Thanks mate, I appreciate the comment and the tact you've taken with it. I didn't make it especially clear on the video (you'll see more in the next one when we tile the roof) but I actually counter batten across the top of the multifoil, which creates the air-gap. I wasn't especially convinced that having multifoil outside the roof structure made all that much sense, and at £100+ per roll I was gritting my teeth a bit, but doing as I was told by the SE who spec'd it all. Ps. We do also multifoil inside the roof as well, to stop cold-bridging and vapour control!

@@OfficeBoyBuilder I was trying to be as respectful as I could and just highlight that there are others ways of doing this. I've been designing roof build ups for 20+ years and I still can't see the sense in the air gap between the foil and the insulation. Underneath you'll 'reduce' cold bridging by using the multifoil, but it won't stop it. I'll wait and see how you do the internals, but the usual spec is that the counter battens squash the foil to almost nothing, so your thickness on the underside of the rafters is reliant on how strong your nail gun is. 150mm PIR between and 75mm PIR below also gets you down to 0.11 and uses no foil and would mean that other than the Velux openings your timbers are almost 100mm away form the internal surface finish. The point stands about the rooflights too, if they make up 50% of the surface area and have a Uw of 1.3 (or worse) then these will make a serious dent on all of the heat loss work around them and bring the average roof down a good deal.

Interesting video, just subscribed. I guess that the gap between the celotex and the super foil will help with radiant heat transfer, but I didn’t see any way for the air to flow out of the top of the roof. May have missed it, does that reduce the effectiveness of the air gap at removing moisture? I’m not an expert at all so keen to understand from any one.

The air-flow is over the top of the multi-foil as we counter battened across the top of that. The little air gap I mentioned is just trapped air which helps with Insulation too, by virtue of reducing convection.

That's good to know, thanks mate - I did give that space around the whole window frame, as it makes it easier to fit them, but I wasn't aware it was for the plasterboarding, as the grooves for the boards are set in for the edges of the frames anyway. Still, the space worked fine 👍

Cheers mate, glad your enjoying it, thanks for supporting the channel!

Cheers! We're doing another layer of multifoil inside the building too! We're almost down to passive house standards I think, except for the air-tightness...!

@@OfficeBoyBuilder but you have 6 large Velux windows so does it not defeat the object? I didn’t see how much other glazing you were installing but be careful not over glaze the extension as you’ll require a SAP calc.

Thankfully we come under the need for a SAP calculation, we did check that 👍 we needed the roof to be as highly spec'd as we could to allow that number of Velux's.

@@OfficeBoyBuilder fair play 👍🏻 good luck with the build.

Best of luck mate. One consideration which I didn't anticipate with this design, is just how much of the view of the garden is blocked from the first floor windows...! The roof projects so far out, and is so high, that we can barely see the end of our garden from one of the bedroom windows. It's not a major concern for us really, but I didn't have any concept of how that might happen before I built it.

@@OfficeBoyBuilder Thanks. Mine isn’t as large as this and it is conventional brick and block. 4 motorised Velux going in and I think I’ll be going for celotex between and under rafters. I’m only after a u value of 0.16. One query - my SE has suggested a steel ridge beam which I can’t understand as the building is only 3.8m so was planning on 1 or maybe 2 x 200 x 50 C16?

Sounds great! I'm not a SE, so I can't comment from a calculations pov, but I understand that the ridge beam does a lot of the work bearing the load, as do walls. We needed the flitch beam as a result to help bear the weight. You could ask about s flitch beam instead, which is still super strong, but costs less and weighs less too!

@@OfficeBoyBuilder what’s a Fitch beam? 🤣

That's the big ridge beam that we installed in the middle/top; it's a flat bar of steel sandwiched in between two bits of timber. We've used three of them on this build so far, and I'm quite a fan of them now. Very strong, much easier to manoeuvre, and far simpler to continue building with, rather than steel.

Cheers mate! Yeah I think that's a good plan, and thankfully I haven't closed it all up yet so I can still add some in at this stage without too much issue. Only having thin PVC vented soffits under there, but no reason not to add the extra strength while I can, and I have the materials.

