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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorward32
    • CommentTimeMar 8th 2017
    I thought my experience with Building Control over my roof insulation might be helpful to others:
    This was a full plans application and after some further information about window and door reveals and wall condensation risk assessments (solid wall + EWI) I got their approval. No question about the roof so I assumed that was all fine.

    The roof was designed by a S.E but he did not specify the coverings and insulation. My build document to BC stated:
    22mm Uditop sarking
    200mm Omnifit Stud between rafters (200mm rafters)
    70mm PIR to the underside of the rafters
    VCL under PIR
    Plasterboard under VCL.

    Inspection was before VCL and plasterboard and went something like this:

    BC said the construction didn’t match the (my) specification and because I hadn’t read it just before he arrived I just accepted this – first mistake.

    BC: there will be a huge condensation risk of the wool sweating behind the sarking.

    ME: there’s a VCL on the inside and the sarking is vapor permeable so where is the moisture going to come from?

    BC: there should be a ventilated air space between the wool and sarking boards. There’s too much insulation! (this was clarified later in the context of “a ventilated air space roof” – 4 days later).

    ME: there isn’t an air space! I have constructed it to the sarking board manufacturers recommendations

    BC: I will need to see the manufacturers data sheet.

    End of visit.

    Emailed Uditop sarking board data sheets, the full plans build specification (which they had from day one) and a condensation risk analysis for the roof build-up.

    Phone conversation with BC officer a couple of days later went something like this:

    While BC was going through my build specification he was struggling to understand the written spec’. I explained the make-up of the roof and how it was as the manufacturers data sheet, and exactly as I had constructed it. [to be fair I didn’t draw the roof make-up and the written description was out of sequence, hence the confusion].

    BC then looked at the manufacturers data sheet and immediately focused on a table talking about board thickness and maximum rafter spacing and said I was using more than 100mm of insulation. The table actually meant I could use 22mm boards up to a maximum rafter span of 850mm while 100mm boards could have a max spacing of 850mm! [I'm losing confidence!]

    BC then moved to the construction examples in the data sheet and asked if I had used their proprietary insulation and I said no (I was tempted to say yes for an easier life because I knew what would follow). He also pointed out that the data sheet did not show PIR boards under the wool insulation

    BC said he couldn't accept this because I have used a hybrid system that is different to the manufacturers data sheet. [manufacturers data sheet show their own insulation products but I had used omnifit as it's bit better insulator but has a similar vapour permeability. I had also included extra insulation below rafters which is not shown on the data sheet - I'm going down!!]

    [I wanted to say they had the full plans 18 months ago so why wasn’t this questioned before I built it– but I held my tongue]

    And finally…

    I directed him to my condensation risk analysis and after I had talked through the numbers he was happy – to a degree.

    BC said he would put copies in the folder and sign it off.

    And finally I get an approval email:
    I am willing to accept the details but must inform you that any defects likely to be caused by interstitial condensation in the future, will be at your own risk.

    Any advice for the future?
    • CommentAuthorDarylP
    • CommentTimeMar 8th 2017
    LA Building Control or AI ? first comes to mind...

    Then, I cannot see the correlation between the CRA and your build-up? you say PIR but the CRA says phenolic...
    BCOs are not generally construction experts, they are reliant on SEs and other consultants to provide the evidence on your behalf to satisfy them that your design meets Building Regulations.

    They have covered themselves with the 'get-out' clause from future claims.
    If you are confident that what you have built is good, then don't worry?:wink:
    • CommentTimeMar 9th 2017
    Well done on getting approval.

    I don't understand the CRA. The external temp of -1 °C seems way warmer than I would think prudent to allow for. The roof surface, whatever it is, should protect you against exposure to night sky temperature, but even sheltered air temperatures are likely to be lower. Where are you?
    • CommentAuthorward32
    • CommentTimeMar 9th 2017
    I'm not particularly worried about the design as it doesn't really deviate significantly from Uditops examples. But the response of BC did rattle me and make me re-examine my previous assumptions.

    As for the CRA, standard practice is to use BS EN 13788:2002 as the basis for weather data and I did notice the -1DegC temperature in January. There is not an easy way to change the outside air temperature so I ran a test with the internal air temperature set at 38DegC. My logic being this would be equivalent to an internal temperature of 20DegC and an outside temperature of -18DegC (using existing weather model) - it would create the same vapour pressure driving force, but correct me if I'm wrong?
    The program still did not report any condensation risk.

    The only way I could get a condensation risk warning was to change the inside RH to 80%. But as I don't have a sauna or indoor swimming pool I doubt this will ever occur.

    An MVHR system is being fitted so I really don't think there will be a problem in the construction, I just thought the encounters of a do-it-all first time builder might help someone else.
    • CommentAuthorEd Davies
    • CommentTimeMar 9th 2017
    Posted By: ward32My logic being this would be equivalent to an internal temperature of 20DegC and an outside temperature of -18DegC (using existing weather model) - it would create the same vapour pressure driving force, but correct me if I'm wrong?
    Equilibrium vapour pressure doesn't scale linearly with temperature; roughly speaking it doubles for each 10 °C increase. So you're playing safe in a way in that the vapour pressure difference you're using is greater but it hurts my head to think about what it's doing to the condensation risk on the way through.
    • CommentAuthorward32
    • CommentTimeMar 10th 2017
    It was LA BC.

    To set my mind at rest I went back to the Dewpoint s/w package to see if I could force condensation conditions. I applied the maximum internal temperature and increased RH until condensation risks were raised. From about 75%RH upwards there is a risk of interstitial condensation on the internal VCL layer, no risks occur further in to the construction.

    You can't do much with the external weather data in the demo version of this s/w so are stuck with average seasonal temperatures, hence -1DegC.

    I'm as confident as I can be that the structure will be free of interstitial condensation so on to the next job.
    • CommentAuthorgyrogear
    • CommentTimeMar 10th 2017
    Posted By: ward32I'm as confident as I can be that the structure will be free of interstitial condensation so on to the next job

    good quote, I have decided to adopt it for *ME* !

    • CommentAuthorlineweight
    • CommentTimeMar 10th 2017
    I've had similar experience with a BC inspector (AI not LA) recently.

    A more "conventional" buildup than yours (no sheeps wool or anything) but with substantial insulation, VCL on inside, and relying on breather membrane. BC guy was obsessed with ventilation above the insulation but didn't really seem to understand the basic principles.

    We left it at "I'll send you the condensation risk analysis", I never actually did because he forgot about it (often the way it is) and the job was signed off without further questioning.

    Meanwhile he failed to notice various fairly serious wrongly constructed roofing details. I did, and hoped he might back me up in my arguments with the builder but he just didn't seem to have much technical knowledge.

    Building regs can be an enfuriatingly box-ticking process... you just have to work out how to turn this to your advantage when you can.
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