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- Internal Solid Wall Insulation (IWI)
- External Solid Wall Insulation (EWI)
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Internal Solid Wall Insulation (IWI)
Important to Know
• Internal wall insulation of any sort should only be carried out if you are sure that the condition of the outer roof, rainwater goods and wall are good; that is, there is no way that water can get into and trapped in the wall from the outside
• Internal wall insulation is best undertaken only if you can be sure that the insulation will be fully continuous, avoiding all thermal bridging and air leakage
• Due to the risks associated with interstitial condensation, which will occur with any internal form of insulation, it is best if the level of insulation is less than typically specified. In other words, aim for modest U-value upgrades, concentrating instead on the continuity of the insulation and airtightness measures installed. This will mean energy savings will be made in reality, even if this is not accurately represented in SAP.
• Because of the risks of solid walls becoming saturated and unable to dry out to the inside, it is critical that the insulation and internal finishes used are fully 'breathable', that is, vapour permeable.
• To help counteract the likely internal temperature fluctuations caused by internal insulation, it is best to use an insulation material with some density. Common synthetic insulations have little or no density - their whole modus operandi is to avoid this – but some insulations are actually quite dense, meaning there is some thermal mass. 'Natural' products, like wood fibre, have some moisture buffering capacity as well. Combined with a hygroscopic internal finish, this will beneficially moderate temperature and humidity swings internally, safeguarding the respiratory health of the occupants; refer back to the diagram in Section 2.2.
Our Guidance vs Conventional Guidance
Topic | Conventional Guidance | Our Guidance |
|---|---|---|
ENERGY | There are various options, but high-performance insulation reduces the thickness required, less space loss & better U-value. | Lambda / U-value is less important than continuity of insulation. There are no gaps in insulation or air/moisture control layers; this leads to several details (e.g., window reveals, floor depths above and below, etc.) Maintained (dry) wall reduces heat loss. Natural insulation reduces embodied energy. |
HEALTH | - | Effective insulation reduces cold spots and mould risk internally & improves comfort. The use of natural insulation reduces respiratory health risks. The use of dense insulation reduced fluctuations in temp / RH. |
FABRIC | DPC or air gap between the internal frame and solid wall. | |
MODELLING | - | As Energy above – emphasis on little details in practice. |
FABRIC | - | As Energy above. |
MOISTURE | VCL internally, usually part of composite board. | Impervious internal finishes are to be removed. Vapour permeable and hygroscopic insulation helps protect the construction from saturation. Continuous VCL protects construction from internal vapour pressure, while the emphasis on maintenance reduces risks from outside. Modest U-values mean reduced risk of interstitial condensation. |
PEOPLE | - | Invest in upgrading services, etc., before installing insulation (less cost and disruption later). |
CONSTRUCTION | - | Proposals combine existing construction with new, working together, rather than separate internal ‘shell.’ |
MAINTENANCE | - | Emphasis on ensuring all external maintenance is carried out first. Electrics in service void. |
SIGNIFICANCE | Cornicing is either removed or left in situ. | Emphasis on continuity of insulation could lead to removing cornicing, etc., but where the internal finish is significant, this should be retained but could be replaced over the insulated finish. |
The issue of internal insulation is where this guide differs most from conventional guidance. Current approaches to internal wall insulation are much more problematic than most people realise, with the potential to do widespread damage to our built heritage whilst not necessarily creating the savings expected.
Conventional guidance focuses on attaining good U-values, primarily through high-performance insulation products. In contrast, BPE has shown that this does not necessarily lead to good performance overall - and therefore, we concentrate more on the continuity of the insulation and airtightness/vapour control membranes to close the gap between anticipated and actual improvement. High levels of internal insulation increase the thermal separation of the original masonry wall, increasing the risk of moisture problems. Therefore, we recommend relatively modest insulation levels and that a ‘breathable’ material be used to reduce risks associated with trapped moisture.
Drawings representing typical (but poor) practice and our recommended approach are shown overleaf and opposite for direct comparison. We should stress that these drawings aim to illustrate the sorts of issues that need to be addressed and, in the case of the typical/poor practice drawing, indicate a ‘worst case’ scenario very much. Numbers relate to the problems/solutions discussed alongside, but we have used these numbers below to help identify particular items on our proposed detail.
There are various common configurations of IWI:
- Insulation is tight against the wall with an internal finish applied directly over, i.e., no air gaps. The disadvantage of this arrangement is that any service boxes installed into the wall will compromise the insulation and could lead to cold spots and condensation/mould locally.
- A cavity is created between the insulation and the wall, usually using battens. The disadvantage of this arrangement is that, in many cases, air leakage from external sources is free to flow behind the insulation and usually finds its way into the building or cools the internal surfaces as a thermal bypass. The potential advantage is that the air movement may dry the external wall, although this cannot be controlled. Unless an additional service void is created on the internal face of the insulation, this arrangement still suffers from the compromise of insulation when installing services boxes.
- Insulation is placed tight against the wall with a service void installed over it. We chose this configuration because it avoids the disadvantages of the other two arrangements.