@@OfficeBoyBuilder A block every 3' o/c would be sufficient, I'm assuming a tiled roof plus a pointed verge so no harm to have the extra support. I've seen lots of barge rafters sag over time even with minimal overhangs. I'm looking forward to your next installment, 👍🔨 🇮🇪

I like to be really scientific and accurate with these sorts of things...🤣

Yeah we have a vented roof the whole way up and over...vented ridge at the roof, and counter-battens will sit across the top of the battens I've run down the length of the roof (you'll see that in the next video). Interestingly the building inspector was fine with 1 timber at the top and bottom of the openings because they're 8*3 timbers with about 4 timberlocks in each side 💪 I was quite happy to put two in, but he said he was happy

Good job it's all covered by the Structural Engineer's professional indemnity and the Building Inspector too who signed it off...I stuck exactly to the brief I was given by the SE.

Cheers mate, really appreciate the support! I'm trying my best to be honest and genuine with my efforts and mistakes! As a keen DIY'er it's taken years of research and I'm still learning along the way. I'd imagine there's tons of stuff that pro's scratch their heads watching me do 🤣

Mate you're absolutely right, but not being s roofer I didn't realise this. Thankfully Steve the roofer I use was happy to add it on and the ridge came out perfectly. Good spot! We had to make up a bit of leadwork around the sill, after cutting it back slightly. Thankfully we've had no leaks or water coming in since doing that, but it was touch and go with the height of the roof, I won't lie!

As a roofer it’s becoming a regular thing with the structural engineer specifying the double ridge beams and not taking in to consideration of fixing top course of tiles and getting the ridge to cover along with keeping a gap for ventilation. In your case your lucky that you had to counter batten and that makes dealing with the wide ridge easier. Looking forward to seeing that video. Keep up the good work 👍🏻

There was loads that I had researched and prepped for, and then loads that I learned and had to adapt when I was building it. Thankfully I worked with a roofer/tiler who I trust and know, so he was happy to walk me through things that I'd missed.

Interestingly the building inspector was fine with 1 at the top and bottom because they're 8*3 timbers with about 4 timberlocks in each side 💪 I was happy to put two in and double them up, but he was fine with 1.

@@OfficeBoyBuilder thanks for the reply all the building inspectors we deal with insist on a double up all 4 side but your reply is very interesting cheers

It is extraordinary the amount of variation there is between building inspectors from different working backgrounds, regions around the country etc!

Always double round ours but there had us doing some mega overkill

Cheers, I can't claim to have come up with that idea, it was my architects suggestion, but it was a bloody good one! Much easier with the PIR, and it also gives me something solid to fix the quilt to on the inside, and cross batten/plasterboard. I think it's must easier to work with, and incredibly warm. Although when you fix it in palace with battens that compression obviously reduces it's effectiveness vs. PIR. On a sloped roof you could probably get away with 20mm stuff really, rather than 50mm. But either way, you've got a bit of a nightmare fixing the stuff into the studs easily (and the plasterboard on top of that!) - I always manage to miss the studs, even though they travel in a straight line! 🤣

I bought this one www.essential-carpenter-tools.co.uk/about I am happy working with metric and imperial, but find metric easier to be more accurate when using smaller increments

Yes, timber frame is considerably lighter, so it doesn't quite have the same level of thermal mass, but the insulation and U-Values are significantly higher than blockwork (for the same thickness of wall).

@@OfficeBoyBuilder Thanks for the reply! Is there any issues with soundproofing or are there ways to rectify this with timber builds?

Our main extension is out the back, which is generally very quiet. The side extension bedroom is at the front, and is also quiet. PIR isn't as good at stopping sound transmission, but often when people are directly knocking or banging on it. For general airborne sound it's absolutely fine. There are also counter-battens on the inside and outside skin of the building (to carry the render board and plasterboard) which provide additional acoustic protection. All told, it's very quiet

Thanks mate. Glad you're enjoying them! I saw the design on an image, and fell in love with the design...it's as basic as that 😂 It does allow a significant more height in the room, which means more light through more windows, but it does restrict your view of the garden from the upstairs windows (because of the projection of the ridge).

@@OfficeBoyBuilder I agree. I hadn't actually considered the view from the upstairs window, but it'll still be a pretty sight. I too decided I wanted to do my extension with dual pitch because of the pictures I saw and the height/light it brought in. I have had planning approved, but thr only issue is now, if I wanted to do a rear first floor extension, I'm told it'll be more difficult to do. Have you or did you consider a first floor rear extension? And if you did, what are you plans?

Nice! We decided very early on that we wouldn't be doing a first floor rear extension. For a start, our room layout just doesn't allow for it. Secondly it's not been done on our street so it could be harder to get through. Thirdly we just don't need the extra space - we now have 5 beds and 3.5 baths. It would be almost impossible to add a single story rear now, and would disrupt the roofline.

