Hello, I would like to insulate and board my loft. Currently i have 270mm rockwool between and over 4x2 rafters. I was intending to take down the ceilings (as i'm doing a full refurb anyhow) fix a 25mm batten top and bottom of existing 4x2 joists to increase the depth to 6", install 140mm rigid foil faced between and 25mm rigid foil faced over the top of the rafters, install the T&G flooring over the top of the 25mm insulation and board the ceilings back up using foil backed board or tacking a vcl to the underside of the joists. This should achieve the required u-value. I guess my question is, is there an easier more cost effective way to do this rather than remove all the ceilings as they are currently larve and plaster. And also if i did choose to do it the above way would i need to leave any vents in the loft at eaves level or install some tile vents?? Thanks in advance.
That's a lot of work and effort. The "required" u-value is whatever you want it to be. I did out my loft with 100mm knauf earthwool in between the loft joists and 70mm PIR in between the rafters with airtec double acting as the VCL. Its another option but a lot of effort in a very hot loft. An easier option would be to screw down 2x2 treated timbers to your existing loft joists and put in whatever PIR will fit..140mm? You should have good ventilation to the loft as the loft will essentially be open to the elements and cold.
Easiest option I think is to cross-lay 4x2s over the existing loft joists and install 2 lots of 100mm rockwool or similar (Knauf earthwool) (a) between the existing joists (b) on top between the additional joists. With 200mm rockwool plus chipboard flooring, it'll be good enough. If you lay the first lot of 100mm insulation between the existing joists, you could add a layer of airtec double before screwing down the cross-laid 4x2s and adding the second layer of insulation. Then chipboard on top of that. I'm assuming the objective of this exercise is to board out the loft but keeping a reasonable amount of insulation? How much headroom is there in the loft?
Yes thats right i need to board out the loft but I'd like to to keep the ceiling level as insulated as possible. i have a very shallow low pitched roof which is why i would like to add minimal depth to the top of the joists.
OK similar situation to me, I have 30 degree roof angle and not much headroom hence why I chose to insulate between the rafters as well. Can you do this?
This was an option. However, should the insulation not all be on ceiling level to bounce heat back off the ceiling? I have no interest in heating the loft itself. Neat job by the way.
There may be some heating inefficiency doing it this way rather than all insulation on the ceiling but I think its minimal. 2/3s of the insulation is in the roof space so the loft stays a nice temperature and much more suited to storing stuff - it won't get cold nor damp. Yes, in winter, you're also heating up the loft space a bit but that space then acts as a kind of massive air gap - a thermal barrier if you like. Its all a matter of compromise. With minimal headspace and the desire to use this space for storage makes this solution pretty darn good IMO. Clothes can be hung up almost unprotected - no dust, no damp.
You're lucky you don't appear to have a need for ventilation in your loft. My loft needs as much ventilation as I can provide it to get rid of the huge amount of condensation that collects on the underside of the tar paper which is just under my tiles. The roof was built before the days of breathable membranes. Before I added my own ventilation holes, my loft used to sweat so much that it actually dripped down onto my loft floor boards. and then down into the insulation under the boards, rendering it almost useless. It's now much better, but still not as dry as I'd like it to be.
Why are you bothering? 270mm is the minimum required to meet building regs and is plenty warm enough, usually one layer of 100mm between joists then one layer of 200mm cross layered over the tops of the joists. What you suggested is a massive upheaval and huge expense when not needed.
It all depends what you are trying to achieve? As Jord say you have plenty of insulation there, you could top it up but the gain will be minimal. If you want to board it, then just go out and buy some loft legs and some boards It will be much cheaper and you wont turn your house into a dust bowl and cover yourself with all sorts of insulation fibres
The main objective is to board the loft, but, keep a similar thermal value. Creating a mess isn’t an issue as the house is currently a building site and not occupied.
do you want to have it as an attic room or just for storage ? If you want it just for storage put the loft legs and chipboard /osb on top.
I've just done this, similar to Dr Bodgit. Cross joists in 4x2, two layers of 100mm insulation, overboarded chip flooring. We then built a stud wall/ceiling frame inside the loft dropping from the bottom of the purlins (which are about 14x4) and with "rafters" in 3x2 set about 300mm inside the roof rafters (so plenty of ventilation still between the roof of my "box" and the actual roof) - in effect a box inside the loft. Lined with PB and some insulation, we have a dry boarded storage area separated from all the dust and any condensation in the loft itself.
Hey Dr Bodgit, I seen alot of people recommending cross laying 2x4 battons. But my 2nd Layer of insulation is 170mm which is more than 6 inches. For this reason - can I use 2x6 battons? My carpenter asked the question whether 2x6 would be too heavy to screw into the existing ceiling joists?
MY Hi Jord86 Thanks for your response - really racking my brains out to try and find an answer. Its a 1930s property. The existing ceiling joists are around 2" by 4" themselves, approx. every 13inches apart - sitting on lath and plastered ceilings. Roof is a Gable Roof. I was thinking of screwing the end batons to the Roof rafters to spread the weight distributions across. Raj
Your carpenter should know what to do mind you, he's seen it in person and we haven't. You need support from your upstairs walls in order to load the existing ceiling joists further, they were only ever designed to take the weight of the ceilings and a small amount of storage.