Preparation
Check the external wall condition.
The first thing is to ensure that the exterior of the wall, including the roof work, guttering, and ground areas, are all in good condition as described elsewhere, keeping the wall dry [14]. Maintenance is always important, of course, but adding internal insulation increases the risks associated with poor maintenance.
Remove cement render
Ideally, cement renders and pointing will be removed and replaced with lime-based alternatives. This is easy to write but far harder and more expensive to carry out in reality and needs to be fully accounted for when weighing up the relative costs of internal and external insulation.
Remove impermeable layers
To ensure that moisture cannot be trapped within the wall, no impermeable layers must be left on the wall before it is insulated. Old wallpapers and paint should be removed to return to a suitable substrate unless of historical value. This will likely be either the masonry itself or a lime-based plaster finish.
Remove and replace the external wall finish first.
If the existing wall finish is to be retained and it is possible/acceptable to remove and replace it, this must be done first. If the existing finish cannot be removed, refer to the alternative detail discussed below.
Extend insulation coverage
Depending on the circumstances, extending the insulation into the floors above and/or below may be necessary, which may mean removing ceiling or floor finishes adjacent to the external wall [2]. Again, this is easier to write than to do and adds to the workload, cost and disruption of the works.
Existing services
Existing cables and other services should be placed within the conduit to be buried within the insulation. However, this will mean a reduction in the depth of insulation, the need to cut around it and an additional penetration through the airtightness layer. Therefore, It is better to loosen/disconnect and be reconnected / re-installed on the warm side of the insulation and airtightness layer once these are installed.
Windows and doors
If windows and doors are to be replaced or repaired, it is best to undertake this before the insulation is installed so that it can be carefully fitted to and sealed against window frames.
Common Issues with Conventional Internal Wall Insulation
ENERGY PROBLEMS
1. heat loss via gaps in the insulation
2. heat loss: no insulation across the floor joist junction with the wall
3. heat loss around the window: insulation is not taken into reveals
4. air leakage around the window
5. increased heat loss due to saturation of wall in places
6. thermal bypass: cold air behind plasterboard cools internal surfaces, increasing radiant heat loss
7. thermal bypass: cold air flowing behind plasterboard draws away heat within the fabric to the outside
8. high-performance insulation with high embodied energy
COMFORT AND HEALTH PROBLEMS
9. discomfort due to draughts
10. condensation and mould forming on cold surfaces
11. internal insulation reduces access to thermally massive /hygroscopic surfaces, increasing fluctuation in temperature and humidity, with accompanying increased health risks
12. reduced IAQ: increased presence of mould spores, increased humidity, VOCs, etc., from synthetic materials
BUILDING FABRIC AND CONSERVATION PROBLEMS
13. loss of original cornicing / plaster/linings, both a 14 conservation loss and a resource/ waste issue, more to landfill
14. opportunities for maintenance and repair not taken, with conservation and practical implications. (downpipe fixing loose, downpipes blocked /water flowing into the hole created, crack in cill, cracks in render and missing / boss render.) this also creates H&S issues if masonry falls
15. combination of interstitial condensation, gaps in insulation, rainwater penetration, lack of breathability, and capillary action create moisture spread between insulation and wall - ideal conditions for rot
16. increased risk of rot/insect attack of timber-safe lintels
17. increased risk of rot/insect attack on floor joist ends
18. saturation of wall leading to increased risk of leaching of salts, failure of mortar, spalling of masonry to outer faces
Recommended Internal Wall Insulation Solution
ENERGY SOLUTIONS
1. there are no gaps in insulation; all joints/corners are taped
2. insulation taken across floor depth and taped against all joists
3. insulation taken into all reveals.
4. the window is fully taped/sealed to the surrounding wall (no air leakage)
5. wall remains dry: better insulation
6. no thermal bypass: no cold air entering, insulation tight to wall: internal surfaces remain warm
7. no thermal bypass: no cold air behind plasterboard, so no heat loss
8. natural, hygroscopic insulation with lower embodied energy
COMFORT & HEALTH SOLUTIONS
9. no draughts, greater comfort regardless of air temperature
10. all surfaces are relatively warm: less risk of condensation and mould
11. relatively dense and hygroscopic insulation offers both thermal and moisture balancing mass, so temperature and RH fluctuations from loss of access to masonry are not as problematic
12. minimalreductiontoIAQthrough use of natural materials, non-toxic finishes, more balanced RH, less risk of mould
BUILDING FABRIC AND CONSERVATION SOLUTIONS
13. loss of original fabric is still an issue, but refer to alternatives for an approach which avoids all removal of original finishes
14. external maintenance is carried out first. Downpipe cleared, fixing repaired, render repaired, all pointing and render in lime to allow the wall to dry out, crack in cill repaired. No H&S issues with insecure masonry
15. vapour permeable insulation + equalising coat reduces risks of interstitial condensation; condensation that does occur is diffused within construction and can dissipate
16. reduced risk of rot/insect attack of timber-safe lintols (dry wall, warm surfaces, moisture can dissipate internally)
17. reduced risk of rot/insect attack of floor joist ends, same reasons as above
18. the wall remains relatively dry, thus reducing the risk of leaching of salts, leaching of mortar, spalling of masonry to outer faces via freeze/thaw action, etc.
