intended to work as a VCL. DPM was just an example of something stronger than 'ordinary' VCL

[Kenbob007]

@Redbeard, thank you 🙏

[MikeGrahamT21]

Keep seeing this stuff popping up to use in conjunction with PIR, anyone come across it? http://www.gapogroup.com

[Kenbob007]

@MikeGrahamT21 not me, I'm afraid...