Thanks mate, I appreciate the comment and the tact you've taken with it. I didn't make it especially clear on the video (you'll see more in the next one when we tile the roof) but I actually counter batten across the top of the multifoil, which creates the air-gap of 25mm+ the whole way across, up and over the insulation and the whole roof

The span was 5.5m and we used 8x3 timbers 👍

Big old boy then. My strada book doesn't even go that big lol

So it was quite a handy set-up, our Architect and SE were all combined, along with the planning consultants who got us through the planning/build process. We had 1 conversation with 1 person and then they drafted in the specific people from their team for each element, as required. So when I say our SE, I'm referring to our architect really, but they're all from the same company.

@@OfficeBoyBuilder ahh that makes sense then! Very handy. Timber framed extensions/ buildings are great as long as they are detailed and built correctly I'd be interested to hear if you had a cost comparison done between brick and timber frame? The US and NZ have been building timber framed house many more years than us and are very good at it. They get a bad name in the UK as a certain house builder beginning with P can't seem to build them properly!

Yeah the US and NZ are light-years ahead of us! I did look at brick and block, and I'm couldn't see it being drastically cheaper really, save for the labour cost, where I'm saving the entire lot by doing the work myself...it is also quicker, and time=money (although this isn't an investment property, so that equation is less impactful here).

I'm not sure the exact brand of multifoil, it's just the generic stuff from Travis Perkins, which was the best price I could find locally. That also acts as a vapour control barrier, so we're putting that all on the inside of the roof as well.

@@OfficeBoyBuilder thanks. I’ll take a look as I’m at a similar stage to you in my self build extension on my bungalow and now need to add insulation to the inside. Nice vlog by the way.

I didn't make it especially clear on the video (you'll see more in the next one when we tile the roof) but I actually counter batten across the top of the multifoil, which creates the air-gap, as it's a warm roof

@@OfficeBoyBuilder Tidy build though.

I know Lol! I lost a stone in a month just shifting them all round 🤣💪

@@OfficeBoyBuilder looking good though mate

Yes mate, I went round and filled every little gap with expanding foam before laying over with the multifoil. I do the same in all the walls and floors too 👍

@@OfficeBoyBuilder nice one. When i did mine i cut everything very tight. But still had gaps. Which were harder to seal cos i cut the insulation so tight! My mate who does it for a living says they cut it undersized. Then seal around the entire thing. Easier to get the nozzle in. Wish i had done that.

Yeah I was recommended to do that by a chippy I worked with. I deliberately cut it 5mm short the whole way round. Invariably it just about squeezes in anyway because the stuff's designed and manufactured in Satan's personal workshop, but the expanding foam definitely helps with the gaps.

Not seen a design like that for cold deck. Surely multifoil isn't it breathable. Looks good though. FYI festool insulation saws are amazing. Bit over kill if you wouldn't use it alot. Use a straight edge and sharp knife on a flat board to cut multifoil. Are you using Tyvec airguard smart on the inside?

I asked the architect about vapour control internally, he said that any additional layer of multifoil inside the roof would help with insulation and also be a vapour barrier too. The Festool saws look brilliant, but I'm not on their battery or hoover system, and although there's been tons of the stuff to cut this time, I couldn't justify the cost.

That's interesting, I hadn't heard that...perhaps that's for custom built places, rather than extensions? Total cost is around £100-110 for the finished product (2 storey side and single storey rear), give or take.

Buildings insurance is normally higher for timber frame

Thats a really good price considering the amount of space gained. We are looking at potentially a single story out of brick and block then the 2nd story timber framed, purely for speed of installation Really good project though, thoroughly enjoying the series

You're 100% right mate, no offence taken! I'm normally an absolute stickler for mess and keeping things tidy and safe etc. As shown in previous videos. Later in some up-coming videos you'll probably see me hoovering out dust and cleaning in between all the joists! We were building at such a pace, and there were only two of us, so it built up a lot quicker than we had time to clear. We did end up spending a decent bit of time having a full tidy up later on to make it safe and easier to take stock of materials.

I'm honestly not sure about the specific timber U Value, but the whole roof, once finished, will be 0.11

@@OfficeBoyBuilder Unless another layer of PIR is going under the rafters I doubt the U value considers the timber cold transfer.

There's another layer of the multifoil underneath....I didn't do the calculation, the SE did, but I don't think they included the velux's in that calculation, I think that's the roof element

HahHa. Joking. U= 0.11 w/m2 C° the roof wichtig styrofoam. Building like in the 80s.