Installation
Equalising plaster
Most systems require an ‘equalising’ plaster layer initially [6]. This may not be necessary if the substrate is smooth and level. Still, this layer usually fulfils several roles, including drawing moisture from the wall, filling out irregularities in the wall surface or simply providing an adhesive surface against which to affix the insulation boards temporarily.
Window reveals
Window reveals should receive as much insulation as is feasible [3], assuming this will always be less than the wall itself, and the wall between floor joists beneath should be insulated and made airtight first [2] so that floorboards can be replaced, allowing for the safe installation of the rest of the walls
Masonry partition walls
Masonry partition walls represent a thermal bridge, and removing these is rarely appropriate, safe or cost-effective. Apart from simply accepting this and acknowledging the risks of cold spots here, conventional wisdom suggests that these walls should be insulated internally for at least 1m ‘back’ from the external wall. It should be carried out where this is technically possible and acceptable. Not being subject to the complex moisture issues of exterior walls, it is possible in these situations to use thinner and non-breathable insulation, which helps to minimise the ‘step’ in the wall surface that this creates.
Insulation boards
Insulation boards can applied to the main areas of the wall. There are various ways in which these can be affixed, sometimes a mix of both adhesive and mechanical fixings [8]. The boards must fit together neatly with no gaps, including at all corners.
Air barrier and vapour control
There is some debate about the value of an air barrier and vapour control membrane. In our view, as long as the ventilation system provides a reliable under-pressure, meaning that moisture is always being removed from the property, it is usually acceptable to go without a specific membrane. Sealing all of the insulation joins is important, so these should be carefully taped using a suitable tape recommended by the manufacturer [1].
If in doubt, model the wall performance.
If in doubt about using a membrane, it may be wise to model the wall using one of the various hygrothermal modelling tools, such as ‘WUFI’, ‘DELPHIN’ or others. A number of manufacturers offer this as evidence of the efficacy of their products. If a membrane is used, it must be an ‘intelligent’ type, which allows vapour back into the property at low pressure. If a conventional membrane is used, there is a risk that moisture in the wall can become trapped.
Re-affix services
Once the insulation (and membrane, if required) is installed, re-affix all services within a service void. In this way, none of the services need to penetrate the membranes or insulation, meaning the performance is safeguarded.
Apply finishes
Any conventional finish can then be affixed to the battens, including plasterboard, timber lining, or some form of mesh and lime or clay plaster finish and finished with decoration as necessary. Remember that conventional paints tend to be impervious to moisture, so it is important to use vapour-permeable paints, especially if used on a plaster finish applied directly to insulation.
Alternatives
Insulation injected behind lath and plaster
• A form of insulation trialled by Historic Environment Scotland was to inject a variety of insulations behind existing lath and plaster. Results were mixed but mostly acceptable.
- The main concern is that it is difficult to control quality with a likelihood of awkward, hard to reach areas being missed leading to cold spots.
+ On the plus side, this technique allows almost all of the existing finish (usually lath and plaster) to be retained which is a valuable resource saving and conservation gain.
+ In areas where the existing finish has significance, then this is an ideal technique to balance energy saving with conservation, with minimal disruption.
• As in the main example, it is very important when undertaking this technique that the external face of the wall to be insulated is well maintained.
Insulating plaster
Another technique is to install an insulating plaster directly onto the prepared wall.
+ This technique has some advantages, mainly that it represents only one or two operations and so is likely to be much cheaper, quicker and simpler to undertake. The modest U-value improvement are not a problem (although they may be for SAP / EESSH compliance).
- But this technique suffers from compromises where service boxes are installed within the depth of insulation. Where this can be avoided, for example through surface mounted services, it may offer a good solution to IWI in many cases.
Health and Safety
Removal of existing finishes can be of some concern. Beyond simple generation of dust, additional concerns surround animal hair which is sometimes found in traditional plaster and there are issues surrounding the removal of lead paint.
• If the furniture of occupants is left in the rooms, then it is important that these are protected from dust and rooms sealed while works are undertaken to stop the spread of dust to other spaces in the property.
• It is assumed that works can be carried out from inside and so there are no issues associated with working from height, nor confined spaces.
• In some cases, we recommend that the insulation extends into the depth of the floor beneath a room and in these situations, it will be necessary to exercise greater care around the gaps in working platform created.
How much is enough?
• Depending greatly on the masonry wall itself. We would recommend a depth of insulation of between 40mm and 80mm. Using 60mm woodfibre with a lambda value of around 0.045W/mk on a typical 500mm stone wall should give a U-value of around 0.45W/m2K or less.
• If this is continuous across all obstacles and penetrations then this is adequate.
• Depending on the circumstances of the retrofit this may need to be agreed with building control (who may require lower U-values) and bear in mind that most within the industry would consider this an inadequate return – on paper – for the effort and cost.